A multi-class approach for ranking graph nodes: models and experiments with incomplete data After the phenomenal success of the PageRank algorithm, many researchers have extended the PageRank approach to ranking graphs with richer structures beside the simple linkage structure. In some scenarios we have to deal with multi-parameters data where each node has additional features and there are relationships between such features. This paper stems from the need of a systematic approach when dealing with multi-parameter data. We propose models and ranking algorithms which can be used with little adjustments for a large variety of networks (bibliographic data, patent data, twitter and social data, healthcare data). In this paper we focus on several aspects which have not been addressed in the literature: (1) we propose different models for ranking multi-parameters data and a class of numerical algorithms for efficiently computing the ranking score of such models, (2) by analyzing the stability and convergence properties of the numerical schemes we tune a fast and stable technique for the ranking problem, (3) we consider the issue of the robustness of our models when data are incomplete. The comparison of the rank on the incomplete data with the rank on the full structure shows that our models compute consistent rankings whose correlation is up to 60% when just 10% of the links of the attributes are maintained suggesting the suitability of our model also when the data are incomplete. Ranking algorithms are essential tools for searching in large collections of data and without them it would be extremely difficult to find the desired information. Following the introduction and the success of PageRank and similar ranking algorithms [8, 16] , researchers have extended such techniques to a multitude of domains [4, 5, 9, 12, 17] .. In this paper we consider the setting in which the data consist of a collection of linked items, where each item has a set of additional attributes (features). In this setting we assume that the ranking of items with common attributes are mutually influenced. Many important problems are instances of this general framework. In bibliographic ranking items are scholar papers and their citations give the linkage structure. For each paper its associated features are its authors, the journal where it appears, subject classification and so on. In patent data items are patents linked by the citations to older patents. To each patent we can associate inventors, firm, examiner, technologies, etc.. Other examples are social or twitter graphs -where we have information about the status, the geographical location, the education, etc., of users. In healthcare data we have patients, doctors, treatments, diseases, etc.. With a little abuse of notation in the following we informally use the term "multigraph" to denote this kind of relationships between items and features, while other authors identify this kind of graph with as heterogeneous information networks [26] .. In this paper we describe different models for representing the multigraph structure of a network, and analyze different techniques for assigning weights to features and to use these weights in the ranking process. These weights capture the importance that each link confers to the linked object. We then build a fast and stable numerical method for computing the ranking score according to our models. The proposed algorithm is obtained by combining two non-stationary methods (BCGStab [21] and TFQMR [21] ) and a final phase of iterative refinement.. We perform many tests on two large datasets of patent data extracted from the US patent office: the first dataset consists of all the patents granted in the period 1976-1990 (roughly 2.5 Million patents), and the second of those issued between 1976 and 2012 (almost 8 Million patents). The experiments aim at understanding the role of the parameters involved in the algorithm and the differences between the various models while comparing the results with those returned by PageRank and the ordering induced by the citation count.. We briefly investigate also the robustness of our models when data are incomplete and unrecoverable. In this setting our goal is to use all the information available without advantaging players (items or features) with more complete data respect to those where some information is missing. We treat unknown values as zeroes, in the sense that we do not distinguish between missing (not available) or absent (not existent) features. This choice is the simplest one and the one implemented in patent repositories and in many citation databases such as Scopus, Mathscinet where, for example, a citation is not attributed to anyone when the name of an author has been misspelled. To evaluate the robustness of our ranking schemas on possibly incomplete data, following the approach in related literature [31, 15] , we randomly remove features from items with an assigned probability. Our experiments show that, even removing up to half of the features, the ranks provided by our algorithm highly correlate to the ranks computed on the complete data. As expected, as more and more features are removed, the ranks converge to the rank obtained using only the linkage structure.. Finally, we tested the robustness of our models with respect to the granularity of the features. For example if we are dealing with bibliographic data we can group papers into subject classes where the granularity can be subject macro areas (Math, Computer Science, etc.) or finer classifications (Algebra, Number Theory, Calculus, Algorithms, Data Bases, etc.). In this context it is desirable that, when using a finer classification, the sum of the ranks of topic A subtopics is close to A's rank computed using the coarser classification. Experiments with the US patent dataset show that most of our models have such desirable features.. The paper is organized as follows. In Section 1.1 we formally introduce the problem we are considering in the paper; in Section 1.2 we motivate our study and connect the techniques and the algorithm we propose with the existing literature. In Section 2 we briefly present some models discussing how extra information and features can be added to the citation structure to improve ranking and possible weighting criteria for such features. In our models the ranking is obtained approximating the Perron vector of a suitable stochastic matrix.. In Section 3 we discuss different ways for approximating the Perron vector showing that it can be obtained computing the solution of a linear system. In Section 4 we discuss different methods for the numerical solution of such linear system and we describe the databases used for the experiments. In Section 5 we report an extensive numerical testing to compare the different models in terms of convergence for missing data and consistence for class aggregation. Section 6 contains the conclusion and some discussions about possible improvements of the models. In this paper we propose several models for ranking multi-parameters data on the basis of the linkage structure. We assume the citation matrix is enriched with other attributes (features) that can be represented by multi-class models. We use the attributes to improve the ranking process and as a by product we obtain a ranking of the attributes as well. After describing the models and different weighting strategies for measuring the influence of each feature in the ranking process, we describe an algorithm for computing the rank based on an iterative scheme which combines non-stationary and stationary methods. We test some of the numerical methods on two large datasets of US patents, and we address issues such as stability and convergence of the algorithm applied to each model, convergence with incomplete data, and consistence for class aggregation. In particular, the experimental part on large datasets shows that these techniques can be used in real applications where we have objects with multiple attributes and where some information can be missing due to the errors or incompleteness of the data. To search among the ordered list of objects one has to add a searching module and retrieve only objects relevant to a given query in analogy to what is done in the context of web search engines.. As a future research we plan to address the problem of spam introducing mechanisms for penalizing self-citations and spammers. A possible approach to deal with cheating could consists in appropriately weighting citations and in modifying the main diagonal of the diagonal blocks to mitigate the influence of spammers on the final rank.. Another challenging future work is the incorporation of a preprocessing phase aimed at recovering missing entries. Unfortunately automatic techniques such the one proposed in [15] do not seem straightforwardly applicable to our case, but maybe an attempt to recover data based on similarity of data in specific domains such as bibliographic data can be employed. For particular problems, such as bibliographic ranking, static indicators for journals such as Impact Factor or Mathematical citation quotient, are available. We plan to investigate how this information can be used in our scheme for improving the ranking process or as a starting point to reduce the number of iterations.