Improving the Extraction and Expansion Method for Large Graph Coloring Graph coloring is one of the most studied combinatorial optimization problems. This paper presents an improved extraction and expansion method (IE 2 COL) initially introduced in [47] . IE 2 COL employs a forward independent set extraction strategy to reduce the initial graph G. From the reduced graph, IE 2 COL triggers a backward coloring process which uses extracted independent sets as new color classes for intermediate subgraph coloring. The proposed method is assessed on 20 large benchmark graphs with 900 to 4000 vertices. Computational results show that it provides new upper bounds for 6 graphs and matches consistently the current best known results for 12 other graphs. Let G = (V, E) be an undirected graph with vertex set V and edge set E. A subset I of V is an independent set if no two adjacent vertices belong to I. A legal k-coloring of G is a partition of V into k independent sets (color classes). The graph k-coloring problem is to find a legal k-coloring of G for a given k. The graph coloring problem is to determine the smallest integer k (the chromatic number χ(G)) such that there exists a legal k-coloring of G. Notice that the graph coloring problem can be approximated by solving a series of k-coloring problems with increasing or decreasing k values [18] .. Graph k-coloring is a well-known NP-complete problem [21] and has a number of practical applications related to printed circuit testing [20] , scheduling [31] , register allocation [11] , timetabling [5] , frequency assignment [40] and bag rationalization [22] . In the general case, exact solution methods can be used only to solve problems of relatively small size. As a matter of fact, there are graphs with as few as 125 vertices that can not be solved optimally even by using the current best exact algorithms [29, 34] . For larger graphs, heuristics and metaheuristics are usually preferred to find approximate solutions. Comprehensive surveys of the most significant coloring methods can be found in [18, 34] .. There are a large number of heuristic approaches for graph coloring: greedy construction (DSATUR [3] , RLF [31] ), tabu search [2, 12, 24, 27, 37] , iterated local search and variable neighborhood search [1, 8] , simulated and quantum annealing [7, 29, 41] , variable space search [28] , scatter search [25] , multiagent fusion search [43] , ant colony optimization [10, 4, 36, 44] , and evolutionary or population based hybrid search [13, 17, 19, 23, 32, 33, 38] . These coloring algorithms are based on diverse solution strategies and have led to continually improved results. Among these algorithms, population-based heuristics are certainly among the most competitive approaches. Nevertheless, large graphs with more than 900 vertices always represent a real challenge for any existing coloring algorithm.. A basic approach to deal with large graphs is to apply the general principle of "reduce-and-solve". Before the coloring process, this approach first removes, during a preprocessing step, some large independent sets from the original graph to obtain a reduced graph (called residual graph). Since each independent set can form a color class, to obtain a coloring of the initial graph, it suffices to find a legal coloring for the residual graph. This approach were explored with success in early studies like [15, 27, 29, 35] .. Very recently, an improvement has been proposed to enhance this basic independent set extraction approach [46] . The extraction phase was enhanced by removing at a time a maximal collection of disjoint independent sets of maximal size instead of only one independent set. The resulting EXTRACOL algorithm has obtained new improved colorings for several large and very large graphs (DSJC1000.9, C2000.5, C2000.9, C4000.5). However, extracting independent sets as a preprocessing technique suffers an inevitable limitation. Actually, if an independent set is wrongly extracted such that it is not part of the optimal coloring, the mistake can never be repaired. To remedy this difficulty, the work of [47] introduced an expansion phase which allows the coloring process to reconsider each extracted independent set on an one-by-one basis. The resulting E2COL algorithm has improved the best-known results for two very large graphs (C2000.5 and C4000.5).. This paper further extends these previous studies by proposing additional strategies, leading to the improved extraction and expansion algorithm (IE 2 COL). We report experimental studies of IE 2 COL on the set of 20 largest and most challenging benchmark graphs (with 900 to 4000 vertices) from the DIMACS and COLOR02/03/04 competitions. These results show that the proposed algorithm obtains new upper bounds for 6 graphs (flat1000 76 0, C2000.5, C4000.5, C2000.9, WAP04, WAP07) and matches consistently the current best-known results for 12 other graphs. Section 2 presents the proposed algorithm. Section 3 is dedicated to extensive computational evaluations and comparisons. Section 4 investigates some key components of the proposed approach, followed by the concluding section.. 2 Improved extraction and expansion coloring (IE 2 COL). The proposed IE 2 COL algorithm is based on and extends the basic extraction and expansion method of [47] and can be summarized by the following general procedure composed of three phases.. (1) The extraction phase simplifies the initial graph G by removing iteratively large independent sets (as well as the corresponding edges) from the original graph. To be effective, each iteration removes a collection of disjoint independent sets of the same size (the largest possible) according to the method developed in [46] . This phase stops when the residual graph contains no more than a fixed number of q vertices. The independent set extraction method is discussed in Section 2.2. (2) The initial coloring phase applies a graph coloring algorithm (the memetic algorithm presented in [32] ) to the residual graph G z to determine a (k − t)-coloring where t is the number of extracted independent sets. If a legal (k − t)-coloring C = {c 1 , ..., c k−t } for G z is found, then C plus the t independent sets extracted during the phase 1 constitutes a legal k-coloring of the initial graph G, return this k-coloring and stop. Otherwise, continue to phase 3 to trigger the expansion and backward coloring phase. The memetic coloring algorithm applied to G z and intermediate subgraphs (phase 3) is discussed in Section 2.3. (3) The expansion and backward coloring phase extends the current subgraph G ′ by adding back some extracted independent sets S to obtain an extended subgraph G ′′ . Then the coloring algorithm is run on G ′′ by starting from the current coloring of G ′ extended with the independent sets of S as new color classes. Once again, if a legal coloring is found for the subgraph G ′′ , this coloring plus the remaining independent sets forms a legal k-coloring of the initial graph G and the whole procedure stops. Otherwise, one repeats this expansion and backward coloring phase until no more independent set is left or a legal coloring is found for the current subgraph under consideration. Possible strategies to select independent sets for expansion are discussed in Sections 2.4 and 4.2.. The proposed IE 2 COL algorithm, designed for the graph k-coloring problem, implements this general approach and is described in Alg. 1 . In what follows, we show how the main components of IE 2 COL are implemented. In this paper, we have presented a general method for the graph vertex coloring problem able to handle large graphs. This method combines an independent set extraction phase with an expansion and backward coloring phase. The extraction phase relies on a dedicated strategy to identify and remove large independent sets from the initial graph. The expansion-coloring phase provides a way of reconsidering extracted independent sets as additional color classes for the purpose of progressive coloring optimization.

