the connection matrix theory for morse decompositions the connection matrix theory for morse decompositions is introduced the connection matrices are matrices of maps between the homology indices of the sets in the morse decomposition the connection matrices cover in a natural way the homology index braid of the morse decomposition and provide information about the structure of the morse decomposition the existence of connection matrices of morse decompositions is established and examples illustrating applications of the connection matrix are provided

differential algebraic systems as differential equations on manifolds based on the theory of differential equations on manifolds existence and uniqueness results are proved for a class of mixed systems of differential and algebraic equations as they occur in various applications both the autonomous and nonautonomous case are considered moreover a class of algebraically incomplete systems is introduced for which existence and uniqueness results only hold on certain lower dimensional manifolds this class includes systems for which the application of ode solvers is known to lead to difficulties finally some solution approach based on continuation techniques is outlined

approximate counting uniform generation and rapidly mixing markov chains the paper studies effective approximate solutions to combinatorial counting and uniform generation problems using a technique based on the simulation of ergodic markov chains it is shown that for self reducible structures almost uniform generation is possible in polynomial time provided only that randomised approximate counting to within some arbitrary polynomial factor is possible in polynomial time it follows that for self reducible structures polynomial time randomised algorithms for counting to within factors of the form 1 n are available either for all fl e r or for no fi e r a substantial part of the paper is devoted to investigating the rate of convergence of finite ergodic markov chains and a simple but powerful characterisation of rapid convergence for a broad class of chains based on a structural property of the underlying graph is established finally the general techniques of the paper are used to derive an almost uniform generation procedure for labelled graphs with a given degree sequence which is valid over a much wider range of degrees than previous methods this in turn leads to randomised approximate counting algorithms for these graphs with very good

punch stretching of sheet metal and differential equations on manifolds the goal of mathematical modelling of sheet metal forming processes is to provide predictive tools for use in the design of stamping processes and the selection of sheet materials most current approaches to finite element modelling of large deformation elastic plastic sheet metal forming problems use a rate form of the virtual work equilibrium equations and a single field finite element representation of the displacement components called the incremental method this approach does not produce approximations which satisfy the discrete equilibrium equations at all times and consequently it demands small time steps to insure stability and numerical accuracy this paper describes a variant of the mixed method in which displacements stresses effective strain and pressures are all given separate finite element representations the equilibrium equations in non rate form are discretized to produce a system of algebraic equations which are coupled with the constitutive equations and then integrated using state of the art numerical software when used to model rate sensitive sheet materials in hydrostatic bulging plane strain punch stretching and hemispherical punch stretching the new approach proved to be between 6 and 26 times as fast as the old incremental method

the connection map for attractor repeller pairs on montre que l application de connexion est additive les proprietes de l application connexion conduisent a une caracterisation des ensembles invariants isoles avec des points critiques hyperboliques comme une paire attracteur repulseur

approximating probabilistic inference in bayesian belief networks is np hard abstract it is known that exact computation of conditional probabilities in belief networks is np hard many investigators in the ai community have tacitly assumed that algorithms for performing approximate inference with belief networks are of polynomial complexity indeed special cases of approximate inference can be performed in time polynomial in the input size however we have discovered that the general problem of approximating conditional probabilities with belief networks like exact inference resides in the np hard complexity class we develop a complexity analysis to elucidate the difficulty of approximate probabilistic inference more specifically we show that the existence of a polynomial time relative approximation algorithm for major classes of problem instances implies that np p we present our proof and explore the implications of the result

nested sequences of index filtrations and continuation of the connection matrix abstract in this paper we prove the existence of nested sequences of index filtrations for convergent sequences of admissible semiflows on a metric space this result is new even in the context of flows on a locally compact space the nested index filtration theorem implies the continuation of homology index braids which in turn implies the continuation of connection matrices in the infinite dimensional conley index theory

the connection matrix in morse smale flows ii given a connection matrix for a morse smale flow on a compact manifold if there are no periodic orbits of equal or adjacent indices related in the partial order we show that the periodic orbits can be replaced by doubly connected rest points in such a way that the given connection matrix induces the unique connection matrix for the resulting flow it follows that for this class of flows all nonuniqueness in the connection matrix is a consequence of the continuation theorem for connection matrices

