a normalized schur cohn stability test for the delta operator based polynomials the author previously 1997 proposed two delta operator based stability tests or more generally zero location tests those tests establish two families of such tests each spanning from the discrete time to the continuous time with the delta operator providing smooth transitions between the two domains in this paper a third family is proposed specifically the normalized schur cohn test in the discrete time domain is transformed into the delta operator domain resulting in a new delta operator test the limit of this new test as the sampling interval vanishes is shown to be the pham le breton test 1991 in the continuous time domain its relationships with the well known routh test and others are studied a numerical example shows the advantage of the new test for fast sampling

the analysis of rates using poisson regression models models are considered in which the underlying rate at which events occur can be represented by a regression function that describes the relation between the predictor variables and the unknown parameters estimates of the parameters can be obtained by means of iteratively reweighted least squares irls when the events of interest follow the poisson distribution the irls algorithm is equivalent to using the method of scoring to obtain maximum likelihood ml estimates the general poisson regression models include log linear quasilinear and intrinsically nonlinear models the approach considered enables one to concentrate on describing the relation between the dependent variable and the predictor variables through the regression model standard statistical packages that support irls can then be used to obtain ml estimates their asymptotic covariance matrix and diagnostic measures that can be used to aid the analyst in detecting outlying responses and extreme points in the model space applications of these methods to epidemiologic follow up studies with the data organized into a life table type of format are discussed the method is illustrated by using a nonlinear model derived from the multistage theory of carcinogenesis to analyze lung cancer death rates among british physicians who were regular cigarette smokers

autoregressive age period cohort models summary age period cohort analysis of vital data has received much attention recently and it is already well known that the exact linear relation of the three time factors creates a non identifiability problem previous studies have shown that the curvature terms of these factors are estimable but the linear trends are not however little attention has been paid to the possibility that the effects due to cohort and or period might change through time stochastically rather than deterministically and hence display a stochastic trend in this paper we model the cohort effects as an ar 1 process and use lung cancer mortality data from 1966 to 1990 for males in taiwan as an example the parameters are identifiable in the proposed model and the estimates are found to be stable however the assumption made in the model should be carefully considered before using our methodology

subdivision based mixed methods for isogeometric analysis of linear and nonlinear nearly incompressible materials this paper addresses the use of isogeometric analysis to solve solid mechanics problems involving nearly incompressible materials the present work is focused on extension of two field mixed variational formulations in both small and large strains to isogeometric analysis inf sup stable displacement pressure combinations for mixed formulations are developed based on the subdivision property of nurbs stability and convergence properties of the proposed displacement pressure combinations are illustrated by computing numerical inf sup constants and error norms the performance of the proposed formulations is assessed by studying several benchmark examples involving nearly incompressible and incompressible elastic and elasto plastic materials in both small and large strain regime

coping with extra poisson variability in the analysis of factors influencing vaginal ring expulsions the vaginal ring is one method of contraception currently under investigation and development by the human reproduction programme of who one factor that reduces its acceptability is that some women experience involuntary expulsions of the ring who have collected prospective data on 1005 women from 19 centres worldwide to investigate various aspects of ring use including factors that might affect expulsions for each subject the data include number of expulsions age parity ponderal index number of days of ring use and health care centre to model the factors influencing expulsion the number of expulsions was first considered as a poisson variate with mean directly proportional to the log of the duration of ring use such a model gave a poor description of the data and it appeared that the number of expulsions was considerably over dispersed for an assumed poisson variate this paper examines a variety of strategies for coping with the resulting lack of fit of the model it turns out that a truncated poisson model gives a better description of the expulsion data and that health care centre and parity influence the rates

variational and projection methods for the volume constraint in finite deformation elasto plasticity this paper focuses on the treatment of volume constraints which in the context of elasto plasticity typically arise as a result of assuming volume preserving plastic flow projection methods based on the modification of the discrete gradient operator b often proposed on an ad hoc basis are systematically obtained in the variational context furnished by a three field hu washizu principle the fully nonlinear formulation proposed here is based on a local multiplicative split of the deformation gradient into volume preserving and dilatational parts without relying on rate forms of the weak form of momentum balance this approach fits naturally in a formulation of plasticity based on the multiplicative decomposition of the deformation gradient and enables one to exactly enforce the condition of volume preserving plastic flow within the framework proposed in this paper rate forms and incrementally objective algorithms are entirely bypassed