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


Nonlocal Image and Movie Denoising Abstract Neighborhood filters are nonlocal image and movie filters which reduce the noise by averaging similar pixels. The first object of the paper is to present a unified theory of these filters and reliable criteria to compare them to other filter classes. A CCD noise model will be presented justifying the involvement of neighborhood filters. A classification of neighborhood filters will be proposed, including classical image and movie denoising methods and discussing further a recently introduced neighborhood filter, NL-means. In order to compare denoising methods three principles will be discussed. The first principle, "method noise", specifies that only noise must be removed from an image. A second principle will be introduced, "noise to noise", according to which a denoising method must transform a white noise into a white noise. Contrarily to "method noise", this principle, which characterizes artifact-free methods, eliminates any subjectivity and can be checked by mathematical arguments and Fourier analysis. "Noise to noise" will be proven to rule out most denoising methods, with the exception of neighborhood filters. This is why a third and new comparison principle, the "statistical optimality", is needed and will be introduced to compare the performance of all neighborhood filters.The three principles will be applied to compare ten different image and movie denoising methods. It will be first shown that only wavelet thresholding methods and NLmeans give an acceptable method noise. Second, that neighborhood filters are the only ones to satisfy the "noise to noise" principle. Third, that among them NL-means is closest to statistical optimality. A particular attention will be A. Buades · B. Coll · J.-M. Morel ( ) CMLA (Mathematics), Ecole Normale Supérieure, de Cachan, 61 av. President Wilson, Cachan 94235, France e-mail: morel@cmla.ens-cachan.fr paid to the application of the statistical optimality criterion for movie denoising methods. It will be pointed out that current movie denoising methods are motion compensated neighborhood filters. This amounts to say that they are neighborhood filters and that the ideal neighborhood of a pixel is its trajectory. Unfortunately the aperture problem makes it impossible to estimate ground true trajectories. It will be demonstrated that computing trajectories and restricting the neighborhood to them is harmful for denoising purposes and that space-time NL-means preserves more movie details. A common framework for the study of the neighborhood filters has been presented. We have proposed three principles for the comparison of denoising methods evaluating the loss of image structure, the creation of artifacts, and the complete usage of image self-similarity. After a structural comparison of non-local denoising methods with other classes, we have shown that movie denoising can avoid the explicit computation of an optical flow estimate. What is left to be done? The very recent extensions of NL-means we mentioned in the introduction point towards the involvement of still more sophisticated statistical instruments. Beyond denoising, we have seen that the association with each pixel of a probability distribution weighting its similarity with the other pixels of the image may become a central tool in image analysis. This rich structure unfolds image information and should be used for algorithm which simultaneously analyze and process images. The recent work of points towards this direction. These authors use NLmeans as a segmentation tool.

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 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 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 B I 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 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


WESTT (workload, error, situational awareness, time and teamwork): an analytical prototyping system for command and control Abstract Modern developments in the use of information technology within command and control allow unprecedented scope for flexibility in the way teams deal with tasks. These developments, together with the increased recognition of the importance of knowledge management within teams present difficulties for the analyst in terms of evaluating the impacts of changes to task composition or team membership. In this paper an approach to this problem is presented that represents team behaviour in terms of three linked networks (representing task, social network structure and knowledge) within the integrative WESTT software tool. In addition, by automating analyses of workload and error based on the same data that generate the networks, WESTT allows the user to engage in the process of rapid and iterative ''analytical prototyping''. For purposes of illustration an example of the use of this technique with regard to a simple tactical vignette is presented. The field of command and control (C2) is undergoing significant changes as a result of the gradual adoption of modern information and networking technologies. Indeed, it is argued that the advent of network enabled capability (NEC), as it is termed in the UK, will transform the way in which UK armed forces operate: ''It offers a new way of not just 'doing things better' but of 'doing better things ''' (MoD 2005) . It is envisaged that NEC, by linking sensors, effectors, decision makers and other individuals will through the rapid and timely sharing of information create widespread shared understanding of a situation that will in turn allow swifter actions, based on better-informed decisions, to be made (MoD 2005) . Another benefit of NEC will be increased flexibility and agility within teams as a result of flexibility within information sharing and communications to create: ''…highly responsive, well integrated and flexible joint force elements that have assured access to and unprecedented freedom of manoeuvre within the entire battlespace. '' (MoD 2004) . Whilst NEC will be based on the roll-out of interconnected technologies, the locus of its effect will be upon human cognition and decision making to enable better actions to be made (see Houghton and Baber 2005) .. The situation created by the adoption of NEC technology poses considerable challenges for the analyst hoping to understand and evaluate the sociotechnical systems it produces: First, the emphasis on agility and flexibility suggests there are many possible system designs we might wish to consider at any one time, therefore a system allowing the rapid modelling and then reconfiguration of that model is required. This general approach is termed analytical prototyping and is based on the concept that initial system descriptions can be quantatively explored in order to evaluate the potential benefits of modification Stanton 1999, 2001 ). Second, the importance attached to the possession and fluid sharing of information prompts us to look to find a way of usefully representing and understanding the distribution and use of information within the sociotechnical systems that NEC will create. Our proposed solution to this problem takes the form of linking what we call ''knowledge objects'' to individuals and tasks to produce a systems-level rendering of knowledge use. Knowledge objects are discussed in more detail later in this paper, but in essence they represent forms of knowledge that must be in some way accessed or owned for a task to be successfully completed. Third, the integration of sensors, effectors and humans suggests the requirement for the production of a systems-level depiction of NEC that considers the interactions of both humans and technological items. Our approach to this issue is to represent all entities as ''agents'' within the system that have essentially the same characteristics whether human or non-human (that is, they can possess and share knowledge objects, can be said to carry out certain tasks or operations and can form nodes in social networks linked to other agents through communications). By adopting this approach one can therefore investigate the impacts of augmenting or even replacing elements of teams with new technology on a like-for-like basis and how these changes alter the system as a whole.. The WESTT (workload, error, situational awareness, time and teamwork) software tool has a range of possible applications that are not necessarily limited to the military sphere. Specific uses include the analysis of field data and the evaluation of current practice; comparison between actual performance and design/doctrine; evaluation of changes to current practice through modelling; and evaluation of performance in training and virtual environments. Whilst our approach was initially motivated with regard to military operations, we believe the WESTT tool can be used widely in the study of any organization where C2 and complex team/collaborative performance are important elements (e.g., emergency services operations and disaster management). Whilst WESTT is an integrative approach, bringing together many metrics and modes of representation of team activity, one is by no means