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Infrared and visible image fusion technology based on directionlets transform The article provides an infrared and visible image fusion algorithm based on directionlets transform. The registered original images were decomposed into the low-frequency and high-frequency coefficients by directionlets transform. Getting the mean of the low-frequency coefficients, applying the local variance maximum principle to the high-frequency coefficients, thereby the fusion coefficients of the fused image can be acquired. Finally, the fused image was obtained using inverse directionlets transform. The experiment indicates that the fusion algorithm can extract the original image features better. Compared with the traditional fusion algorithms, the fusion algorithm presented in this article provides better subjective visual effect, and the standard deviation and entropy value would be somewhat increased. Infrared and visible image fusion is part of multi-source image fusion. Multi-source image fusion is the process of acquiring unified description towards the formation of high-performance perception system with employing different types of image sensors, and combing two of more kinds of image information effectively. It is the technology with comprehensive and optimized treatment for multi-information's acquisition, presentation, and internal relations [1] .. Because the approaches of the infrared detectors to obtain measured wavelength range of the target information are different and with different imaging modalities, the infrared image and the visible image's information are very different and complementary. Through the fusion of the infrared image and visible image, plus synthesizing the complementary and redundant information of the two types of images, the fused image's object's contour can be clearer than the original one with the characteristics of information-rich, and easy to identify, and the image sensor's perception towards the environment can also be extended.. Image fusion is mainly divided into three levels, namely the pixel-level fusion, feature fusion, and decision-level fusion. This article only focuses on the pixel-level fusion. There are many existing pixel-level image fusion methods, including the weighted average method [2] , the tower decomposition method [3] , principal component analysis (PCA) method [4] as well as fusion based on wavelet transform methods. The wavelet transform has become an important tool in image fusion method for its excellent feature of time frequency analysis [5] . However, the advantages of wavelet transform are mainly embodied in the analysis and processing of the one-dimensional piecewise smooth function or bounded variation function. When the wavelet transform was applied to two-dimensional or higher-dimensional domain, the one-dimensional separable wavelet which is spanned by the one-dimensional wavelet has limited direction; therefore, it cannot have optimal representation of high-dimensional function containing the line or surface singularity. Therefore, the wavelet transform can only reflect the signal point singularity, as far as the lines singularity and surface singularity contained in the linear and edge characteristics of the two-dimensional image is concerned, they are hard to be reflected with the wavelet transform approach. In the multi-resolution decomposition fusion algorithm, the adoptions of the wavelet transform fusion algorithms tend to introduce high-frequency noise, which would affect the quality of the fused image.. For the above-mentioned drawbacks of the wavelet transform, directionlets transform is anisotropic transform proposed by Velisavljevic et al. [6] , which is based on integer lattice. The directionlets still use the onedimensional filter group, but with the base function of multi-directional anisotropy, the directionlets have a detachable filter and critical structure, and are able to be fully reconstructed, thus, theoretically it has more advantages than the general wavelet transform, and the other second generation wavelet transform [6] .. This article has applied directionlets transform to image fusion experiments. The test results show that it can blend the edges of the image information fairly well, and subjectively more in line with human visual characteristics and the objective evaluation is also superior to other image fusion method. The article puts forward an infrared and visible fusion algorithm based on the directionlets transform. Compared with the traditional wavelet transform, directionlets can preserve the infrared and visible image's feature information, enhance the fused image's space detail representation ability, and improve the fused image's information. The experiment proves that the approach in this article is better than the wavelet transform and Laplace pyramid decomposition, etc.

O O B I O O O O B I O O O O O O B I O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Bio and health informatics meets cloud : BioVLab as an example The exponential increase of genomic data brought by the advent of the next or the third generation sequencing (NGS) technologies and the dramatic drop in sequencing cost have driven biological and medical sciences to data-driven sciences. This revolutionary paradigm shift comes with challenges in terms of data transfer, storage, computation, and analysis of big bio/medical data. Cloud computing is a service model sharing a pool of configurable resources, which is a suitable workbench to address these challenges. From the medical or biological perspective, providing computing power and storage is the most attractive feature of cloud computing in handling the ever increasing biological data. As data increases in size, many research organizations start to experience the lack of computing power, which becomes a major hurdle in achieving research goals. In this paper, we review the features of publically available bio and health cloud systems in terms of graphical user interface, external data integration, security and extensibility of features. We then discuss about issues and limitations of current cloud systems and conclude with suggestion of a biological cloud environment concept, which can be defined as a total workbench environment assembling computational tools and databases for analyzing bio/medical big data in particular application domains.Keywords: Bioinformatics, Cloud computing, Big data, Workflow, User interface, Data integration, Analysis, SecurityHigh throughput, massive parallel sequencing technologies, called the next generation sequencing (NGS) technologies, were first introduced in the late 1990s. The leap of sequencing technology by NGS dropped sequencing cost dramatically and caused the exponentially increasing volume of data. Comparing NGS to the traditional Sanger sequencing technology that was used for the first human genome project, a three billion dollar project over a span of 10 years, the cost and time for sequencing a single human genome has dropped by 3,000,000 and 3000 times respectively[1,2]. As a result, the nucleotide sequence resource in the EMBL-Bank DB has approximately doubled over the past 5 years [3, 4] . The sequencing throughput rate has increased five fold per year, whereas computer performance generally follows the Moore's Law, doubling only every 18 or 24 months. Thus the gap The analysis of big data in the bio and medical fields requires very big computing power and many research facilities and clinics are unable to afford such computing facilities. Thus cloud computing has gained great interest in the bio and medical fields to analyze big data and a growing number of bioinformatics cloud systems have been developed. In this paper we surveyed current bioinformatics cloud systems and discussed design issues. To handle big data of multiple types, analysis tasks require use of appropriate tools and databases from a vast number of tools and databases. Thus simply using cloud computing does not resolve the issue of big data analysis in the bio and medical fields. We discussed this issue by introducing a concept of cloud environment for specific scientific research goals. Chae (XSEDE) . In this role he is responsible for specialized scientific community interfaces to large scale cyberinfrastructure. Marru is also the principal research systems architect within Pervasive Technology Institute of Indiana University and is a founding member of the Apache Airavata project management committee. Marru's current research focuses on working alongside multidisciplinary experts in the field of eScience to developed open community driven web and service-based scientific workflow and gateway systems for individual research, collaboration, outreach, and cross-disciplinary collaboration. These systems assist scientists to focus on their research without the need to be distracted my emerging information technology and cyberinfrastructure.