Moment instabilities in multidimensional systems with noise

We present a systematic study of moment evolution in multidimensional stochastic difference systems, focusing on characterizing systems whose low-order moments diverge in the neighborhood of a stable fixed point. We consider systems with a simple, dominant eigenvalue and stationary, white noise. When the noise is small, we obtain general expressions for the approximate asymptotic distribution and moment Lyapunov exponents. In the case of larger noise, the second moment is calculated using a different approach, which gives an exact result for some types of noise. We analyze the dependence of the moments on the system's dimension, relevant system properties, the form of the noise, and the magnitude of the noise. We determine a critical value for noise strength, as a function of the unperturbed system's convergence rate, above which the second moment diverges and large fluctuations are likely. Analytical results are validated by numerical simulations. We show that our results cannot be extended to the continuous time limit except in certain special cases.Comment: 21 pages, 15 figure

Separability as a modeling paradigm in large probabilistic models

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 185-191).Many interesting stochastic models can be formulated as finite-state vector Markov processes, with a state characterized by the values of a collection of random variables. In general, such models suffer from the curse of dimensionality: the size of the state space grows exponentially with the number of underlying random variables, thereby precluding conventional modeling and analysis. A potential cure to this curse is to work with models that allow the propagation of partial information, e.g. marginal distributions, expectations, higher-moments, or cross-correlations, as derived from the joint distribution for the network state. This thesis develops and rigorously investigates the notion of separability, associated with structure in probabilistic models that permits exact propagation of partial information. We show that when partial information can be propagated exactly, it can be done so linearly. The matrices for propagating such partial information share many valuable spectral relationships with the underlying transition matrix of the Markov chain. Separability can be understood from the perspective of subspace invariance in linear systems, though it relates to invariance in a non-standard way. We analyze the asymptotic generality-- as the number of random variables becomes large-of some special cases of separability that permit the propagation of marginal distributions. Within this discussion of separability, we introduce the generalized influence model, which incorporates as special cases two prominent models permitting the propagation of marginal distributions: the influence model and Markov chains on permutations (the symmetric group). The thesis proposes a potentially tractable solution to learning informative model parameters, and illustrates many advantageous properties of the estimator under the assumption of separability. Lastly, we illustrate separability in the general setting without any notion of time-homogeneity, and discuss potential benefits for inference in special cases.by William J. Richoux.Ph.D

Moment-linear stochastic systems and their applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 263-271).Our work is motivated by the need for tractable stochastic models for complex network and system dynamics. With this motivation in mind, we develop a class of discrete-time Markov models, called moment-linear stochastic systems (MLSS), which are structured so that moments and cross-moments of the state variables can be computed efficiently, using linear recursions. We show that MLSS provide a common framework for representing and characterizing several models that are common in the literature, such as jump-linear systems, Markov-modulated Poisson processes, and infinite server queues. We also consider MLSS models for network interactions, and hence introduce moment-linear stochastic network (MLSN) models. Several potential applications for MLSN-in such areas as traffic flow modeling, queueing, and stochastic automata modeling-are explored. Fur- ther, we exploit the quasi-linear structure of MLSS and MLSN to analyze their asymptotic dynamics, and to construct linear minimum mean-square-error estimators and minimum quadratic cost controllers. Finally, we study in detail two examples of MLSN, a stochastic automaton called the influence model and an aggregate model for air traffic flows.by Sandip Roy.Ph.D