required to necessarily use all of its elements. This paper has described the rationale behind and use of the WESTT software tool. It is envisaged that WESTT will provide a novel and useful means of representing team activity, and will be particularly beneficial for exploring future configurations in command and control system structure. Because the analyses and metrics are largely automated, WESTT can be used as an analytical prototyping tool allowing one to rapidly compare different approaches by manipulating data in the main data-table. By supporting analysis at several levels, it is possible to explore the effects of changes in system structure, the introduction or removal of knowledge objects (which might be operating procedures, cultural knowledge or tactical information) and the replacement or augmentation of human agents with technology. One way of understanding how the various networks are linked is to consider that within WESTT, for each agent (that is, a human or technological entity) we have the following information available: what they have done or will be doing (operation sequence diagram); what they know (knowledge objects embedded in a propositional network); who they know (social network diagram and metrics); what their modes of failure might be at various points (error); and broadly what their capacity is for adapting to a different schedule of tasks is (OSD/critical path analysis of time/workload). To answer a design question using WESTT we might want to select from a subset of these attributes for a subset of agents. Alternatively we may survey the overall structures of knowledge, social nets and tasks to gain an appreciation of the functioning of the system as an entity in itself.. As an integrative tool, many individual aspects of WESTT have already been subject to extensive testing, academic discussion and validation. For example, there is a large literature discussing the uses of social network modelling (e.g., Wasserman and Faust 1992) and many of the workload, error and time metrics are in common use by human factors practitioners. The use of ''plug in'' databases of task time and error also allow one to employ pre-validated and reliability-tested estimates.. Validation of WESTT as a whole has so far consisted of discussions with subject matter experts regarding the face validity of the outputs and usefulness of comparing different types of representations as an aid to the investigation by analysts of complex datasets (this has taken place with regard to police, fire and power industry scenarios). In addition to this we are currently following two more formal approaches to validation. First, we are comparing WESTT predictions to the outputs delivered by simulation tools (such as MicroSAINT). Second, and Fig. 6 Estimated propositional network for an urban operations scenario perhaps more importantly given an analysis tool's ultimate duty is to produce metrics and data that accord with reality itself, we are involved in ongoing experimental work looking at the effects of different network structures and technologies upon the activities of teams of human participants in different games and scenarios and comparing the outputs of WESTT analyses against those real world patterns of performance. Future directions for the development of WESTT include adding more metrics as appropriate and in particular ''headline figures'' which more succinctly describe the performance of the system as a whole.

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


Normal Forms for Spiking Neural P Systems Summary. The spiking neural P systems are a class of computing devices recently introduced as a bridge between spiking neural nets and membrane computing. In this paper we prove a series of normal forms for spiking neural P systems, concerning the regular expressions used in the firing rules, the delay between firing and spiking, the forgetting rules used, and the outdegree of the graph of synapses. In all cases, surprising simplifications are found, without losing the computational universality -sometimes at the price of (slightly) increasing other parameters which describe the complexity of these systems. The spiking neural P systems (in short, SN P systems) were recently introduced in [2] , and then investigated in [7] and [8] , thus incorporating in membrane computing [6] ideas from spiking neurons, see, e.g., [1] , [3] , [4] .. In short, an SN P system consists of a set of neurons placed in the nodes of a graph, representing synapses. The neurons send signals (spikes) along synapses (edges of the graph). This is done by means of firing rules, which are of the form E/a c → a; d, where E is a regular expression, c is the number of spikes consumed by the rule, and d is the delay from firing the rule and emitting the spike. The rule can be used only if the number of spikes collected by the neuron is "covered" by expression E, in the sense that the current number of spikes in the neuron, n, is such that a n ∈ L(E), where L(E) is the language described by expression E. In the interval between firing a rule and emitting the spike, the neuron is closed/blocked, it does not receive other spikes and cannot fire again. There also are rules for forgetting spikes, of the form a s → λ (s spikes are just removed from the neuron). Starting from an initial distribution of spikes in the neurons and using the rules in a synchronized manner (a global clock is assumed), the system evolves. A sequence of transitions among configurations of an SN P system, starting in the initial configuration, is called a computation. One of the neurons is designated as the output neuron and its spikes can also exit the system. The sequence of steps when the output neuron sends spikes to the environment is called the spike train of the computation.. An SN P system can be used as a computing devices in two main ways: as a number generator and as a generator and transducer of infinite sequences of bits. In the first case, considered in [2] and [7] , one associates a set of numbers with a spike train in various ways: considering the distance between the first k spikes of a spike train, or the distances between all consecutive spikes, in both cases taking into account all intervals or only considering intervals that alternate (ignoring every second one), accepting only halting or only infinite computations. In this last case, one can naturally associate an infinite binary sequence with a spike train by writing 0 for a step when no spike exits the system and 1 for a step when a spike is emitted by the output neuron.. In the first interpretation of SN P systems, as devices computing sets of natural numbers, it was proven in [2] , [7] that Turing completeness is achieved if no bound is imposed on the number of spikes present in the neurons, and a characterization of semilinear sets of numbers is obtained if a bound is imposed on the number of spikes present in neurons during a computation.. In the proofs of these results, all features of the SN P systems as briefly introduced above were used: regular expressions describing languages different from a * , delays d different from 0, forgetting rules, while the synapse graphs of the systems involved in the proofs have outdegree four. The question of improving these proofs from these points of view was formulated as a research topic in the papers [2] , [7] . We contribute here to this topic with several results, some of them surprising: we prove universality: (i) with regular expressions of the form E = a + (hence telling nothing else about the number of spikes from the neuron other than the fact that some spikes do exist) or of the form a i for some i ≥ 1, (ii) rules without delay, i.e., of the form E/a c → a; 0, and (iii) without using forgetting rules (this last result solves an open problem from [2] , asking whether the universality is preserved even when forgetting rules are not used -the answer proves to be affirmative). We do not know whether these normal forms can be combined: in the proofs of (i) and (iii) we use delays, in the proofs of (ii) and (iii) we use non-trivial regular expressions, while in the proofs of (i) and (ii) we use forgetting rules. What can be combined with all these three normal forms is the next condition: each neuron has only two outgoing synapses (hence the synapse graph has the outdegree two). This is of a clear interest in our framework, because the spikes in an SN P system can be increased only by means of multiple outgoing synapses -the result mentioned above shows that the minimal outdegree suffices.. In the next section we introduce a few technical prerequisites, then (Section 3) we recall from [2] , [7] the definition of spiking neural P systems and fix the notation we use. In Section 4 we give the result about the possibility of working with no delay, the next section (Section 6) describes the power of systems that are not allowed to use forgetting rules, then (Section 5) we also bound the outdegree of SN P systems without losing the universality. Section 7 gives the normal form about the regular expressions from the firing rules. The paper ends with some open problems and research topics discussed in Section 8.