isogeometric analysis of nearly incompressible solids this paper describes the use of isogeometric methods to solve problems in finite deformation solid mechanics in which compressible and nearly incompressible behavior may be encountered the work is based on a three field variational structures in which displacements mean stress and volume effects are independently approximated using this approach and tensor product nurbs approximations it is shown how the solution can be conducted in a standard approach where main variables are organized at nodes control points this leads to an efficient procedure to solve large problems of a general form without the need to develop special solution strategies the approach is demonstrated on some standard test problems copyright 2010 john wiley sons ltd

lattice filters for rls estimation of a delta operator based model a lattice filter algorithm is derived for the recursive least squares rls estimation of the parameters of the input output model of a plant based on the delta operator formulation the lattice derivation presented allows a numerically efficient implementation particularly for large orders the order recursive property of the lattices is ideally suited for order determination techniques and applications where the order of the plant may change during operation an example concerning parameter identification for a fourth order plant with measurement noise is given

empirical comparisons of proportional hazards and logistic regression models we compare parameter estimates from the proportional hazards model the cumulative logistic model and a new modified logistic model referred to as the person time logistic model with the use of simulated data sets and with the following quantities varied disease incidence risk factor strength length of follow up the proportion censored non proportional hazards and sample size parameter estimates from the person time logistic regression model closely approximated those from the cox model when the survival time distribution was close to exponential but could differ substantially in other situations we found parameter estimates from the cumulative logistic model similar to those from the cox and person time logistic models when the disease was rare the risk factor moderate and censoring rates similar across the covariates we also compare the models with analysis of a real data set that involves the relationship of age race sex blood pressure and smoking to subsequent mortality in this example the length of follow up among survivors varied from 5 to 14 years and the cox and person time logistic approaches gave nearly identical results the cumulative logistic results had somewhat larger p values but were substantively similar for all but one coefficient the age race interaction the latter difference reflects differential censoring rates by age race and sex

geometrically non linear enhanced strain mixed methods and the method of incompatible modes a class of assumed strain mixed finite element methods for fully non linear problems in solid mechanics is presented which when restricted to geometrically linear problems encompasses the classical method of incompatible modes as a particular case the method relies crucially on a local multiplicative decomposition of the deformation gradient into a conforming and an enhanced part formulated in the context of a three field variational formulation the resulting class of mixed methods provides a possible extension to the non linear regime of well known incompatible mode formulations in addition this class of methods includes non linear generalizations of recently proposed enhanced strain interpolations for axisymmetric problems which cannot be interpreted as incompatible modes elements the good performance of the proposed methodology is illustrated in a number of simulations including 2 d 3 d and axisymmetric finite deformation problems in elasticity and elastoplasticity remarkably these methods appear to be specially well suited for problems involving localization of the deformation as illustrated in several numerical examples

delta operator formulated discrete time approximations of continuous time systems given a continuous time system a technique to directly obtain an approximate delta operator formulated discrete time system spl delta system is presented for this purpose the analog of the well known boxer thaler integrators q forms applicable to shift operator formulated discrete time systems q systems are derived for spl delta systems next using these spl delta forms a method to obtain an approximate spl delta system of a given continuous time system is derived this algorithm is easily implementable in a computer with little computational burden it is shown that as sampling time decreases the spl delta system thus obtained yields the given continuous time system further verifying the close equivalence between this formulation and continuous time systems two examples illustrating advantages that may be gained by utilizing these spl delta forms in digitizing analog systems are also included

approximate case influence for the proportional hazards regression model with censored data a method is presented for approximating the influence of individual cases upon regression coefficient estimates obtained from the cox proportional hazards model observations can thus be identified which may greatly influence statistical inferences regarding the effects of prognostic factors upon survival time an example from a cancer clinical trial is given

isogeometric analysis of nearly incompressible large strain plasticity abstract we study the behavior of nurbs based isogeometric analysis on problems of large deformation plasticity we evaluate the performance of standard nurbs elements and elements based on the f formulation of elguedj et al 2008 1 our experience shows that standard measures of evaluation employed in the literature namely displacements at selected locations and graphs of reaction forces versus displacements are often misleading metrics in problems of plasticity on the other hand stress distributions in the form of contour plots are in our experience revealing measure of element performance we also determine that the concept of mesh locking which has dominated investigations of low order elements is not a relevant issue for higher order nurbs elements for problems of large deformation plasticity however standard higher order nurbs elements of type qk of continuity class c k 1 k 2 typically exhibit spurious stress oscillations whereas the f elements of type qk qk 1 produce good results in all cases