the computational complexity of probabilistic inference using bayesian belief networks research note abstract bayesian belief networks provide a natural efficient method for representing probabilistic dependencies among a set of variables for these reasons numerous researchers are exploring the use of belief networks as a knowledge representation in artificial intelligence algorithms have been developed previously for efficient probabilistic inference using special classes of belief networks more general classes of belief networks however have eluded efforts to develop efficient inference algorithms we show that probabilistic inference using belief networks is np hard therefore it seems unlikely that an exact algorithm can be developed to perform probabilistic inference efficiently over all classes of belief networks this result suggests that research should be directed away from the search for a general efficient probabilistic inference algorithm and toward the design of efficient special case average case and approximation algorithms

an optimal approximation algorithm for bayesian inference abstract approximating the inference probability pr x x e e in any sense even for a single evidence node e is np hard this result holds for belief networks that are allowed to contain extreme conditional probabilities that is conditional probabilities arbitrarily close to 0 nevertheless all previous approximation algorithms have failed to approximate efficiently many inferences even for belief networks without extreme conditional probabilities we prove that we can approximate efficiently probabilistic inference in belief networks without extreme conditional probabilities we construct a randomized approximation algorithm the bounded variance algorithm that is a variant of the known likelihood weighting algorithm the bounded variance algorithm is the first algorithm with provably fast inference approximation on all belief networks without extreme conditional probabilities from the bounded variance algorithm we construct a deterministic approximation algorithm using current advances in the theory of pseudorandom generators in contrast to the exponential worst case behavior of all previous deterministic approximations the deterministic bounded variance algorithm approximates inference probabilities in worst case time that is subexponential 2 log n d for some integer d that is a linear function of the depth of the belief network

dem a new computational approach to sheet metal forming problems many current approaches to finite element modelling of large deformation elastic plastic forming problems use a rate form of the virtual work equilibrium equations and a finite element representation of the displacement components called the incremental method this approach produces a three field formulation in which displacements stresses and effective strain are dependent variables next the formulation is converted to a one field displacement formulation by an algebraic time discretization which uses a low order explicit time stepping procedure to integrate the equations this approach does not produce approximations which satisfy the discrete equilibrium equations at all times and moreover the advantage of the single field algebraic formulation is realized at the expense of very small time steps needed to produce stability and accuracy in the numerical calculations this paper describes a variant of the mixed method in which all three field variables displacements stresses and effective strain are given finite element representations the discrete equilibrium equations then generate a nonlinear system of algebraic equations whose solutions represent a manifold while the constitutive equations form a system of ordinary differential equations a commercially available variable time step variable order code is then used to integrate this differential algebraic system when applied to the problem of hydrostatic bulging of a membrane the new approach requires far less computer time than the incremental method

existence of trajectories joining critical points for a symbiotic system via the conley index abstract in this paper we prove the existence of trajectories joining critical points for ecological systems with three species by constructing the index pair for the isolated invariant set and using the conley index theory

connected simple systems and the conley index of isolated invariant sets the object of this paper is to present new and simplified proofs for most of the basic results in the index theory for flows simple explicit formulae are derived for the maps which play a central role in the theory the presentation is self contained

homotopy invariants of repeller attractor pairs ii continuation of r a pairs on considere une famille continue a 1 parametre de flots dans le plan parametrise par 0 1 on etablit un cadre qui rend precise la notion de famille continue de paires de repulseur attracteurs on montre qu il y a des equivalences naturelles entre les suites d indices de 2 paires r a le long d un parcours de paires r a

approximating the permanent a randomised approximation scheme for the permanent of a 0 1s presented the task of estimating a permanent is reduced to that of almost uniformly generating perfect matchings in a graph the latter is accomplished by simulating a markov chain whose states are the matchings in the graph for a wide class of 0 1 matrices the approximation scheme is fully polynomial i e runs in time polynomial in the size of the matrix and a parameter that controls the accuracy of the output this class includes all dense matrices those that contain sufficiently many 1 s and almost all sparse matrices in some reasonable probabilistic model for 0 1 matrices of given density for the approach sketched above to be computationally efficient the markov chain must be rapidly mixing informally it must converge in a short time to its stationary distribution a major portion of the paper is devoted to demonstrating that the matchings chain is rapidly mixing apparently the first such result for a markov chain with genuinely c

morse type inequalities for dynamical systems and the witten laplacian we provide an analytic proof of morse type inequalities for vector flelds determining a morse decomposition with normally hyperbolic dynamics in the demonstration we reduce the problem to the gradient case using a morse bott lyapunov function and