O O O O O 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 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 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 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 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 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


A type system for certified binaries A certified binary is a value together with a proof that the value satisfies a given specification. Existing compilers that generate certified code have focused on simple memory and control-flow safety rather than more advanced properties. In this article, we present a general framework for explicitly representing complex propositions and proofs in typed intermediate and assembly languages. The new framework allows us to reason about certified programs that involve effects while still maintaining decidable typechecking. We show how to integrate an entire proof system (the calculus of inductive constructions) into a compiler intermediate language and how the intermediate language can undergo complex transformations (CPS and closure conversion) while preserving proofs represented in the type system. Our work provides a foundation for the process of automatically generating certified binaries in a type-theoretic framework. Proof-carrying code (PCC), as pioneered by Necula and Lee [1996] and Necula [1997] , allows a code producer to provide a machine-language program to a host, along with a formal proof of its safety. The proof can be mechanically checked by the host; the producer need not be trusted because a valid proof is incontrovertible evidence of safety.. The PCC framework is general because it can be applied to certify arbitrary data objects with complex specifications [Necula 1998; Appel and Felten 2001] . For example, the Foundational PCC system [Appel and Felty 2000] can certify any property expressible in Church's higher-order logic. Harper [2000] and Burstall and McKinna [1991] call all these proof-carrying constructs certified binaries (or deliverables). A certified binary is a value (which can be a function, a data structure, or a combination of both) together with a proof that the value satisfies a given specification.. Unfortunately, little is known on how to construct or generate certified binaries. Most existing certifying compilers [Necula and Lee 1998; Colby et al. 2000] have focused on simple memory and control-flow safety only. Typed intermediate languages [Harper and Morrisett 1995] and typed assembly languages ] are effective techniques for automatically generating certified code; however, none of these type systems can rival the expressiveness of the actual higher-order predicate logic (which could be used in any Foundational PCC system).. In this article, we present a type-theoretic framework for constructing, composing, and reasoning about certified binaries. Our plan is to use the formulaeas-types principle [Howard 1980 ] to represent propositions and proofs in a general type system, and then to investigate their relationship with compiler intermediate and assembly languages. We show how to integrate an entire proof system (the calculus of inductive constructions [Paulin-Mohring 1993; Coquand and Huet 1988] ) into an intermediate language, and how to define complex transformations (CPS and closure conversion) of programs in this language so that they preserve proofs represented in the type system. Our approach builds upon a large body of previous work in the logic and theorem-proving community (see Barendregt and Geuvers [1999] and Barendregt [1991] for a good summary), and makes the following new contributions: -We show how to design new typed intermediate languages that are capable of representing and manipulating propositions and proofs. In particular, we show how to maintain decidability of typechecking when reasoning about certified programs that involve effects. This is different from the work done in the logic community which focuses on strongly normalizing (primitive recursive) programs. -We maintain a phase distinction between compile-time typechecking and run-time evaluation. This property is often lost in the presence of dependent types (which are necessary for representing proofs in predicate logic). We achieve this by never having the type language (see Section 3) dependent on the computation language (see Section 4). Proofs are instead always represented at the type level using dependent kinds.. -We show how to use propositions to express program invariants and how to use proofs to serve as static capabilities. Following Xi and Pfenning [1999] , we use singleton types [Hayashi 1991 ] to support the necessary interaction between the type and computation languages. We can assign an accurate type to unchecked vector (or array) access (see Section 4.3). Xi and Pfenning [1999] can achieve the same using constraint checking, but their system does not support arbitrary propositions and (explicit) proofs, so it is less general than ours. -We use a single type language to typecheck different compiler intermediate languages. This is crucial because it is impractical to have separate proof libraries for each intermediate language. We achieve this by using inductive definitions to define all types used to classify computation terms. This in turn nicely fits our work on (fully reflexive) intensional type analysis [Trifonov et al. 2000 ] into a single system. -We show how to perform CPS and closure conversion on our intermediate languages while still preserving proofs represented in the type system. Existing algorithms Harper and Lillibridge 1993; Minamide et al. 1996; Barthe et al. 1999] all require that the transformation be performed on the entire type language. This is impractical because proofs are large in size; transforming them can alter their meanings and break the sharing among different languages. We present new techniques that completely solve these problems (Sections 5-6). -Our type language is a variant of the calculus of inductive constructions of Paulin-Mohring [1993] and Coquand and Huet [1988] . Following Werner [1994] , we give rigorous proofs for its meta-theoretic properties (subject reduction, strong normalization, confluence, and consistency of the underlying logic). We also give the soundness proof for our sample computation language. See Sections 3-4, the Appendix, and the companion technical report for details.. As far as we know, our work is the first comprehensive study on how to incorporate higher-order predicate logic (with inductive terms and predicates) into typed intermediate languages. Our results are significant because they open up many new exciting possibilities in the area of type-based language design and compilation. The fact that we can internalize a very expressive logic into our type system means that formal reasoning traditionally done at the meta level can now be expressed inside the actual language itself. For example, much of the past work on program verification using Hoare-like logics may now be captured and made explicit in a typed intermediate language.. From the standpoint of type-based language design, recent work [Harper and Morrisett 1995; Xi and Pfenning 1999; Walker 2000; Crary and Weirich 2000; Trifonov et al. 2000 ] has produced many specialized, increasingly complex type systems, each with its own meta-theoretical proofs, yet it is unclear how they will fit together. We can hope to replace them with one very general type system whose meta theory is proved once and for all, and that allows the definition of specialized type operators via the general mechanism of inductive definitions. For example, inductive definitions subsume and generalize earlier systems for intensional type analysis [Harper and Morrisett 1995; Crary and Weirich 1999; Trifonov et al. 2000] .. We have a prototype implementation of our new type system in the FLINT compiler [Shao 1997; Shao et al. 1998 ], but making the implementation realistic still involves solving many remaining problems (e.g., efficient proof representations). Nevertheless, we believe our current contributions constitute a significant step toward the goal of providing a practical end-to-end compiler that generates certified binaries. We presented a general framework for explicitly representing propositions and proofs in typed intermediate or assembly languages. We showed how to integrate an entire proof system into our type language and how to perform CPS and closure conversion while still preserving proofs represented in the type system. Our work is a first step toward the goal of building realistic infrastructure for certified