design of simple low order finite elements for large strain analysis of nearly incompressible solids a simple four node quadrilateral and an eight node hexahedron for large strain analysis of nearly incompressible solids are proposed based on the concept of deviatoric volumetric split and the replacement of the compatible deformation gradient with an assumed modified counterpart the formulation developed is applicable to arbitrary material models the closed form of the corresponding exact tangent stiffnesses which have a particularly simple structure is derived it ensures asymptotically quadratic rates of convergence of the newton raphson scheme employed in the solution of the implicit finite element equilibrium equations from a practical point of view the incorporation of the proposed elements into existing codes is straightforward it requires only small changes in the routines of the standard displacement based 4 node quadrilateral and 8 node brick a comprehensive set of numerical examples involving hyperelasticity as well as multiplicative elasto plasticity is provided it illustrates the performance of the proposed elements over a wide range of applications including strain localisation problems metal forming simulation and adaptive analysis

consistent tangent operators for rate independent elastoplasticity abstract it is shown that for problems involving rate constitutive equations such as rate independent elastoplasticity the notion of consistency between the tangent stiffness operator and the integration algorithm employed in the solution of the incremental problem plays a crucial role in preserving the quadratic rate of asymptotic convergence of iterative solution schemes based upon newton s method within the framework of closest point projection algorithms a methodology is presented whereby tangent operators consistent with this class of algorithms may be systematically developed to wit associative j 2 flow rules with general nonlinear kinematic and isotropic hardening rules as well as a class of non associative flow rules are considered the resulting iterative solution scheme preserves the asymptotic quadratic convergence characteristic of newton s method whereas use of the socalled elastoplastic tangent in conjunction with a radial return integration algorithm a procedure often employed results in newton type of algorithms with suboptimal rate of convergence application is made to a set of numerical examples which include saturation hardening laws of exponential type

f bar based linear triangles and tetrahedra for finite strain analysis of nearly incompressible solids part i formulation and benchmarking this paper proposes a new technique which allows the use of simplex finite elements linear triangles in 2d and linear tetrahedra in 3d in the large strain analysis of nearly incompressible solids the new technique extends the f bar method proposed by de souza neto et al int j solids and struct 1996 33 3277 3296 and is conceptually very simple it relies on the enforcement of near incompressibility over a patch of simplex elements rather than the point wise enforcement of conventional displacement based finite elements within the framework of the f bar method this is achieved by assuming for each element of a mesh a modified f bar deformation gradient whose volumetric component is defined as the volume change ratio of a pre defined patch of elements the resulting constraint relaxation effectively overcomes volumetric locking and allows the successful use of simplex elements under finite strain near incompressibility as the original f bar procedure the present methodology preserves the displacement based structure of the finite element equations as well as the strain driven format of standard algorithms for numerical integration of path dependent constitutive equations and can be used regardless of the constitutive model adopted the new elements are implemented within an implicit quasi static environment in this context a closed form expression for the exact tangent stiffness of the new elements is derived this allows the use of the full newton raphson scheme for equilibrium iterations the performance of the proposed elements is assessed by means of a comprehensive set of benchmarking two and three dimensional numerical examples

maximum likelihood estimation for cytogenetic dose response curves in vitro dose response curves are used to describe the relation between chromosome aberrations and radiation dose for human lymphocytes the lymphocytes are exposed to low let radiation and the resulting dicentric chromosome aberrations follow the poisson distribution the expected yield depends on both the magnitude and the temporal distribution of the dose a general dose response model that describes this relation has been presented by kellerer and rossi 1972 current topics on radiation research quarterly 8 85 158 1978 radiation research 75 471 488 using the theory of dual radiation action two special cases of practical interest are split dose and continuous exposure experiments and the resulting dose time response models are intrinsically nonlinear in the parameters a general purpose maximum likelihood estimation procedure is described and estimation for the nonlinear models is illustrated with numerical examples from both experimental designs poisson regression analysis is used for estimation hypothesis testing and regression diagnostics results are discussed in the context of exposure assessment procedures for both acute and chronic human radiation exposure

robustness of isogeometric structural discretizations under severe mesh distortion abstract this paper investigates higher order and higher continuity functions in isogeometric structural analysis under distortion of the control and physical meshes first the concepts behind isogeometric analysis are briefly reviewed the effect of perturbing control points upon the geometrical mapping is observed in the one dimensional setting the concept is extended to multiple dimensions with a series of cases where the interior of a cube is distorted in different ways and patch tests are performed to assess accuracy a circular fillet patch is introduced through the appropriate degeneration of a control mesh in order to address the problem of modeling small features in finite element models the fillet patch is tested with two problems of linear elasticity and is shown to be accurate the investigation is further extended to the setting of large deformation nearly incompressible nonlinear elasticity with two examples which make use of the recently proposed f projection method