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B B I B I B B I B I B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B I O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O B I O O O O O B O O O O O O O O O O O B O O O O O O O O B I O O O B O O O O O O O O O O O O O O O O O O O B I O O O O O O B I B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Agent negotiation on resources with nonlinear utility functions Abstract. To date, researches on agent multi-issue negotiation are mostly based on linear utility functions. However, the relationship between utilities and resources is usually saturated nonlinear. To this end, we expand linear utility functions to nonlinear cases according to the law of diminishing marginal utility. Furthermore, we propose a negotiation model on multiple divisible resources with two phases to realize Pareto optimal results. The computational complexity of the proposed algorithm is polynomial order. Experimental results show that the optimized efficiency of the proposed algorithm is distinctly higher than prior work. Allocation of multiple divisible resources under incomplete information is always an interesting problem where negotiation is one of essential approaches, such as the work of Luo et al [1] , Saha et al [2] , Chevaleyre et al [3] , Fatima et al [5] . They proposed Pareto optimal negotiation, envy-free negotiation, and welfare optimal negotiation in their researches. In our past work [6] [7] [8] , we also studied some problems of multi-issue negotiation.. However, prior researches mostly did not take explicit utility functions, or directly regarded resources as agents' utilities. Therefore, only linear utility functions were considered. However, there are much cases where utility functions are nonlinear. For example, $100 will produce lots of utilities if an agent has not a bean, while $100 will produce few utilities if that agent has a capital of a hundred million dollars. The utilities of autonomous agents are some kinds of subjective belief, and resources are objective things, i.e. resources generally have firing effect on utilities.. So the relationship between utilities and resources is usually saturated nonlinear just as indicated by Wooldridge [9] . Furthermore, the relationship should follow the law of diminishing marginal utility according to microeconomics, i.e. the increments of utilities decrease along with the increments of resources. Recently, some researches gradually paid more attention to nonlinear utility functions such as [11, 12, [21] [22] [23] [24] .. We try to expand linear utility functions to nonlinear cases in this paper. This work is motivated by the sigmoid function of artificial neuron of artificial neural network. In particular, we propose an improved nonlinear utility function to describe the relationship between utilities and resources. The proposed function not only follows the law of diminishing marginal utility, but also is consistent with the assumption of Wooldridge et al.. Sequentially, we bring forward a negotiation model on multiple divisible resources. In particular, we firstly take resources as indivisible units and reach a preliminary agreement using strict alternation in the first phase. Information about the preferences and firing rate of the opponent is also obtained at the same time. Subsequently, we divide resources using a greedy algorithm in the second phase and realize Pareto optimal results.. The first contribution of this paper is the proposal of a new utility function which follows the law of diminishing marginal utility while it is mostly ignored by previous work. Moreover, in the first phase of negotiation, strict alternation provides not only a preliminary agreement but also some useful information for agents. Therefore, history information is not necessarily required. Secondly, this model can generate Pareto optimal results for all negotiation agents. The computational complexity of the proposed algorithm is polynomial order and it is usually lower than that of Fatima et al. Experimental results show that the optimized efficiency of the proposed algorithm has an advantage over the work of Fatima et al. The application of this work includes many similar domains such as allocation of band width, allocation of computational resources, and allocation of business etc.. The rest of this paper is organized as follows. Section 2 describes the basic notions about negotiation scenes and utility functions. We propose a negotiation model and a feasible optimal algorithm in section 3. Section 4 discusses negotiation with complete information, and section 5 discusses negotiation under incomplete information. Experimental settings and results are provided in section 6. Background and related work is provided in section 7. Conclusions are drawn in the last section. This paper proposed an improved nonlinear utility function motivated by firing function of artificial neuron of artificial neural network. The proposed utility func-tion follows the law of diminishing marginal utility. The negotiation model presented in this paper can deal with the instances of incomplete information, and Pareto optimal solutions will be obtained as a result. Experimental results show that the efficiency of optimal approach has an advantage over prior work.. The future work may improve the first phase of negotiation to reach a more rational preliminary agreement. A new optimal approach is also required to maximize the utilities of two agents, as well as maximizing the utilities of agent a.. Houkuan Huang, Professor and Ph. D. supervisor, Senior member of China Computer Federation, Beijing Jiaotong University, Beijing, China. His interests of researches include artificial intelligence, data mining, and machine learning.. Received: March 6, 2012 ; Accepted: December 3, 2012.

O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Deterministic and stochastic models of enzymatic networks - applications to pharmaceutical research The intracellular transducing device consists of complex networks of enzymatic reactions. Unfortunately, the mathematical models commonly used to describe them are still unsatisfactory and unreliable, even at the level of reproducing simple reaction schemes. The improvement of mathematical models is necessary and can follow different approaches still poorly employed, such as the modeling of spatial structures and phenomena, time delays, stochastic perturbations, only to cite the most relevant ones. In this paper we show some recent results related to the total quasi-steady-state approximation (tQSSA), in a deterministic scenario. Moreover, we show some possible applications of the tQSSA in a stochastic scheme. Every year, pharma companies invest billions of dollars on lead compound research, with the aim of drug discovery and commercialization. Most of these costs concern laboratory experiments (in vitro and in vivo) and clinical tests. One of the main goals of the so-called 'Systems Biology' is to help the pharmaceutical research in dramatically reducing the costs of the research phases (mostly preclinical ones) preceding drug commercialization and the time needed to complete all these phases.. The completion (in some sense) of the Human Genome Project, aimed at sequencing the complete human genome, raised great excitation in the genetics community, because this "primary" (genomic) information was believed to be the first step towards the unification between proteomic and genomic data and its use in medical, clinical and pharmaceutical fields. This effort would have brought complete information not only to basic research but also to medical applications to find the best treatment for human disease.. The expected results, human gene mapping and specific drug design to modulate gene expression and protein targets, targeted gene engineering and so on, have been disappointing. As can been seen in [1] (see Fig. 1 of that paper), a constant decline in pharmaceutical production began at the middle of the seventies and persisted throughout the "genomic era". The goal of having a complete list of human genes and, consequently, of the complete list of potential gene and protein targets has not been reached, because of the relatively low number of human genes (more or less one-fourth of the expected number) and of the high complexity of gene regulation and protein domain folding. The proteomic level is, in fact, more complex than expected and many "emergent" properties of protein interactome cannot be inferred from the knowledge of the primary structure of a genome. Another level of complexity is the gene transcription, whose general rules escape our efforts in the study of regulation regions surrounding the genes. The potential targets of drugs have exponentially grown in number, and the