programming and certifying compilation.. • Z. Shao et al. Our type system is fairly concise and simple with respect to the number of syntactic constructs, yet it is powerful enough to express all the propositions and proofs in the higher-order predicate logic (extended with induction principles). In the future, we would like to use our type system to express advanced program invariants such as those involved in low-level mutable recursive data structures.. Our type language is not designed around any particular programming language. We can use it to typecheck as many different computation languages as we like; all we need is to define the corresponding kind as an inductive definition. We hope to evolve our framework into a realistic typed common intermediate format. The proof of Progress of λ H relies critically on the adequacy of the representation of meta-proofs of natural numbers being in the less-than relation, that is, that for closed A and B the kind LT A B is inhabited if and only if A and B represent natural numbers related by less-than. In the case of the less-than relation and LT, this fact was proved in Lemma 4.1. However, it must be kept in mind when considering extensions of λ H that since CIC and TL are more expressive than higher-order predicate logic, adequacy of the representations of metaproofs does not hold in general; hence, the existence of a term of the kind of the proposition does not imply that there is a meta-proof of the proposition. For instance, the ability to eliminate inductive kinds in TL allows analysis of proof derivations-a technique which allows the construction of proof terms without counterpart in standard meta-reasoning. This issue does not arise for first-order proof representations (whose constructors have no parameters of a function kind) such as LT, and we do not expect it to be a concern in practice. In cases when it does arise, it could be resolved by using the underlying consistent logic of CIC in place of the meta-logic; for instance, in our presentation the question of adequacy is raised because the operational semantics of λ H is defined in meta-logical terms, but this question would be moot if λ H and its semantics were defined as CIC terms. To eliminate the interaction with the meta-logic, this approach should be applied all the way down to the hardware specification (as done in some PCC system [Appel and Felty 2000] ); we plan to pursue this in the future. The language λ H is intended only as an illustration of the expressiveness of type systems based on TL. As we showed in Section 4.4, type conversions can be programmed in λ H ; however, it is also easy to extend λ H with a type conversion construct cast, which allows conversion between any types which the programmer can prove are in a given relation of equivalence. The strongest such equivalence relation in TL is represented by Eq, and in this case the typing rule for cast is ;. The dynamic semantics of cast is trivial:. The proof of the soundness of this extension is based on the observation (following from Theorem 3.3, the Church-Rosser property of TL) that if the judgment · B : Eq A A is derivable (which is what we have in the corresponding case of the proof of Subject Reduction), then the normal form B of B is an application of refl to some kind equivalent to and to some type A 1 . But the kind of this application is then Eq A 1 A 1 , while the kind of B is Eq A A , so either A = A 1 , or there is an application of rule CONV in the derivation for B , with a proof of A = βηι A 1 in the premise, and similarly for A vs. A 1 . Thus we can obtain a proof that A = βηι A , and the rest of the meta-proof is the same as for E-CONV. Our type language is a variant of the calculus of constructions [Coquand and Huet 1988] extended with inductive definitions (with both small and large elimination) [Paulin-Mohring 1993; Werner 1994] . We omitted parameterized inductive kinds and dependent large elimination to simplify our presentation, however, all our meta-theoretic proofs carry over to a language that includes them. We support η-reduction in our language while the official Coq system does not. The proofs for the properties of TL are adapted from Geuvers [1993] and Werner [1994] (which in turn borrows ideas from Altenkirch [1993] ); the main difference is that our language has kind-schema variables and a new product formation rule (Ext, Kind) which are not in Werner's system. The Coq proof assistant provides support for extracting programs from proofs [Paulin-Mohring 1993] . It separates propositions and sets into two distinct universes Prop and Set. We do not distinguish between them because we are not aiming to extract programs from our proofs, instead, we are using proofs as specifications for our computation terms. Burstall and McKinna [1991] proposed the notion of deliverables, which is essentially the same as our notion of certified binaries. They use dependent strong sums to model each deliverable and give its categorical semantics. Their work does not support programs with effects and has all the problems mentioned in Section 2.3.. Xi and Pfenning's DML [Xi and Pfenning 1999] is the first language that nicely combines dependent types with programs that may involve effects. Our ideas of using singleton types and lifting the level of the proof language are directly inspired by their work. DML does not support explicit proofs in its type language; any assertions (or constraints) must be resolved fully automatically in order to ensure decidable typechecking. As a result, DML's assertion language only allows integer linear inequalities. Our system, on the other hand, allows arbitrary propositions and proofs. An assertion in our system can use any integer constraints but a certified program must explicitly provide proofs on how these constraints are satisfied. Our system is best suited for use in compiler typed intermediate languages while the DML type system is more suitable for use in a source programming language. Another difference is that DML does not define the kind as an inductive definition so it does not support intensional type analysis [Trifonov et al. 2000] and it is unclear how it can preserve proofs during compilation.. We have discussed the relationship between our work and those on PCC, typed assembly languages, and intensional type analysis in Section 1. Inductive definitions subsume and generalize earlier systems on intensional type analysis [Harper and Morrisett 1995; Crary and Weirich 1999; Trifonov et al. 2000] ; the type-analysis construct in the computation language can be eliminated using the technique proposed by Crary et al. [1998] .. The work presented in this article showed one way of having types and proofs coexist in an intermediate language for certified binaries, that is, by embedding predicates and proofs directly into types. Another possibility, which we did not address, is to embed types into the logic which proofs are carried outessentially using pre-and post-conditions as in Hoare logic to express type invariants. Unfortunately, Hoare logic does not work well with higher-order functions, for example, it is unclear how to describe an assertion that a formal parameter (of another function) has a function type (as simple as int→int). Foundational PCC [Appel and Felty 2000] requires explicit construction of the fixed point (using index-based semantic model) to support higher-order functionswhich is probably too complex for compiler intermediate languages.. Concurrently with our work, Crary and Vanderwaart [2001] recently proposed a system called LTT, which also aims at adding explicit proofs to typed intermediate languages. LTT uses Linear LF [Cervesato and Pfenning 1996] as its proof language. It shares some similarities with our system in that both are using singleton types [Xi and Pfenning 1999] to circumvent the problems of dependent types. However, since LF does not have inductive definitions and the Elim construct, it is unclear how LTT can support intensional type analysis and type-level primitive recursive functions [Crary and Weirich 2000] . In fact, to define as an inductive kind [Trifonov et al. 2000] , LTT would have to add proof-kind variables and proof-kind polymorphism, which could significantly complicate the meta-theory of its proof language. LTT requires different type languages for different intermediate languages; it is unclear whether it can preserve proofs during CPS and closure conversion. The power of linear reasoning in LTT is desirable for tracking ephemeral properties that hold only for certain program states; we are working on adding such support into our framework.