improved finite word length characteristics in digital control using delta operators this paper examines some of the consequences of finite word lengths in digital control it is shown that in many cases of practical importance the usual shift operator formulation is inferior to an alternative formulation which we designate the delta operator approach this latter approach is shown to give better coefficient representation and less roundoff noise in many cases we thus argue that the shift operator and its associated z transform can be replaced by delta operators and their associated transform which we designate a transform an added advantage of this approach is that discrete designs and transforms converge to their continuous time counterparts as the sampling rate is increased

the finite cell method for nearly incompressible finite strain plasticity problems with complex geometries abstract in this paper the performance of the finite cell method is studied for nearly incompressible finite strain plasticity problems the finite cell method is a combination of the fictitious domain approach with the high order finite element method it provides easy mesh generation capabilities for highly complex geometries moreover this method offers high convergence rates the possibility to overcome locking and robustness against high mesh distortions the performance of this method is numerically investigated based on computations of benchmark and applied problems the results are also verified with the h and p version finite element method it is demonstrated that the finite cell method is an appropriate simulation tool for large plastic deformations of structures with complex geometries and microstructured materials such as porous and cellular metals that are made up of ductile materials obeying nearly incompressible j 2 theory of plasticity

delta levinson and schur type rls algorithms for adaptive signal processing in this paper we develop delta operator based levinson and schur type on line rls algorithms such algorithms have the potential of improved numerical behavior for ill conditioned input data these new algorithms are obtained by a unified transformation on the existing q operator based ones we first show that the conventional lattice structure can be naturally derived when the backward delta operator is used with this operator levinson and schur algorithms for the stationary stochastic model in q domain can easily be transformed into the delta domain then same transformation will be applied to the q domain on line levinson and schur type rls algorithms to obtain the delta domain counterparts their normalized versions as well as a systolic array architecture implementing the new delta schur rls algorithm are proposed extension to the equal length multichannel case is also given computer simulations show the expected numerical advantages of the delta based algorithms for fast sampled data in real time over the q domain ones under finite precision implementation

a delta least squares lattice algorithm for fast sampling most shift operator based adaptive algorithms exhibit poor numerical behavior when the input discrete time process is obtained from a continuous time process by fast sampling this includes the shift operator based least squares lattice algorithm we develop a delta least squares lattice algorithm this algorithm has a low computational complexity compared with the delta levinson rls algorithm and shows better numerical properties compared with the shift least squares lattice algorithm under fast sampling computer simulations show that the new algorithm also outperforms an existing delta least squares lattice algorithm

two dimensional delta domain lattice filter and its application to adaptive image enhancement in this correspondence a new 2 d lattice filter structure in the so called delta domain which avoids possible numerical instabilities that may occur in traditional q domain based algorithms has been developed this structure has been used to implement an adaptive lattice noise canceller very successful results have been obtained even under very low signal to noise conditions

a delta mywe algorithm for parameter estimation of noisy ar processes we develop a delta operator based modified yule walker equation algorithm mywe for parameter estimation of a noisy autoregressive ar process the methodology in developing this new algorithm is similar to the previous works on pure ar processes computer simulation results are given to show the improvement of performance in estimating ar parameters in white noise over the q operator mywe algorithm

detection of influential observation in linear regression a new measure based on confidence ellipsoids is developed for judging the contribution of each data point to the determination of the least squares estimate of the parameter vector in full rank linear regression models it is shown that the measure combines information from the studentized residuals and the variances of the residuals and predicted values two examples are presented

testing for overdispersion in poisson and binomial regression models abstract in this article a method for obtaining tests for overdispersion with respect to a natural exponential family is derived the tests are designed to be powerful against arbitrary alternative mixture models where only the first two moments of the mixed distribution are specified various tests for extra poisson and extra binomial variation are obtained as special cases the use of a particular test may be motivated by a consideration of the mechanism through which the overdispersion may arise the common occurrence of extra poisson and extra binomial variation has been noted by several authors however the poisson and binomial models remain valid in many instances and because of their simplicity and appeal it is of real interest to ascertain when they apply this paper develops a unifying theory for testing for overdispersion and generalizes tests previously derived including those by fisher 1950 collings and margolin 1985 and prentice 1986 it also shows the pearson statistic to be a sc