genomic information is largely insufficient to lead to drug discovery and therapy design.. Succinctly put, the genomic approach alone is not a successful way to deal with the drug design task. At least, we can say that no beneficial boost has been experienced by drug research during the period of human genome sequencing. The expected result was the dramatic reduction of the time and cost of the so-called preclinical phase of drug production, the one in which bioinformatic, computational and theoretical methods would have helped to select the promising gene and protein targets.. The recent estimate of the costs for new drug development (see [2] , Fig. 2 ), based on data of the eighties, for the preclinical and clinical studies and trials, is of more than 800M US $ for the total course of drug commercialization (a period of about ten years). But a more recent estimate raises these costs to more than 1000M US $ for each successful drug.. Finally, the success rate of the trials is, in the best actual cases, no more than 30% (see [2] , Table 1 ). It can be estimated that a mean of only 10% of the tested new compounds will be commercialized (with a good profit for the pharmas and a beneficial effect on public healthy), and the amount of nonsuccessful research is easily evaluated.. These data highlight the need for new conceptual tools for effective drug discovery. In other words, it is not the large-scale extension of (otherwise) successful experimental and computational tools to improve the pharmaceutical research. A conclusion can be drawn from these data: since the research on new drugs is guided by empirical and trial methods, it would be more efficient to develop new tools for the preclinical phase of the research, i.e. the introduction of new powerful theoretical schemes, based on the huge quantity of experimental data on cell biology and biochemistry. These new tools, which are of most interest in the field of Systems Biology, are mathematical models and computer software to accurately reproduce the intracellular and intercellular phenomena in a quantitative way. A robust predictive mathematical model must be used to address the experimental activity following initial research stages. In other words, the experimental activity must be targeted on a more massive production of quantitative data (kinetic constants, molecular concentrations, geometrical data on the cell structure, etc.) which will be used for the mathematical model construction. The accuracy of these data, together with reliable proteomic data (for example, protein interaction map data) will allow the formulation of mathematical models of increasing complexity and large-scale on the various aspects of the intra and intercellular phenomena. This "virtual cell" project, which has been successfully begun at the end of nineties [3, 4] on some selected well-known intracellular signal transduction pathways (MAP kinase cascade, NFkB pathway and others), needs an extension of the present understanding of protein interaction events, for example a reconsideration of the well-known Michaelis-Menten reaction scheme is needed to be reconciled with the highly connected structure of intracellular signal transduction pathways.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Constraint-Based Deadlock Checking of High-Level Specifications Establishing the absence of deadlocks is important in many applications of formal methods. The use of model checking for finding deadlocks in formal models is often limited. In this paper we propose a constraint-based approach to finding deadlocks employing the ProB constraint solver. We present the general technique, as well as various improvements that had to be performed on ProB's Prolog kernel, such as reification of membership and arithmetic constraints. This work was guided by an industrial case study, where a team from Bosch was modeling a cruise control system. Within this case study, ProB was able to quickly find counter examples to very large deadlock-freedom constraints. In the paper, we also present other successful applications of this new technique. Experiments using SAT and SMT solvers on these constraints were thus far unsuccessful. Formal modelling of discrete event systems is an important tool in order to verify their correct functioning. Among others we may want to verify (a) termination of certain components, (b) avoidance of unsafe states or (c) absence of deadlocks. In the formal models discussed in this article property (c), absence of deadlocks, is considered the principal property. In fact, in the industrial applications that have motivated the work described in this paper the first two properties (a) and (b) play only a small role.. The classic approach to locating deadlocks is model-checking. Model-checking can provide fast feedback, but is also associated with known problems: in many applications the state space is either infinite or much too large to explore exhaustively. Furthermore, model checking is particularly problematic when the out-degree of certain states is very large.. In this paper we describe a successful application of constraint solving to verify absence of deadlocks. In the industrial case study that has started this work, a team from Bosch attempts to develop a deadlock-free formal model of a cruise control system. For this application, constraint solving typically finds counter examples to deadlock-freedom constraints of more than 30 A4 pages in under two seconds, while model checking was unsuccessful. It turns out that this approach -on top of being much less sensitive to the size of the state space-has additional benefits. It exploits safety properties that have been specified (having the positive side effect of encouraging their specification) and it can be easily related to verification by formal proof. Model-checking can succeed showing absence of deadlocks even if this cannot be verified by formal proof using all specified safety properties. Constraint solving will only succeed if also a proof can be found: it is not based on model execution. In Event-B (Abrial 2010) , the formal method we have used in the case studies, this helped to achieve a more comprehensive methodology of verification. In the end, it is the mix of constraint solving, model checking and proof that advanced the case study of Bosch using ProB (Leuschel and Butler 2008) and Rodin . Rodin is a software tool for formal modelling with Event-B. It is equipped with editors, a proof obligation generator and some theorem provers. ProB is also available as a plug-in for Rodin providing animation, model-checking and constraint-solving facilities. The ProB kernel provides a constraint-solver for the basic datatypes of B (and Z) and the various operations on it. As such, it supports booleans, integers, userdefined base types, pairs, records and inductively: sets, relations, functions and sequences. These datatypes and operations are embedded inside B (and Z) predicates, which can make use of the usual connectives (∧, ∨, ⇒, ⇔, ¬) and typed universal (∀x.P ⇒ Q) and existential (∃x.P ∧ Q) quantification.. ProB integrates various constraint solvers in its kernel: • Integers are represented using Prolog integers. To implement arithmetic constraints, ProB uses the SICStus CLP(FD) finite-domain library (Carlsson and Ottosson 1997 ).. • Elements of basic sets are represented internally as terms of the form fd(Nr,T), where T is the type name and Nr is the number of the element. Equality is then implemented simply using unification. Disequality is generally implemented using the disequality operator of CLP(FD). Thus, disequality can also sometimes deterministically instantiate its arguments. E.g., given the user-defined type S={a,b}, the predicate x /= a will force the value of x to be b.. • The constraints for the more complicated types have been written in Prolog with co-routines. Note that ProB employs various set-representations: AVL-trees for fully known sets (to be able to deal with large sets arising in industrial applications, cf. )), closures to represent certain sets symbolically and Prolog lists for partially known sets. A feature that distinguishes ProB is that it not only deals with simple sets, but also allows sets of sets, relations, etc. Generally, co-routines are used to block non-deterministic computations. A non-deterministic computation provides an estimate of the number of solutions to the ProB waitflags store and obtains a Prolog variable on which it can block (called a waitflag). This waitflag will be instantiated by the enumeration process, which will unblock computations with the least number of solutions first and will also take care of labeling the CLP(FD) variables.. • Finally, the boolean predicate solver is also written in Prolog using co-routines.. We describe some aspects of its implementation in more detail below. We did not reuse the SICStus boolean constraint solver mainly due to the treatment of undefined predicates and in order to link the labeling with our other solvers.