O B I O O O O O O O O B O O O O O O B 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 B B 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 B O O O O B O O O O O O O O O O O O O O B I O O O O O O O B O O O O O O O O B O B O O O O O O O O O O O O O O B 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 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 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 I I O O O 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 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 B 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 O O O 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 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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Simulating forest plantation co-management with a multi-agent system Co-management of forest resources is a process of governance that enables all relevant stakeholders to participate in the decision-making processes. Illegal logging and forest degradation are currently increasing, and logging bans are ineffective in reducing forest degradation. At the same time interest in forest plantations and concern about poverty problems of neighboring people whose livelihoods depend on forest services and products continue to increase rapidly. Governments have identified the development of small forest plantations as an opportunity to provide wood supplies to forest industries and to reduce poverty. However, the development of small plantations is very slow due to an imbalance of power and suspicion between communities and large companies. This paper proposes a framework for linking social, economic and biophysical dynamics to create multi-agent simulations and explore scenarios of collaboration for plantations. Multi-agent simulation is a branch of artificial intelligence that offers a promising approach to dealing with multi-stakeholder management systems, such as common pool resources. It provides a framework which allows analysis of stakeholders' (or agents') interactions and decision making. Each stakeholder has explicit communication capacities, behaviors and rationales from which emerge specific actions. The purpose of this modeling is to create a common dynamic representation to facilitate negotiations for growing trees. A system of governance involving multi-stakeholders, especially local communities and wood based industries appearing to offer the most promising pathway to accelerating plantation development, local community poverty alleviation and forest landscape improvement. Principle 22 of the 1992 Rio Declaration on Environment and Development highlights the importance of local people and their participation in sustainable development. In forest plantations, this applies to local communities living in or near forest plantations. The research concludes that MAS can be used to envision scenarios of forest plantation development over the long term involving multi-stakeholders. Although, we need a deeper study, the current results already show that setting up a sawmill adapted for processing small logs from plantations would be a huge incentive for smallholders to develop plantations, including areas far from roads. The sawmills would increase the wood's value. The outcome, besides improved landscape management, is also increased income for smallholders. This is because without a sawmill, smallholders move to sites close to roads and leave their remote plots; as a result, wood supply to industries, smallholder incomes and forest areas are all reduced. MAS is able to represent the interaction among stakeholders and tree dynamics in the plantation systems. The impact of any policy initiative (such as co-management by stakeholders in managing a complex forest) may take many years to be manifested and frequently becomes evident beyond the period of research and monitoring. In this case, MAS, through one of its tools, namely CORMAS, can be used to examine the long term impact of possible scenarios. The primary goal of modeling is not to give solutions but to learn about the situation.. Simulation experiments with the model showed that the present situation, where the smallholders mainly depend on the brokers and there is no sawmill, cannot sustain the existing plantations. SAFODA's strategy of plant-cut-replant cannot produce its expected revenue. This situation is apparent in landscape, wood supply, and financial problems for smallholders and SAFODA. None of them are able to solve the problem by themselves in this value chain of wood products. To increase its revenue, SAFODA depends on the interaction with the government of Sabah State. The smallholders need to organize themselves or to widen their perspective on international markets of certified products in order to improve their welfare.. We can learn from the model that a sawmill is needed to solve the problems of the smallholders. However, this cannot happen automatically. Fig. 4 describes the organization level of the forest plantation model. The plantation governance social group can play an important role in changing the situation. Citizens, the forest development agency and profit makers can interact and influence the government to introduce a new policy initiative of giving incentives to the profit makers and others to establish a sawmill. It should be noted the government is only involved in one social group, but has an important role in changing the whole situation. If the government shows incentives to establish a sawmill then we believe sawmills and timber plantations will emerge in the area.. SAFODA may ask the government what its primary mission is. If the agency takes responsibility for self-financing, the government should allow the agency to implement different strategies to maintain its financial sustainability. This involves implementing a patch assessment strategy in order to plant, or not to plant, pulp or timber plantations. The government should inform the agency of this flexibility; otherwise the agency's financial crisis will continue.. The stakeholders can interact to solve the global problems of decreasing plantation area, as well as wood pulp supply, the failure of an improvement in the smallholders' welfare and the financial crisis of SAFODA. We use the term 'global problems' to distinguish them from local problems which include problems of individual stakeholders. The global problems emerge from the stakeholder interactions.. The way SAFODA behaves is primarily a result of the interaction between SAFODA and the government. This happens because SAFODA belongs to the government, and therefore SAFODA follows the government's instructions. SAFODA implements the strategy of plant-cut-replant. The rule of implementing what the government orders is always stronger that other rules. The feedback of the interaction cannot make the other rules stronger than the government's order. Therefore, the key to change is the government's behavior. If the government gives SAFODA more flexibility to plant pulp plantations, then SAFODA may achieve better financial performance.. The government interacts with all the other agents. The government pays great attention to public interests, which are the global problems in this case. If the interactions can change the internal rules of the government in such a way that it is more adaptive to the situation then appropriate action may occur. Its rules can change through the reinforcement and reproduction rule mechanisms. Feedback from the citizens and companies plays a major role in making the government more adaptive.. Smallholders, who are taking the role of citizen, hope for a better price for their wood. Analysis of the value chains of the wood pulp reveals that there is no way to increase its price for the smallholders unless there is better price at the harbor for export. The smallholders need to interact with international markets, which are traditionally not connected with them. In the model we did not represent a social group of international trading, in which smallholders and importers from various countries such as China and Indonesia are members. Demand for pulp wood is rising in China due to the development of a giant pulp and paper industry. Asia pulp and paper, one of the largest pulp and paper industries in the world and located in Indonesia, also has an enormous demand for pulp wood.. Connecting the smallholders directly to international markets means shortening the value chains of wood pulp. The role of the brokers is always important. However, if the smallholders have a better perspective on their product market, it will enforce the distribution of the profits. This also applies to SAFODA. In our model, SAFODA receives a net revenue about million RM 1.8 while the pulp broker receives million RM 3.5. The broker receives about twice as much money as SAFODA does. Meanwhile, the smallholders' net revenue is only several hundred RM.. As in the top-down scenario, sawmill establishment can increase the sustainability of the plantations. The smallholders, when taking the role of citizen, can suggest that government make the situation more conducive to private company investment in establishing a sawmill in the area. They can also form a cooperative to jointly establish a sawmill. Collective action by the smallholders can solve their problems. More interaction among them is necessary to have a collective plan and collective action.. The smallholders, in the role of citizen, can campaign for awareness of environmental problems. Illegal logging, which has spread all over the island of Borneo, has decreased the price of wood in general. The use of certified forest products is in the preliminary stages. Wood coming from the smallholders is certain to achieve easy certification because it is the result of a community based effort to plant trees. Reducing illegal logging and using certified forest products will increase the wood price, and will also increase the welfare of the smallholders.. The way the model is organized gives us an insight that can be used anywhere in the world for forest plantations like those in Indonesia. Thus, the model also acts as a general model from which a site-specific model can be derived through calibration and adaptation. At the micro-level we observe the interaction of agents with various roles. At the macro-level we observe the global problems that emerge from the stakeholders' interactions.. The use of organization centered MAS or OCMAS increases the generality and modularity of the model. However, in this simulation we did not examine the impact of the stakeholders' genetic algorithms on reproducing 'offspring' rules in order to fit and survive better in the far future. This is because the problems of governance of forest plantations, as we observed, were more concerned with the short-term results directed by government policies. A further study of the use classifiers of the genetic algorithm as well as its results in the context of multi-stakeholder forest plantations is strongly recommended. Stakeholders were identified according to the following criteria: proximity to the forest, legal or traditional rights over the forest, dependency on the forest, and knowledge of forest management [16] . Stakeholder characteristics were recognized through field visits and group discussions [17] . Researchers facilitated the discussions to establish stakeholder identities, their rationale, and their behavior and actions. These characteristics formed the basis for the MAS model developed subsequently. Other data were obtained from SAFODA and related literature.. There are four key phases in the development of a model [18] : i.e. (a) Forming a conceptual model is to state the model's objectives, bound the system of interest, categorize its components, identify relationships, and to describe the expected patterns of the model's behavior; (b) Specifying the model is to identify the functional forms of the model's equations, estimate the parameters, and to represent it in CORMAS; (c) Evaluating the model is to re-assess the logic underpinning the model, and compare model predictions with expectations; (d) Using the model is to develop scenarios.