mixed stress displacement isogeometric collocation for nearly incompressible elasticity and elastoplasticity abstract we propose a mixed stress displacement isogeometric collocation method for nearly incompressible elastic materials and for materials exhibiting von mises plasticity the discretization is based on isogeometric analysis iga with non uniform rational b splines nurbs as basis functions as compared to conventional iga galerkin formulations isogeometric collocation methods offer a high potential of computational cost reduction for higher order discretizations as they eliminate the need for quadrature in the proposed mixed formulation both stress and displacement fields are approximated as primary variables with the aim of treating volumetric locking and instability issues which occur in displacement based isogeometric collocation for nearly incompressible elasticity and von mises plasticity the performance of the proposed approach is demonstrated by several numerical examples

on the orthogonal subgrid scale pressure stabilization of finite deformation j2 plasticity the use of stabilization methods is becoming an increasingly well accepted technique due to their success in dealing with numerous numerical pathologies that arise in a variety of applications in computational mechanics in this paper a multiscale finite element method technique to deal with pressure stabilization of nearly incompressibility problems in nonlinear solid mechanics at finite deformations is presented a j2 flow theory plasticity model at finite deformations is considered a mixed formulation involving pressure and displacement fields is used as starting point within the finite element discretization setting continuous linear interpolation for both fields is considered to overcome the babuska brezzi stability condition a multiscale stabilization method based on the orthogonal subgrid scale osgs technique is introduced a suitable nonlinear expression of the stabilization parameter is proposed the main advantage of the method is the possibility of using linear triangular or tetrahedral finite elements which are easy to generate and therefore very convenient for practical industrial applications numerical results obtained using the osgs stabilization technique are compared with results provided by the p1 standard galerkin displacements linear triangular tetrahedral element p1 p1 standard mixed linear displacements linear pressure triangular tetrahedral element and q1 p0 mixed bilinear trilinear displacements constant pressure quadrilateral hexahedral element for 2d 3d nearly incompressible problems in the context of a nonlinear finite deformation j2 plasticity model

isogeometric finite element data structures based on bezier extraction of t splines we develop finite element data structures for t splines based on bezier extraction generalizing our previous work for nurbs as in traditional finite element analysis the extracted bezier elements are defined in terms of a fixed set of polynomial basis functions the so called bernstein basis the bezier elements may be processed in the same way as in a standard finite element computer program utilizing exactly the same data processing arrays in fact only the shape function subroutine needs to be modified while all other aspects of a finite element program remain the same a byproduct of the extraction process is the element extraction operator this operator localizes the topological and global smoothness information to the element level and represents a canonical treatment of t junctions referred to as hanging nodes in finite element analysis and a fundamental feature of t splines a detailed example is presented to illustrate the ideas copyright 2011 john wiley sons ltd

a hybrid high order method for finite elastoplastic deformations within a logarithmic strain framework we devise and evaluate numerically a hybrid high order hho method for finite plasticity within a logarithmic strain framework the hho method uses as discrete unknowns piecewise polynomials of order k 1 on the mesh skeleton together with cell based polynomi als that can be eliminated locally by static condensation the hho method leads to a primal formulation supports polyhedral meshes with non matching interfaces is free of volumetric locking the integration of the behavior law is performed only at cell based quadrature nodes and the tangent matrix in newton s method is symmetric moreover the principle of virtual work is satisfied locally with equilibrated tractions various two and three dimensional benchmarks are presented as well as comparison against known solutions with an industrial software using conforming and mixed finite elements

on properties of information matrices of delta operator based adaptive signal processing algorithms in this paper we study the properties of information matrices of the difference operator or the so called delta spl delta operator based algorithms for adaptive signal processing we show that the conditioning of a transformed information matrix in the spl delta domain is always better than that of the original information matrix in the conventional q domain for ill conditioned problems the results obtained in this paper give an explanation of the advantages of using delta operator algorithms for adaptive signal processing that have been developed the analysis in this paper also helps to clarify the problem about the effect of spl delta in the delta operator algorithms and justify the use of sampling interval as spl delta in most cases when the ill conditioning is caused by fast sampling of continuous time systems