O O O O O O O O O O B O O O O B O B I O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Classes of two-dimensional languages and recognizability conditions Abstract. The paper deals with some classes of two-dimensional recognizable languages of "high complexity", in a sense specified in the paper and motivated by some necessary conditions holding for recognizable and unambiguous languages. For such classes we can solve some open questions related to unambiguity, finite ambiguity and complementation. Then we reformulate a necessary condition for recognizability stated by Matz, introducing a new complexity function. We solve an open question proposed by Matz, showing that all the known necessary conditions for recognizability of a language and its complement are not sufficient. The proof relies on a family of languages defined by functions.Mathematics Subject Classification. 68Q45, 68Q70. The increasing interest for pattern recognition and image processing has motivated, in the sixties, the research on languages of pictures or two-dimensional languages, and nowadays this is a research field of great interest. Since then, many approaches have been presented in the literature in order to generalize the well-founded theory of formal languages of strings, and various models of finite automata, grammars, logics and regular expressions have been proposed. In [13, 14] the family REC of recognizable picture languages was defined as an equivalent of Keywords and phrases. Two-dimensional languages, unambiguity, complement.. the class of recognizable (or regular) string languages. This definition takes as starting point a characterization of recognizable string languages in terms of local languages and projections (cf. [11] ). A two-dimensional language is said recognizable when it is the alphabetic projection of a local language defined in terms of a finite set of 2 × 2 pictures called tiles; the recognition is given by a so called tiling system.. REC family inherits several properties from the class of regular string languages. A crucial difference lies in the fact that REC family is not closed under complementation: there are languages in REC whose complement is not in REC [14] . It is then important to take into account also the class co-REC of languages whose complement is in REC. The strict inclusion REC ⊂ (REC ∪ co-REC) holds even in the unary case [22] and it fits the fact that the definition of recognizability by tiling systems is intrinsically non-deterministic. The notion of determinism on tiling systems have been recently discussed in [4, 6, 20] .. The non-closure of REC under complementation motivated the introduction of unambiguous two-dimensional languages, whose family is denoted UREC [13] . Informally, a picture language belongs to UREC when it admits an unambiguous tiling system, that is if every picture has a unique pre-image in its corresponding local language. In [3] , the proper inclusion of UREC in REC is proved; it holds true in the unary case too (see [1] ). In other words there exist in REC inherently ambiguous languages. An open question is whether UREC is closed under complementation or not. Its answer depends on the following open problem, where L denotes the complement of language L. Question 1. Does L ∈ REC and L / ∈ REC imply that L / ∈ UREC? Question 1 was firstly stated in [25] . The converse is actually an open question too: Does L ∈ REC\UREC imply that L / ∈ REC? Note that positive answers to both Question 1 and the converse mean that UREC=REC ∩ co-REC and that UREC is the largest subset of REC closed under complementation. Also note that such question is related to some difficult problems on complexity classes [7] .. All the inherently ambiguous languages known in the literature are infinitely ambiguous, in the sense that it is not possible to recognize them by a tiling system, in such a way that each picture has a fixed number of pre-images at most (see Sect. 2 for more details). The question whether this is always the case or not is open. Let us state it as follows. Question 2. Does there exist a language L ∈ REC\UREC such that L is finitely ambiguous?. In this paper we will answer Questions 1 and 2 in some particular cases, where languages involved have "high complexity", as specified in the following. We will introduce a class HP ⊆ co-REC\REC of not-recognizable languages and a class HK ⊆ REC\UREC of ambiguous recognizable languages, whose languages are "hard" with respect to some complexity functions. We will show that:. If L ∈ REC and L ∈ HK then L is infinitely ambiguous. Let us emphasize that it is not known whether the inclusions HP ⊆ co-REC\REC and HK⊆ REC\UREC are strict or not. No example (nor a candidate) exists showing the inclusions are strict. Hence in the case HP = co-REC\REC and/or HK = REC\UREC, our results would be an answer to Questions 1 and/or 2, in their general setting.. The introduction of classes HP and HK is motivated by some necessary conditions for languages in REC and in UREC, respectively, stating that: if L ∈REC then the size of some permutation matrices associated to L cannot grow so quickly; and if L ∈UREC then the rank of the same matrices associated to L cannot grow so quickly. Here "HP" stands for "High Permutation" and "HK" stands for "High ranK". In the literature, all examples of languages that witness the strict inclusions UREC ⊂ REC ⊂ (REC ∪ co-REC) have been provided applying the necessary conditions we have just mentioned.. The main difficulty in this framework is that there are no characterizations of REC and UREC, that could be easily and fruitfully applied. A question that naturally arises is whether these conditions are also sufficient, so that they could characterize the classes. In this paper we will show that the considered conditions, that are necessary for the recognizability of a language or its complement, unfortunately are not sufficient.. Let us give some more details on the ideas on which the mentioned necessary conditions are based, since our results will be basically related to them.. In [21] Matz isolated a technique to obtain necessary conditions for the belonging of a picture language to REC. It consists in reducing the problem from two dimensions to one dimension and applying some known lower bounds on string automata. More precisely, for any recognizable picture language L and integer m, he considered the string language L(m) of all pictures in L of fixed height m. He proved that, if L ∈ REC, then it is possible to associate to any tiling system recognizing L a family {A m }, where, for some constant c, each A m is an automaton accepting L(m) with c m states at most. Finally, he used the extended fooling set technique as a lower bound on the size of a string automaton [8, 18] .. Later on, in [3] , Matz's technique was used together with a lower bound on the size of unambiguous string automata. This provided a necessary condition for unambiguous picture languages, based on the growth of some parameters of the Hankel matrices of the string languages L(m). Recently, in [15, 16] , the idea of finding necessary conditions for picture languages, using Matz's technique, has been considered again introducing some complexity functions of Hankel matrices of L(m). It provided a necessary condition for belonging to REC ∪ co-REC, by rephrasing Cervelle's one (see [10] ), and a necessary condition for belonging to REC, using the fooling set technique [17] (instead of the extended fooling set technique) as a lower bound on the size of a string automaton.. Moreover, in 1998, Matz wondered whether the necessary condition for REC in [21] is also sufficient, in order to have a handy characterization of REC. He proposed as a counter-example the language of pictures over {a, b} with same number of a's as b's, that satisfies his condition. But then Reinhardt [26] proved that the language is recognizable. Since then, other candidate languages have been proposed as counter-examples, but they always failed. Here we negatively answer the open question, after rephrasing Matz's condition in the framework of Hankel matrices.. The analogous question regarding other necessary conditions for REC and for REC ∪ co-REC can be considered. We prove that all these conditions are not sufficient. The counter-examples are given by a family of languages defined by functions. Recently another counter-example showing that Matz's condition is not sufficient was independently found (see [9] ) by a completely different technique. This technique has provided a family of binary languages, whereas our counterexample is a family of unary languages.. The paper is organized as follows. After giving the basic definitions and results on two-dimensional languages in Section 2, in Section 3 we recall the above mentioned necessary conditions for two-dimensional languages and introduce the classes HP and HK. Section 4 contains the main results concerning Questions 1 and 2. In Section 5 we introduce a new complexity function of picture languages that allows us to reformulate Matz's condition in terms of the growth of this function. At last in Section 6 we show that the considered recognizability conditions are not sufficient.. Some results of this paper can be found in [2] .