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 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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 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 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 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 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 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 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 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 B 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


Multiple password interference in text passwords and click-based graphical passwords The underlying issues relating to the usability and security of multiple passwords are largely unexplored. However, we know that people generally have difficulty remembering multiple passwords. This reduces security since users reuse the same password for different systems or reveal other passwords as they try to log in. We report on a laboratory study comparing recall of multiple text passwords with recall of multiple click-based graphical passwords. In a one-hour session (short-term), we found that participants in the graphical password condition coped significantly better than those in the text password condition. In particular, they made fewer errors when recalling their passwords, did not resort to creating passwords directly related to account names, and did not use similar passwords across multiple accounts. After two weeks, participants in the two conditions had recall success rates that were not statistically different from each other, but those with text passwords made more recall errors than participants with graphical passwords. In our study, click-based graphical passwords were significantly less susceptible to multiple password interference in the short-term, while having comparable usability to text passwords in most other respects. Special consideration is required to design usable, understandable, and manageable security features. At first glance, it seems like applying standard usability and Human-Computer Interaction Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. (HCI) principles should suffice, but security constraints make this problematic. Most importantly, some design features that might make a system more usable would also make it less secure. Addressing these security weaknesses can too easily render the software unusable again. Even worse, one might argue that an unusable security system is inherently insecure, since users will then misuse or bypass the security mechanisms. One must also consider how the design affects the observable behaviour of legitimate users, in case such behaviour could be exploited by attackers. The challenge is to design software that is both secure and usable [10] .. In this paper, we address an important issue in user authentication software: the memorability of multiple passwords. Authentication software supports legitimate users in gaining access to systems or resources by verifying their credentials. We focus on passwords, the most common form of credentials. The problem with passwords is making them easy for legitimate users to remember, but difficult for attackers to guess. Alternatives to passwords include physical tokens or biometrics; these also have problems, such as cost, management, and privacy, which we will not address in the paper. As passwords are the most common method of authentication, the password problem is important, and is made worse by the increasing number of users and the number of different systems they access [15, 18] . In particular, users now need to remember not just one password, but many. This places a significant memory load on users, leading them to choose (and reuse) simple passwords that are easy for attackers to guess. Despite this reality, there has been little work investigating the issues relating to multiple passwords.. Our current work is motivated by recent proposals for alternative kinds of passwords, particularly click-based graphical passwords [3, 39] . In such systems, the user does not enter a text password using a keyboard, but instead clicks on particular points on an image. Such graphical passwords are intended to take advantage of the human ability to more easily recognize and recall images than textual information [29] . We wished to study whether this approach had advantages over text passwords when multiple distinct passwords were necessary. We were concerned about the potential for multiple password interference, where remembering a password for one system might affect the user's memory of a password for another system. As described in the cognitive psychology literature [2] , memory interference is "the impaired ability to remember an item when it is similar to other items stored in memory".. Our study was conducted in a laboratory setting where 65 participants were assigned to use either textual or graphical passwords. They created six distinct passwords for several different "accounts", and later had to recall the passwords for each account in a different order than they were created. Twenty-six of those participants also returned after two weeks to test recall of these passwords. In the case of graphical passwords, each account was associated with a different image, so participants had one image per password. Literature on memory research acknowledges that ceiling effects, such as high success rates that mask differences between conditions, are a problem which must be overcome with careful experimental design in order to get meaningful results [22] . While our study does not mirror real-life usage of passwords, the intent of the experiment was to highlight differences in performance.. We found that in the short-term, participants had more difficulty recalling multiple text passwords than multiple graphical passwords. However, after two weeks, both groups had significant difficulty remembering their passwords and there was no statistical difference in success rates. We further found that participants in the text condition could more easily recall their passwords when they used insecure password practices, such as choosing passwords that followed a common pattern or that were obviously associated with account names. For example, 40% of text passwords were obviously related to their associated account. These results constitute evidence for an important advantage inherent in click-based graphical passwords -built-in cueing that helps with memorability -while text password systems encourage users to adopt insecure coping strategies. In post-hoc analysis, we found that males were more successful than females with graphical passwords and that males were more likely to use account-related text passwords. These results align with psychology literature showing that males perform better at visual-spatial tasks and more weakly in linguistic tasks than females [5, 23] . The results of this paper motivate further study in field settings and deeper examination of the underlying human factors issues involved in using these authentication mechanisms.. The remainder of the paper is divided as follows. Section 2 provides background on the type of graphical password system used, memory cueing, and multiple password interference. In Sections 3 and 4, we outline our hypotheses, describe the methodology of our study, and present the results. We discuss validation of our hypotheses in Section 5. Lastly, we offer some discussion and concluding remarks in Sections 6 and 7. Results of our lab study indicate that in the short-term, PassPoints passwords are more robust than text passwords against multiple password interference (assuming distinct background images). Often, the usability of a system is tested in isolation but in the case of passwords this is especially problematic because user behaviour may change as users accumulate passwords. We show that in the short-term, participants could more easily remember multiple click-based graphical passwords than multiple text passwords. Participants in the Text condition made significantly more recall errors and resorted to additional coping strategies such as using account-based passwords or cycling through all of their passwords when trying to recall their password. We believe the memory cueing provided by click-based graphical passwords is at least part of the reason for better user performance and that cueing should be part of any recall-based authentication scheme, where possible.. After two weeks, recall of the passwords in the text and graphical conditions was not statistically different from each other. Given that users were much more familiar with memorizing and recalling text passwords, they were better positioned to remember these passwords. However, this advantage was offset by the graphical passwords' built-in memory cue, which is a more secure memory aid than users' typical text password coping mechanisms. Our results raise an interesting research question for text passwords: can cueing mechanisms be (safely) added to text passwords in order to achieve the same memorability advantages seen in click-based graphical passwords? As is the standard approach in analyzing data from user studies, we used various statistical tests to assess whether differences in the data reflect actual differences between conditions or whether these may have occurred by chance. Three types of statistical tests for significance were used in the present analysis, each intended to determine whether the groups being analyzed were distinct from each other with respect to the factor being tested. We used t-tests to compare variance of the means between two groups, Mann-Whitney tests to compare ordered categorical data (e.g., Likert scale responses, where the choices are discrete and ordered, but it cannot be assumed that participants view all pairs of adjacent levels as equidistant), and Chi-square (χ 2 ) tests to compare non-ordered categorical or nominal data (e.g., comparing login success/fail ratios). In all cases, we regard a value of p < .05 as indicating that the groups being tested are different from each other with at least 95% probability, making the result statistically significant. In the tables, "not significant" indicates that the test revealed no statistically significant differences between the two conditions (i.e., p > .05). Our study included two lab-based sessions. Session 1 took one hour and was completed by all 65 participants. For Session 2, participant returned to the lab and tried to recall their previously created passwords. The second session occurred after two weeks and was completed by 26 participants.