random generation of combinatorial structures from a uniform distribution abstract the class of problems involving the random generation of combinatorial structures from a uniform distribution is considered uniform generation problems are in computational difficulty intermediate between classical existence and counting problems it is shown that exactly uniform generation of efficiently verifiable combinatorial structures is reducible to approximate counting and hence is within the third level of the polynomial hierarchy natural combinatorial problems are presented which exhibit complexity gaps between their existence and generation and between their generation and counting versions it is further shown that for self reducible problems almost uniform generation and randomized approximate counting are inter reducible and hence of similar complexity

connecting orbits for nonlinear differential equations at resonance abstract we study the existence of orbits connecting stationary points for the first order differential equations being at resonance at infinity where the right hand side is the perturbations of a sectorial operator our aim is to prove an index formula expressing the conley index of associated semiflow with respect to appropriately large ball in terms of special geometrical assumptions imposed on the nonlinearity we also prove that the geometrical assumptions are generalization of the well known in literature landesman lazer and strong resonance conditions obtained index formula will be used to derive the criteria determining the existence of orbits connecting stationary points for the heat equation being at resonance at infinity

global topological properties of the hopf bifurcation we study the homotopical and homological properties of the attractors evolving from a generalized hopf bifurcation we consider the lorenz equations for parameter values near the hopf bifurcation and study a natural morse decomposition of the global attractor calculating the cech homotopy type of the lorenz attractor the shape indexes of the morse sets and the morse equation of the decomposition

elastic viscoplastic differential equations on a manifold modelling of in plane stretching of sheet metal the computational method for large deformation plasticity called differential equations on a manifold dem has previously been shown to be effective for axisymmetric and plane strain sheet metal forming problems the method has now been formulated for in plane stretching problems incorporated into a computer code and applied to several problems the code s performance is robust and accurate as evidenced by a comparison with other published results incorporation of an automatic mesh generator significantly reduces data preparation time

index filtrations and the homology index braid for partially ordered morse decompositions abstract on a morse decomposition of an invariant set in a flow there are partial orderings defined by the flow these are called admissable orderings of the morse decomposition the index filtrations for a total ordering of a morse decomposition are generalized in this paper with the definition and proof of existence of index filtrations for admissable partial orderings of a morse decomposition it is shown that associated to an index filtration there is a collection of chain complexes and chain maps called the chain complex braid of the index filtration the homology index braid of the corresponding admissable ordering of the morse decomposition is obtained by passing to homology in the chain complex braid keywords morse decomposition conley index index filtration

approximating probabilistic inference in bayesian belief networks a belief network comprises a graphical representation of dependencies between variables of a domain and a set of conditional probabilities associated with each dependency unless rho np an efficient exact algorithm does not exist to compute probabilistic inference in belief networks stochastic simulation methods which often improve run times provide an alternative to exact inference algorithms such a stochastic simulation algorithm d bnras which is a randomized approximation scheme is presented to analyze the run time belief networks are parameterized by the dependence value d sub xi which is a measure of the cumulative strengths of the belief network dependencies given background evidence xi this parameterization defines the class of f dependence networks the run time of d bnras is polynomial when f is a polynomial function thus the results prove the existence of a class of belief networks for which inference approximation is polynomial and hence provably faster than any exact algorithm

computations of critical groups and applications to asymptotically linear wave equation and beam equation in this paper by using the morse index theory for strongly indefinite functionals developed in nonlinear anal tma in press we compute precisely the critical groups at the origin and at infinity respectively the abstract theorems are used to study the existence multiplicity of nontrivial periodical solutions for asymptotically wave equation and beam equation with resonance both at infinity and at zero