O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Neurally plausible, non-combinatorial iterative independent process analysis It has been shown recently that the identification of mixed hidden independent auto-regressive processes (independent process analysis, IPA), under certain conditions, can be free from combinatorial explosion. The key is that IPA can be reduced (i) to independent subspace analysis and then, via a novel decomposition technique called Separation Theorem, (ii) to independent component analysis. Here, we introduce an iterative scheme and its neural network representation that takes advantage of the reduction method and can accomplish the IPA task. Computer simulation illustrates the working of the algorithm. r Independent component analysis (ICA) (see, e.g., [7] and references therein) aims to recover mixed independent hidden sources. There is a broad range of applications for ICA, e.g., blind source separation and deconvolution [1] , feature extraction [2] , and denoising [6] . ICA algorithms are one-dimensional: all sources are assumed to be independent real valued random variables. However, applications where not all, but only certain groups of the sources are independent may have high relevance in practice. In this case, independent sources can be multidimensional. For example, consider the generalization of the cocktail-party problem, where independent groups of people are talking about independent topics, or that more than one group of musicians are playing at the party. The separation task requires an extension of ICA, which can be called independent subspace analysis (ISA), see, e.g., [13] .. Most practical problems exhibit considerable temporal correlations. In such cases, one may take advantage of independent process analysis (IPA) [5, 3, 10, 4, 11] , a generalization of ISA for auto-regressive (AR) processes: onedimensional sources are assumed in [5] , extensions to ARmixture of Gaussians are presented in [3, 4] , and multidimensional sources are considered in [10, 11] . The IPA task can be rewritten as the identification of linear dynamical systems (LDS) with state space formalization. Further, the identification task can be reduced to the ISA task using linear prediction and the concept of innovations. Typical ISA solutions might need the minimization of mutual information between the estimated processes and the search for (i) the optimal number and (ii) the dimensions of the processes [10] . However, such estimations can be computationally intensive and the dimension search is subject to combinatorial explosion. Recently, a two-phase algorithm that avoided the combinatorial explosion under certain conditions has been introduced for IPA [9] . Here, an iterative scheme that enables neural interpretation is put forth after reviewing the IPA problem. Illustrative numerical simulations are also provided. Recently, it has been shown that for hidden and mixed AR processes, which are driven by non-Gaussian noises, ICA is satisfactory for the estimation of the independent directions, provided that innovations satisfy certain technical conditions. It then followed that combinatorial searches for the hidden processes can be lessened or fully avoided except for degenerate cases. Here, we have introduced an iterative approach and its neural network architecture that iterates in two-phases and enables the finding and the grouping of the hidden components. In one phase, the network makes use of non-linear Hebbian learning rule that estimates the ICA components. In the other phase, delta-rules tune the predictive matrix of the hidden process and the inverse of the ICA matrix that improves the ICA estimation of the iterative procedure. Numerical studies indicate the robustness of the neural algorithm. We shall demonstrate the working of the algorithm. The task was to identify the hidden processes from the mixed AR signals without using our knowledge about the number and the dimensions of the hidden processes. The hidden AR components were defined by their driving noises m i (Eq. (1)), and two tests were carried out. In the first case, the conditions of the separation theorem were satisfied: three pieces of four-dimensional noise sources with spherical distributions were chosen; this is the 4-spherical database, total dimension is 12. In the other case, noise sources were constructed from images of tale figures. These images were considered as density functions and m i s were generated by sampling two-dimensional coordinates proportional to the corresponding pixel intensities. This is the tale database. The F i predictive matrices of the s i AR processes were chosen randomly under the constraint that the AR processes were stable. Mixing matrix A was chosen randomly from the orthogonal group. The two-phase algorithm was applied for the mixed data in batch learning that avoids parameter dependencies for aðtÞ, bðtÞ, and gðtÞ. For speed, FastICA was applied [7] . Coordinates of matrixF were initialized with independent, uniformly distributed random variables over ½0; 1. The normalized Amari-error was used to measure the precision of the estimation. This error takes values in interval [0,1]; 0 (1) means perfect (worst possible) estimation. Fifty independent runs were averaged, sample number T changed between 1000 and 20,000.. The quality of the gathering procedure depends on parameter e. We have studied (i) the Amari-error for a suitable e as a function of the sample and iteration numbers, and (ii) the dynamic range, i.e., the ratio of the highest and lowest 'good e values': we divided interval ½0; F m (F m ¼ max i;j jF ij j) into 200 equal parts. For different sample and iteration numbers in both databases and at each division point we used the gathering procedure to group the ICA elements. For each of the 50 random trials we have computed the Amari-errors separately. For the smallest Amari-error, we collected the corresponding set of e's, these are the 'good e values'. Then we took the ratio of the largest and smallest e values in this set and averaged the ratios over the 50 runs. The average is called the dynamic range. Fig. 1 summarizes our findings for the tale and the 4-spherical tasks. According to the figure, there are good e parameters for theF-'connectedness' already for 1000-2000 samples: our neural method can find the hidden components with high precision. Increasing the sample number the Amari-error decreases, it is 1:54% for the tale and 0:85% for the 4-spherical databases, at 20,000 samples on the average. Two iterations were satisfactory for the optimization, further iterations did not improve the results. The dynamic range is better by a factor about 2.5 for the 4-spherical database that satisfies the conditions of the separation theorem. Using 20,000 samples, the dynamic range was about 10 for the tale database. These results demonstrate the robustness of the neural approach.

O O O O B I I O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Analysis and evaluation of the establishment and maintenance of paths in a MANET used for industrial process monitoring Albero Albero, T.; Sempere Paya, VM.; Mataix Oltra, J. (2013). Analysis and evaluation of the establishment and maintenance of paths in a MANET used for industrial process monitoring. Simulation Modelling Practice and Theory. 37:79-98. doi:10.1016/j.simpat.2013.05.009. Analysis and evaluation of the establishment and maintenance of paths in a MANET used for industrial process monitoring. The use of wireless networks is having an ever greater impact on systems controlling automation, industrial manufacturing, distributed control and supervision. Wireless technologies in particular have penetrated quickly the area of control of public utilities networks, and typical examples of this are remote control and supervision of large water, gas or electricity distribution networks. These types of networks are usually formed of a central station and different remote stations, and remote clients who are able to remotely monitor or even control the working of the system. To improve the use of the system, the remote stations must have the capacity to exchange information between each other and with the central station. The spontaneous generation of wireless networks in remote stations to exchange information between workers is a possible solution. Taking into account the characteristics of this type of monitoring system, the paper has proposed a scenario (number of nodes, area size, mobility) and has modeled, using stochastic activity networks (SAN), the operation of an ad hoc network that uses a reactive routing protocol to determine if the network is able to offer the typical services of these facilities (images or video streaming and alerts). The use of wireless networks is having an ever greater impact on systems controlling automation, industrial manufacturing, distributed control, supervision, and almost every other type of integrated network system due to the simplicity of setting up and extending the network and low maintenance needs. Moreover, there are many potentially interesting applications that can make use of these wireless networks, as described by Willig in [1] , such as coordination between mobile robots or autonomous vehicles and control and monitoring the functioning of machinery and facilities. The characteristics of the ad hoc network studied in this paper are based on those found in a large urban water treatment plant, but it could be the values of another type of facilities such as these to supervise gas or electricity distribution networks.. A good example of wireless technologies having penetrated an area quickly is in the control of public utilities networks, and typical examples of this are remote control and supervision of large water, gas or electricity distribution networks. The traffic on these networks is composed of control