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


Nanomechanical strength mechanisms of hierarchical biological materials and tissues Biological protein materials (BPMs), intriguing hierarchical structures formed by assembly of chemical building blocks, are crucial for critical functions of life. The structural details of BPMs are fascinating: They represent a combination of universally found motifs such as a-helices or b-sheets with highly adapted protein structures such as cytoskeletal networks or spider silk nanocomposites. BPMs combine properties like strength and robustness, self-healing ability, adaptability, changeability, evolvability and others into multi-functional materials at a level unmatched in synthetic materials. The ability to achieve these properties depends critically on the particular traits of these materials, first and foremost their hierarchical architecture and seamless integration of material and structure, from nano to macro. Here, we provide a brief review of this field and outline new research directions, along with a review of recent research results in the development of structureproperty relationships of biological protein materials exemplified in a study of vimentin intermediate filaments. Proteins are the fundamental building blocks of a vast array of biological materials that are involved in critical functions of life, many of which are based on highly characteristic nanostructured arrangements of protein components that include tropocollagen, a-helices or b-sheets (Lakes 1993; Alberts et al. 2002; Wegst and Ashby 2004; Aizenberg et al. 2005) . Bone, providing structure to our body, or spider silk, used for prey procurement, are examples of materials that have incredible elasticity, strength and robustness unmatched by many synthetic materials (Buehler 1961; Hayashi and Lewis 1998; Weiner and Wagner 1998; Arzt et al. 2003; Sarikaya et al. 2003; Buehler 2006a Buehler , 2006b Buehler and Ackbarow 2007) .. Biological protein materials (BPMs) are particularly fascinating as they combine properties such as strength and robustness, self-healing ability, adaptability, changeability, evolvability and many other properties into multifunctional materials at a level still unmatched in synthetic materials. It is believed that the ability to achieve these properties depends critically on the particular traits of these materials, first and foremost their hierarchical architecture and the seamless integration of material and structure, from nano to macro.. Here, we review recent advances in utilising largescale atomistic and molecular modeling that elucidate the deformation and fracture mechanisms of BPMs. We illustrate how a combination of predictive atomistic modelling implemented on large scale computing facilities and theoretical analysis using statistical and continuum theories enables us to carry out highly spatially and temporally controlled experiments to probe the behaviour of protein structures at different hierarchical scales, ranging from single proteins, assemblies of proteins to protein networks, at time scales that range from picoseconds, nanoseconds to microseconds. The systematic traversing across multiple scales enables us to elucidate the scale-dependent properties and enables us to develop structure -property relationships. Clearly, this research strategy must be based on a multidisciplinary approach that combines chemistry, biology and materials science.. The particular focus of the present work is on BPMs that play a mechanical role in physiological systems (Engler et al. 2006) . This is motivated by the emergence of evidence that mechanical cues are central to many biological processes, as for instance in cell differentiation, mechanosensation/transduction, amyloidosis or cellular viral capsid injection (Alberts et al. 2002; Engler et al. 2006) . In some of these processes, mechanical cues directly influence gene regulation that then trigger biochemical signalling cascades that lead to a nonlocal response, controlling the behaviour of larger tissues scales. A particularly important field is thereby the analysis of the nanomechanical properties of participating proteins and protein networks, by describing the processes and behaviours when BPMs are stretched, bent or loaded in compression, and therefore undergo significant changes in shape and molecular structure.. Theories that describe these mechanical processes and properties must explicitly consider the chemical bonding and molecular structure, due to the strong interaction of chemistry and mechanics . In addition, biological processes often involve mechanical properties that are controlled by chemical cues (Alberts et al. 2002) . This combination is the equivalent of the extension of systems biology towards the ultimate material nano-scale at the boundary of chemistry, biology and materials science (Ackbarow and Buehler) . This could help to answer important open questions in the biological sciences, such as the elucidation of the boundaries and characteristics that distinguish a 'material' and 'living organism'. By using an integrated approach of theory and simulation, we have systematically varied the pulling velocity and discovered a change in unfolding behaviour during stretching of AH protein domains. The results of the MD simulations were analysed using the Extended Bell Model, which enables us to calculate energy barrier associated with breaking individual HBs. The results suggest that the unfolding mechanism at fast pulling rates is rupture of a single HB, whereas the unfolding mechanisms at slow pulling rates proceeds by simultaneous rupture of several parallel HBs .. It is apparent that as of today, numerical MD simulations are the only means to directly observe these mechanisms, since experiments are still lacking appropriate spatial and temporal resolution. Advances in computing power has enabled us to carry out direct atomistic simulation of unfolding phenomena, including explicit solvent, at time scales approaching a significant fraction of a microsecond.. In previous atomistic simulations, unfolding forces have been significantly larger than those measured in experiment, likely because they were carried out in the FDM so that forces reach up to several nN for individual AHs. This is clearly an artifact of large pulling speeds, as pointed out earlier (Lu and Schulten 1999; Sotomayor and Schulten 2007) . Our analysis shows that in addition to incorrect force estimates, the observed unfolding mechanism can also be significantly different if the pulling speed is too high. The estimate for v cr provides a maximum pulling rate that could be used in MD studies, in order to still allow a reasonable interpretation of MD results in light of biological relevance. The quantitative value reported here may thus provide guidance to set up other MD simulations.. We note that the SDM is most relevant for biological function. However, the FDM could be important during tissue injuries that may be incurred under large deformation rates (e.g. shock impact, bullets and fractures).

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