the natural variability of vital rates and associated statistics the first concern of this work is the development of approximations to the distributions of crude mortality rates age specific mortality rates age standardized rates standardized mortality ratios and the like for the case of a closed population or period study it is found that assuming poisson birthtimes and independent lifetimes implies that the number of deaths and the corresponding midyear population have a bivariate poisson distribution it is suggested that situations in which explanatory variables are present may be modelled via a doubly stochastic poisson distribution for the number of deaths with mean proportional to the population size and an exponential function of a linear combination of the explanatories such a model is fit to mortality data for canadian females classified by age and year a dynamic variant of the model is further fit to the time series of total female deaths alone by year the models with extra poisson variation are found to lead to substantially improved fits excerpt

an isogeometric method for linear nearly incompressible elasticity with local stress projection abstract in this paper we propose an isogeometric method for solving the linear nearly incompressible elasticity problem the method is similar to the b formulation where the volumetric strain is projected on a lower degree spline space in order to prevent volumetric locking in our method we adopt a local projection on a coarser mesh chosen in order to guarantee optimal convergence moreover the locality of the projector allows to maintain the sparsity of the stiffness matrix that is the efficiency of the method the analysis of the method is based on the inf sup stability of the associated mixed formulation via a macro element technique for spline functions the numerical tests confirm the theory of the method

convergence of an efficient local least squares fitting method for bases with compact support abstract the least squares projection procedure appears frequently in mathematics science and engineering it possesses the well known property that a least squares approximation formed via orthogonal projection to a given data set provides an optimal fit in the chosen norm the orthogonal projection of the data onto a finite basis is typically approached by the inversion of a gram matrix involving the inner products of the basis functions even if the basis functions have compact support so that the gram matrix is sparse its inverse will be dense thus computing the orthogonal projection is expensive an efficient local least squares algorithm for non orthogonal projection onto smooth piecewise polynomial basis functions is analyzed the algorithm runs in optimal time and delivers the same order of accuracy as the standard orthogonal projection numerical results indicate that in many computational situations the new algorithm offers an effective alternative to global least squares approximation

dispersion optimized quadrature rules for isogeometric analysis modified inner products their dispersion properties and optimally blended schemes abstract this paper introduces optimally blended quadrature rules for isogeometric analysis and analyzes the numerical dispersion of the resulting discretizations to quantify the approximation errors when we modify the inner products we generalize the pythagorean eigenvalue theorem of strang and fix the proposed blended quadrature rules have advantages over alternative integration rules for isogeometric analysis on uniform and non uniform meshes as well as for different polynomial orders and continuity of the basis the optimally blended schemes improve the convergence rate of the method by two orders with respect to the fully integrated galerkin method the proposed technique increases the accuracy and robustness of isogeometric analysis for wave propagation problems

goodness of fit in armitage doll poisson models summary we consider testing the adequacy of multistage models of cancer induction for poisson event rates depending on age and exposure to environmental agents the space of time and covariates is partitioned for comparison of observed and fitted event counts over regions of the partition a test of fit based on the quadratic score statistic is proposed to assess overall residual error between observed and fitted counts the limiting distribution of the test statistic is central chi squared under the null hypothesis and noncentral chi squared under local alternatives standardized residuals and tests of trend for individual regions of the partition are given the method is applied to lung cancer incidence rates among british physicians grouped by age by amount of smoking and by decile of estimated risk the relation of the proposed method to chi squared frequency tests and extensions to casecontrol studies are considered

consistent estimation of the order of stochastic control systems a consistent estimate of the order of feedback control systems with unknown matrix coefficients estimated by the least squares method is derived by minimizing a modified version of the bayesian information criterion

circular implementation of vector channel lattices discusses the circular or periodic implementation of vector channel least squares lattice filters the resulting algorithm eliminates matrix inversions and greatly reduces matrix multiplications complete residual and ar coefficient algorithms are provided as well as initialization and other implementation issues a full treatment of the prediction error form of the lattices and its relationship to the residual form is also included

isogeometric collocation for three dimensional geometrically exact shear deformable beams abstract we extend the isogeometric collocation method to the geometrically nonlinear beams an exact kinematic formulation able to represent three dimensional displacements and rotations without any restriction in magnitude is presented without the introduction of the moving frame concept a displacement based formulation is adopted full linearization of the strong form of the governing equations is derived consistently with the underlying geometric structure of the configuration manifold incremental rotations are parametrized through eulerian rotation vectors and configuration updates are performed by means of the exponential map numerical tests demonstrate that the proposed combination of isogeometric collocation method with the chosen rotations parametrization results in an efficient computational scheme able to model complex problems with high accuracy