information and the transmission of images. These types of networks are spreading in metropolitan environments, and are usually formed of a principal node denominated central station, different remote stations, and remote clients who are able to remotely monitor or even control the working of the system, see [2, 3] . To improve the use of the system, facilitate maintenance and to improve the response to alerts and potential risk situations, the remote stations must have the capacity to exchange information on states between each other and with the central station. The spontaneous generation of wireless networks in remote stations where each node in the network is represented by a user that exchange information is a solution to the need for fast and timely information exchange.. Routing in wireless networks and in particularly in ad hoc networks is highly changeable, especially when dealing with mobile nodes (MANETMobile Ad hoc Network), and for this reason, the conventional routing protocols are not appropriate in these environments to provide the mentioned types of traffic. Moreover, the inclusion of wireless networks in industrial environments carries its own particular problems: errors due to interference, which make it difficult to create routes within the established times; breaks in active routes due node mobility; noise from surroundings or from the technology used; and difficulties in finding alternative routes after a loss of communication, which may cause problems when establishing communication as well as errors and losses once the communication is active.. Although there has been a great deal of study carried out on ad hoc networks in recent years, there remain many aspects that need to be studied and improved. There are no formal models that take into account a complete network with knowledge of all the topology and where the nodes are mobile. In this paper, we study node accessibility and how this affects their mobility when offering the service requested. With this objective in mind, we have developed a formal model which represents the behavior of a complete ad hoc network in which the routing protocol has reactive behavior. For the implementation of this reactive protocol, some temporal and functional characteristics of the routing protocol AODV (Ad Hoc On-Demand Distance Vector Routing), see [4] , have been considered.. Modeling and evaluation of ad hoc communication networks is not a new area of study, in fact there has been a huge amount of work on mobile networks published in recent years. Many studies on MANETs have been based on simulation. The most popular simulation tools for ad hoc networks are OPNET, see an example in [5, 6] , ns-2 [7] , and Glomosim, see [8] . However, these are not the only tools available to study this type of network. The tool used to construct the formal model presented in the paper is the publicly available tool Möbius 2.3, [9] which supports the specification of Stochastic Activity Networks models, see [10, 11] .. There have been various studies on Petri Networks, which have evaluated different aspects of ad hoc networks. Authors such as Murata et al. [12, 13] , were the first to model and simulate a MANET with CPN (Colored Petri Net). They modeled in detail the AODV routing protocol. They studied how mobility affects the AODV protocol, but it is important to point out that they did not know the exact topology of the network, they did not know where was located every node in the area. According to Murata, it is possible to model a MANET without information on its exact graphic structure, but in our opinion, this fact makes it impossible to study a break in a route and therefore its recovery. Other authors have used the Fuzzy Petri Net to design an efficient routing algorithm (RRA, Reliable Routing Algorithm) which allows them to find the routing tree from source to destination in an ad hoc network, see [14] . The algorithm used does not need to estimate the topology, it only needs to know the neighbors of each node. The objective is to find the most trustworthy route, but without studying what happens after obtaining one of these routes, unlike the paper under consideration where we study the behavior after the path establishment. This work, [14] , uses neither the AODV protocol's behavior nor that of a complete ad hoc network, but concentrates only on the RRA algorithm. Another approach to studying an ad hoc network is to not build the network node by node, as according to the authors the size would be too large to obtain an exact numerical solution. In [15] , an ad hoc network is presented using Stochastic Petri nets to model the behavior of a node under the work load generated by the entire network. The authors modeled the occupancy of the entry and exit buffers of the nodes in the network, and the rest of the network was modeled compactly. In this work, authors are not representing in detail the behavior of a protocol nor are they considering node mobility.. In the papers cited previously, [12, 13, 14, 15] , the behavior of ad hoc networks was modeled without knowing the exact position of the nodes in the network, as the objective was to evaluate whether or not the protocol could establish communication, or to model an algorithm applied to a routing protocol to see if it would improve the possibility of communication. However, without considering the topology of the network, it is not possible to know the exact route to be taken when requesting a communication between source and destination. Therefore it is impossible to know if a later movement of a node will cause a loss of the route, given that it is not known whether the node that has moved belongs or not to the active route. Once again the need for a formal model of a complete ad hoc network where the topology is known and the nodes are mobile is evident. With such a model it would be possible to know the exact route between source and destination and therefore it would also be possible to study what happens when this is active and one or more of the nodes forming it move.. In our previous works the probability of the source reaching the destination in an ad hoc network was studied in function of radio coverage range [16] , determining single-hop and multi-hop communications and failed attempts at communication in which the destination was unreachable. Furthermore, the effect of mobility in an ad hoc network on the duration and stability of the routes in order to determine how the services were affected was studied, see [17] . The model shown in the present paper presents a more realistic behavior as it includes error rates more representative of the errors that may occur in an industrial environment when wireless communication is used. AODV protocol parameters which intervene in the search for and recovery of routes have been modeled. Furthermore, in this work we have used a more realistic environment taking into account the characteristics of the typical scenarios in which usually appear the networks under evaluation. Therefore, knowing the temporal requirements of industrial networks of metropolitan scope, and knowing how the routing protocol works during transmission of the typical information used in these systems, we can determine whether or not ad hoc mobile networks generated in these environments fulfill these requirements.. Section 2 lists some characteristics and functioning of AODV protocol. In section 3 we give a brief description of the elements of the modeling tool used. The work scenario (environment, work area, call rate, movement rate...) is described in section 4. In section 5 we explain in detail each of the models which form the model as a whole. In section 6 we present the results obtained on reachability, establishment, recovery and maintenance. Finally, in section 7, we present our conclusions and propose future lines of research to be carried out. The model has allowed us to identify whether can be offered the typical services of a system as tested. These services are alerts and images or video streaming. Overall, the user will perceive correctly and just in time an alert sent through the ad hoc networks formed in large facilities for industrial process monitoring. On the other hand, the quality with which the user will perceive a service such as images or video streaming will be poor when the default parameters of the AODV are used. This is due to the continuous cuts produced during display.. If a model of an ad hoc network allows us to evaluate its behavior, obtaining results just by modifying the values of parameters or variables, as well as evaluating different aspects by simply implementing new models, then it is certainly necessary to compare and contrast these results obtained via simulation with real experiments. With this in mind, the results presented in this paper could be compared to those obtained from a real testbed created to test the working of the AODV routing protocol in an ad hoc network, [23] .. The protocol performed at a high level in the tests, showing the capacity to find routes almost instantaneously as in the model and showing cuts in the video streaming transmission with the default parameters of the AODV protocol. The results shown were obtained through simulation, numerical analysis is not possible due to the large state space. All of these experiments have been evaluated for a confidence level of 0.95 and a confidence interval of 0.1. The simulation time used was 6000 t.u. (time units).. The results have been divided into three groups: firstly, the results on reachability of the network when using a scenario such as the one proposed and with a reactive protocol. Secondly, the protocol's behavior in terms of route establishment time, route maintenance time and route recovery time for lost routes. Thirdly, the average time for which the route between source and destination is active.