the continuation theory for morse decompositions and connection matrices the continuation theory for ordered morse decompositions and the indices defined on them the honmology index braid and the connection matrices is established the equivalence between ordered morse decompositions and consistent attractor filtrations is displayed the spaces of ordered morse decompositions for a product parametrization of a local flow are introduced and the local continuation of ordered morse decompositions is obtained via the above described equivalence and the local continuation of attractors the homology index braid and the connection matrices of an admissible ordering of a morse decomposition are shown to be invariant on path components of the corresponding space of ordered morse decompositions this invariance is used to prove that the collection of connection matrices of a morse decomposition is upper semicontinuous over the space of morse decompositions and over the parameter space under local continuation introduction a theorem that describes the behavior of an index under continuous change in its domain is an important component of any index theory the conley index is defined for isolated invariant sets in a local flow and in 1 it is shown that for a product parameterization of a local flow isolated invariant sets continue locally and the conley index is invariant under the local continuation in 10 kurland establishes a corresponding continuation theory for the index defined on attractor repeller pairs it is the aim of this paper to extend these continuation results to a continuation theory for ordered morse decompositions and the indices the homology index braid and the connection matrices defined on them conley s and kurland s continuation theories actually contain more than the local continuation results described above these local results are a part of a broader global continuation scheme in particular in 1 conley defines the space of isolated invariant sets for a product parameterization of a local flow and shows that this space is locally homeomorphic to the parameter space this local homeomorphism then yields the notion of local continuation of isolated invariant sets next it is shown that the conley index is invariant on path components of the space of isolated invariant sets this yields the invariance of the index under local continuation in this paper we use the same approach we define the spaces of ordered morse decompositions for a product parameterization of a local flow the notion of local continuation of morse decompositions and of admissible orderings of morse received by the editors september 20 1987 1980 mathematics subject classification 1985 revision primary 58f35 secondary 58f12 34c35

a new method to find homoclinic and heteroclinic orbits a new series method is provided for continuous time autonomous dynamical systems which can find exact orbits as opposed to approximate ones the method can reduce the connecting orbit problem as a boundary value problem in an infinite time domain to the initial value problem it consists of transforming time to the logarithmic scale substituting a power series around each fixed point of interest for each of the unknown functions into the system and equating the corresponding coefficients when solving for the power series coefficients additional parameters are used in order to find the intersections of the unstable manifold and the stable manifold of the equilibria this paper demonstrates how the new method allows to obtain heteroclinic and homoclinic orbits in some well known cases such as nagumo system stretch twist fold flow or mathematical pendulum

detection of renewal system factors via the conley index let n be an isolating neighborhood for a map f if we can decompose n into the disjoint union of compact sets n 1 and n 2 then we can relate the dynamics on the maximal invariant set inv n to the shift on two symbols by noting which component of n each iterate of a point x inv n lies in we examine a method based on work by mischaikow szymczak et al for using the discrete conley index to detect explicit subshifts of the shift associated to n in essence we measure the difference between the conley index of inv n and the sum of the indices of inv n 1 and inv n 2

morse type index theory for flows and periodic solutions for hamiltonian equations an index theory for flows is presented which extends the classical morse theory for gradient flows on compact manifolds the theory is used to prove a morse type existence statement for periodic solutions of a time dependent periodic in time and asymptotically linear hamiltonian equation

algebraic transition matrices in the conley index theory we introduce the concept of an algebraic transition matrix these are degree zero isomorphisms which are upper triangular with respect to a partial order it is shown that all connection matrices of a morse decomposition for which the partial order is a series parallel admissible order are related via a conjugation with one of these transition matrices this result is then restated in the form of an existence theorem for global bifurcations simple exarnples of how these results can be applied are also presented

implicit differential equations near a singular point abstract this paper presents general and precise results on existence and number of solutions to implicitly defined ordinary differential equations in the vicinity of a singular point where the equation is not equivalent to an explicit one unlike in most if not in all other studies devoted to this problem in the literature it is not assumed that either rank drop occurs on an entire neighborhood of the singular point or that the equation is a scalar one attention is confined to the simplest but most frequently encountered kind of singular points it is shown that such standard singular points split into two complementary classes those from which exactly two distinct solutions emanate and those at which exactly two distinct solutions terminate whether a point of the latter class is eventually encountered is unaffected by slight modifications of the nonsingular initial condition and further evolution of the system governed by the singular ode then requires using a suitable jump condition according to appropriate physical criteria points of the former class are also shown to play a more subtle but equally important role in the dynamics the phenomena described here are relevant in various problems from the sciences such as phase transitions or plasticity they should also be relevant in some aspects of the classical domain of application of differential algebraic equations singular perturbations of ode s

uniform persistence and hopf bifurcations in r n we consider parameterized families of flows in locally compact metrizable spaces and give a characterization of those parameterized families of flows for which uniform persistence continues on the other hand we study the generalized poincare andronov hopf bifurcations of parameterized families of flows at boundary points of r n or more generally of an n dimensional manifold and show that this kind of bifurcations produce a whole family of attractors evolving from the bifurcation point and having interesting topological properties in particular in some cases the bifurcation transforms a system with extreme non permanence properties into a uniformly persistent one we study in the paper when this phenomenon happens and provide an example constructed by combining a holling type interaction with a pitchfork bifurcation