on delta operator schur cohn zero location tests for fast sampling a stability test or more generally a zero location test algorithm similar to the well known schur cohn test is proposed when a complex polynomial formulated in the delta operator is given the real polynomial case falls out naturally as a special case the method used is a simple linear transformation based on the schur cohn test and the proposed algorithm is computationally efficient furthermore it is shown that as the sampling interval vanishes the new algorithm converges to a new continuous time test therefore providing a smooth transition from the shift operator based discrete time algorithm to the continuous time one furthermore a sensitivity analysis shows that the sensitivity of the new test in the delta operator remains finite with respect to parameter perturbation and converges to that of the continuous time one whereas that of the shift operator grows without bound as the sampling interval vanishes examples are given to show the clear numerical advantages over the traditional shift operator based schur cohn test in the case of fast sampling

residual stresses in metal deposition modeling discretizations of higher order abstract this article addresses the research question if and how the finite cell method an embedded domain finite element method of high order may be used in the simulation of metal deposition to harvest its computational efficiency this application demands for the solution of a coupled thermo elasto plastic problem on transient meshes within which history variables need to be managed dynamically on non boundary conforming discretizations to this end we propose to combine the multi level h p method and the finite cell method the former was specifically designed to treat high order finite element discretizations on transient meshes while the latter offers a remedy to retain high order convergence rates also in cases where the physical boundary does not coincide with the boundary of the discretization we investigate the performance of the method at two analytical and one experimental benchmark

improved likelihood inference for discrete data reference stat article 2006 001doi 10 1111 j 1467 9868 2006 00548 xview record in web of science record created on 2006 04 21 modified on 2017 05 12

efficient quadrature for nurbs based isogeometric analysis abstract we initiate the study of efficient quadrature rules for nurbs based isogeometric analysis a rule of thumb emerges the half point rule indicating that optimal rules involve a number of points roughly equal to half the number of degrees of freedom or equivalently half the number of basis functions of the space under consideration the half point rule is independent of the polynomial order of the basis efficient rules require taking into account the precise smoothness of basis functions across element boundaries several rules of practical interest are obtained and a numerical procedure for determining efficient rules is presented we compare the cost of quadrature for typical situations arising in structural mechanics and fluid dynamics the new rules represent improvements over those used previously in isogeometric analysis

effects of exposure misclassification on regression analyses of epidemiologic follow up study data in epidemiologic studies subjects are often misclassified as to their level of exposure ignoring this misclassification error in the analysis introduces bias in the estimates of certain parameters and invalidates many hypothesis tests for situations in which there is misclassification of exposure in a follow up study with categorical data we have developed a model that permits consideration of any number of exposure categories and any number of multiple category covariates when used with logistic and poisson regression procedures this model helps assess the potential for bias when misclassification is ignored when reliable ancillary information is available the model can be used to correct for misclassification bias in the estimates produced by these regression procedures

logistic regression diagnostics a maximum likelihood fit of a logistic regression model and other similar models is extremely sensitive to outlying responses and extreme points in the design space we develop diagnostic measures to aid the analyst in detecting such observations and in quantifying their effect on various aspects of the maximum likelihood fit the elements of the fitting process which constitute the usual output parameter estimates standard errors residuals etc will be used for this purpose with a properly designed computing package for fitting the usual maximum likelihood model the diagnostics are essentially free for the asking in particular good data analysis for logistic regression models need not be expensive or time consuming

mixed poisson likelihood regression models for longitudinal interval count data summary in many longitudinal studies it is desired to estimate and test the rate over time of a particular recurrent event often only the event counts corresponding to the elapsed time intervals between each subject s successive observation times and baseline covariate data are available the intervals may vary substantially in length and number between subjects so that the corresponding vectors of counts are not directly comparable a family of poisson likelihood regression models incorporating a mixed random multiplicative component in the rate function of each subject is proposed for this longitudinal data structure a related empirical bayes estimate of random effect parameters is also described these methods are illustrated by an analysis of dyspepsia data from the national cooperative gallstone study

correspondence analysis applied to grouped cohort data in this paper we investigate application of generalized correspondence analysis gca to model a two way table of rates consisting of a cross classification of case numerators and person time denominators the gca reconstitution formula is proposed as a correlation model for analysing these data this provides two major advantages over the techniques usually applied the number of interaction parameters to be estimated is reduced and moreover they have useful interpretation the singular values and the scores are measures of row column modification interaction effects in addition the scores allow graphical display of these effects an asymmetrical plot is proposed for optimal representation of interaction parameters as distances finally two examples are presented to illustrate the method of analysing a two way table of covariate by exposure and a two way table derived from a cross classification of two exposure factors