O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O


Perceived performance effects of ICT in manufacturing SMEs Purpose. We aim to investigate whether ICT resources, including investment and use of specific types of ICT as well as innovative work practices, have a positive impact on several dimensions of firm performance, taking into account both direct and indirect effects and both short and long-term effects.Design/methodology/approach. An empirical study using data from a sample of 267 Spanish manufacturing SMEs was conducted. We have used path analysis to study direct and indirect ICT impacts. OLS regression is the estimation method employed. SMEs and point to the need for persistence in use so as to obtain good results in some areas.They also stress the positive effects of adopting ICTs together with organizational innovation. 5 Evaluating Information and Communication Technologies (ICT) investment payoff constitutes a key concern for companies in managing their ICT resources. Companies are compelled to find ways to evaluate the specific impact of different types of ICT on the several dimensions of their performance.. ICT payoff becomes a crucial issue, particularly in the case of Small and Medium-size Enterprises (SMEs). SMEs may differ from larger firms in the way they address ICT adoption (Lucchetti and Sterlacchini, 2004; Haug et al., 2011) . However, the majority of papers on ICT effects relate to large firms and the evidence is far from conclusive.. What appears to be generally lacking from the literature on ICT payoff in SMEs are studies exploring how different types of ICT adopted by SMEs impact on different areas of performance. Studies have shown that ICT effects vary according to the type of technology being used and its degree of adoption (Lucchetti and Sterlacchini, 2004; Nevo et al., 2010; Boothy et al., 2010) . In addition, many analyses of ICT impact on firm performance have demonstrated the existence of complementarities between technological and organizational changes (Brynjolfsson and Hitt, 2000; Boothy et al., 2010; Cao, 2010; Cozzarin and Percival, 2010) .. The lack of unambiguous evidence has also prompted researchers to analyze the effects on business processes as well as the effects on the organization as a whole (Dedrick et al., 2003; Melville et al., 2004; Liang et al., 2010) . Many authors have suggested that one of the reasons for inconsistent findings about ICT payoffs might be the existence of lag effects since ICT payoff may require time to materialize. However, most of the studies that take time lags into account relate to large organizations (Brynjolfsson and Hitt, 2000; Brynjolfsson et al., 2002; Devaraj and Kohli, 2003; Lee and Kim, 2006; Nevo et al., 2010) or specific sectors (Das et al., 2011) .. Our research contributes to the debate about whether ICT creates value for SMEs performance, providing empirical evidence of the relationships between information technologies, organizational innovation and firm performance from a sample of 267 Spanish manufacturing SMEs. We aim to investigate whether ICT resources, including investment and use of specific types of ICT, along with innovative work practices, correlate positively with several dimensions of firm performance, taking into account both direct and indirect effects and both short (less than 1 year of ICT implementation) and long-term effects (more than 5 years).. The present research extends the scope of the analysis of the prior literature by focusing on a broad range of technologies, a variety of performance measures and several time lags to study ICT adoption impacts in SMEs. In addition, we have analyzed ICT effects using perceptual measures (Vehovar, 2007; Tallon and Kraemer, 2007; San-Jose et al., 2009) to complement the results obtained in previous quantitative analyses.. The paper is organized as follows. An overview of the relevant literature regarding ICT impact on firm performance is provided in section two. The conceptual framework and research hypotheses are presented in section three; data and methodology are outlined in section four. Section five comprises the main results of our study. The final section presents the main conclusions and discusses issues for further research. This paper provides empirical evidence on the direct and indirect impact of ICT on different dimensions of performance in Spanish SMEs. These relationships are explored within the framework of the resource-based view using path analysis and taking into account time-lags of the effects, as well as different types of technologies. In addition, we have also studied whether complementarities between technological and organizational resources may affect the managers´ perception of ICT impact on firm performance.. Our findings show that ICT has a significant impact on the improvement of external and internal communication. Nevertheless, ICT direct impact on operational and final performance is limited. Impact on these two performance dimensions is mainly indirect though communication improvement. These results highlight the importance of indirect effects to understand ICT payoffs. This result is valid for the three types of ICT studied.. As expected, ICT impacts on perceived performance correlate positively with the experience of the company in using different types of technologies. We have also found that managers' perceptions of ICT impact are related to the adoption of new work practices. As for ICT, performance also depends positively on the time-period from when new work practices were first implemented. At least one year of previous adoption is needed for ICT impact on communication improvement to be noticed.. Our results highlight the relevance of using perceptual measures in the research on ICT payoff. Although this type of variables may present disadvantages over objective measures, it may provide new insights to better identify the processes through which ICT impacts on firm performance. For example, as shown in the paper, the relevance of improvements in external and internal communication to explain ICT effects on firm performance can only be empirically identified using perceptual measures. Our research confirms previous findings in the literature with regard ICT value for firms. The finding of a positive link between ICT and firm performance is especially important, given the large share of SMEs in the economic activity of most countries. Therefore, ICT adoption in SMEs should be encouraged in order to improve their competitive position.. Our findings also suggest that competitive advantage can be the result of the adequate combination of different types of information technologies and work practices. To take full benefits of ICT investment, firms should make efforts to adopt organizational practices aimed at increasing teamwork and worker participation in decision-making. Complementarities found between both types of innovations suggest that managers should not consider ICT adoption isolatedly, but along with additional changes in job design.. Positive ICT impacts have been found both in the short and long-term. Nonetheless, these beneficial effects appear to be larger the longer these technologies have been in use. This result suggests ICT adoption is a complex process requiring continuous attention and surveillance from managers. To reap potential full benefits, a long-term approach should guide firms. Therefore, short-term pressures are not the best scenario for ICT investment.. One of the most robust findings lies on the positive impact of ICT on communication and the minor direct effect on operational and overall performance. As a consequence, it might be inferred that firms with larger coordination needs may benefit more from ICT adoption. This would be the case of firms dealing with more frequent and complex information flows usually associated with a large number and variety of agents involved.. Finally, this paper provides empirical evidence for a country in which the level of ICT adoption is below the European average and where ICT payoff for SMEs has been hardly analyzed (Sánchez, et al., 2006) . Within this context, and given that the literature has generally addressed large firms in countries with higher ICT penetration, our findings may be of interest to other researchers to identify how managers in SMEs perceive the benefits derived from their ICT investment. Our results point to the significance of enhancing ICT adoption among SMEs and to the need for specific policies to overcome their lag, as compared with large firms.. Some limitations on this research should also be mentioned. The selected geographical area where the survey was conducted may be considered too limited. Further research should consider extending the geographical scope of the sample, including other regions and countries. It is also noteworthy that, as we used cross-sectional data, it has not been possible to study lagged effects for the intensity of the implementation of ICT resources and new work practices. Although interesting results have been obtained from our time analysis, it would be interesting to test whether the effects detected are also registered for the intensity of both ICT and organizational innovations by using panel data. Another potential limitation is related to the use of perceptual measures provided by the person who has made the decision about ICT adoption, what could give rise to a halo effect. Note: The effects are assessed on a 0 (no effect) to 10 (very significant) scale.

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