tests for detecting overdispersion in poisson regression models abstract poisson regression models are widely used in analyzing count data this article develops tests for detecting extra poisson variation in such situations the tests can be obtained as score tests against arbitrary mixed poisson alternatives and are generalizations of tests of fisher 1950 and collings and margolin 1985 accurate approximations for computing significance levels are given and the power of the tests against negative binomial alternatives is compared with those of the pearson and deviance statistics one way to test for extra poisson variation is to fit models that parametrically incorporate and then test for the absence of such variation within the models for example negative binomial models can be used in this way cameron and trivedi 1986 lawless 1987a the tests in this article require only the poisson model to be fitted two test statistics are developed that are motivated partly by a desire to have good distributional approximations for computing significance levels simu

b and f projection methods for nearly incompressible linear and non linear elasticity and plasticity using higher order nurbs elements abstract this paper presents projection methods to treat the incompressibility constraint in small and large deformation elasticity and plasticity within the framework of isogeometric analysis after reviewing some fundamentals of isogeometric analysis we investigate the use of higher order non uniform rational b splines nurbs within the b projection method the higher continuity property of such functions is explored in nearly incompressible applications and shown to produce accurate and robust results a new non linear f projection method based on a modified minimum potential energy principle and inspired by the b method is proposed for the large deformation case it leads to a symmetric formulation for which the consistent linearized operator for fully non linear elasticity is derived and used in a newton raphson iterative procedure the performance of the methods is assessed on several numerical examples and results obtained are shown to compare favorably with other published techniques

an isogeometric analysis approach to gradient dependent plasticity summary gradient dependent plasticity can be used to achieve mesh objective results upon loss of well posedness of the initial boundary value problem due to the introduction of strain softening non associated flow and geometric non linearity a prominent class of gradient plasticity models considers a dependence of the yield strength on the laplacian of the hardening parameter usually an invariant of the plastic strain tensor this inclusion causes the consistency condition to become a partial differential equation in addition to the momentum balance at the internal moving boundary one has to impose appropriate boundary conditions on the hardening parameter or equivalently on the plastic multiplier this internal boundary condition can be enforced without tracking the elastic plastic boundary by requiring c1 continuity with respect to the plastic multiplier in this contribution this continuity has been achieved by using nurbs as shape functions both for the plastic multiplier and for the displacements one advantage of this isogeometric analysis approach is that the displacements can be interpolated one order higher making it consistent with the interpolation of the plastic multiplier this is different from previous approaches which have been exploited the regularising effect of gradient plasticity is shown for one and two dimensional boundary value problems this article is protected by copyright all rights reserved

vector channel lattice filters and identification of flexible structures a novel derivative of a least squares lattice filter is presented and applied to the identification of flexible structures the vector channel lattice given derived in an infinite dimensional history space without matrix manipulations or geometric arguments can constrain the autoregressive ar coefficients for several outputs to be the same numerical results for a simulated flexible structure compare the vector channel lattice to the standard lattice filter these results show that the frequencies and damping ratios for the most significant modes can be identified adaptively with lattice filters

estimation of continuous time ar process parameters from discrete time data the problem of estimating continuous time autoregressive process parameters from discrete time data is considered the basic approach used here is based on replacing the derivatives in the model by discrete time differences forming a linear regression and using the least squares method such a procedure is simple to apply computationally flexible and efficient and may have good numerical properties it is known however that all standard approximations of the highest order derivative such as repeated use of the delta operator gives a biased least squares estimate even as the sampling interval tends to zero some of our previous approaches to overcome this problem are reviewed then two new methods which avoid the shift in our previous results are presented one of them which is termed bias compensation is computationally very efficient finally the relationship of the above least squares approaches with an instrumental variable method is investigated comparative simulation results are also presented

a framework for finite strain elastoplasticity based on maximum plastic dissipation and the multiplicative decomposition part ii computational aspects computational aspects of the formulation of elastoplasticity proposed in part i of this work employing a multiplicative decomposition and based on the principle of maximum plastic dissipation are examined in detail in particular the implementation of the proposed hyperelastic j2 flow theory is shown to reduce to a trivial modification of the classical radial return algorithm which is amenable to exact linearization this results in a closed form expression for the consistent elastoplastic tangent moduli the algorithmic treatment of the general case is considered subsequently it is emphasized that objective rates and incrementally objective algorithms play no role in the present approach a number of numerical experiments are presented that illustrate the excellent performance of the proposed formulation