# Guide Dynamics & Stochastics

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This is called the Quasi-White approximation. The Ornstein-Uhlenbeck process OU was proposed to model the velocity of a particle executing Brownian motion its position is then obtained by integration. It is the only stationary Markovian process that is Gaussian and a diffusion process. Its realizations are continuous, and successive values are correlated exponentially.

This latter property makes the OU process a "colored" noise.

Different scalings of the OU process are used in the literature. For example, in Fig. It is also referred to as the random telegraph signal.

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L'Heureux and Kapral In this case it is possible to obtain a linear equation which describes the time-evolution of the probability density. This equation is more complicated than the Fokker-Planck equation see below , but the stationary density can still be obtained analytically, at least for the case where there is only one dynamical variable. In contrast, no exact evolution equation for the probability density of the state variable can be obtained for the case of the OU noise. Also, there exists a limit of dichotomous noise that is white shot noise - which is a sequence of delta-function spikes - from which it is possible to transform to Gaussian white noise.

This distinction concerns stochastic differential equations that involve Gaussian white noise.

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Gardiner Two main interpretations are used in the literature, known as the Ito and Stratonovich interpretations. It is important to establish what interpretation or "calculus" one assumes at the outset of an analysis, as this may influence the resulting form of the stochastic differential equation SDE. It will further affect the kind of calculus to use upon making variable changes. The Stratonovich calculus obeys the usual laws of calculus such as for changes of variables , but this is not the case for the Ito calculus.

Nevertheless it is possible to convert from one form of calculus to the other, and to restate an SDE in one form into the other as well as their corresponding Fokker-Planck equations - see below. The properties obtained with both calculi are identical when the Gaussian white noise is additive. Further, it is necessary to use a numerical integration method Kloeden and Platen that is compatible with the chosen calculus in order to match up simulation to theory.

For example, the explicit Euler method is compatible with the Ito interpretation. In the following text the Ito calculus is used. One may be interested in the behavior of individual trajectories, features of which can be compared to those from experimental measurements, or in the evolution of probability densities. The SDE approach is concerned with the former, and involves either exact or approximate analytical solutions, or numerical solutions; the Fokker-Planck approach or more generally, the Chapman-Kolmogorov approach - see below focusses on time-dependent probability densities.

The SDE approach can also be used to compute densities relevant to the latter approach. The SDE of the type seen in Eq. Such generators are in fact high-dimensional chaotic systems. Such algorithms must be "seeded", i. One of the results of the theory of Gaussian white noise is that the random term in Eq.

Appropriate averages of the relevant properties can be computed over many realizations, each with a different seed.

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More precise numerical algorithms for solving SDE's are also available, and are easily generalized to multiplicative noise processes of the type shown in Eq. Sancho et al.

One can study how an ensemble of initial conditions, characterized by an initial density, propagates under the action of the SDE Eq. Note that the probabilistic aspect of the problem has been shifted from individual realizations of a SDE to a probability density that evolves according to a deterministic linear evolution equation.

Setting the left hand side of Eq. One can also solve the Fokker-Planck equation numerically, or adopt the Langevin approach, and estimate the density directly from realizations of the SDE. This is a statement of the fact that only the present state is necessary to compute the future state; the past is irrelevant for this computation.

## Computation and Combinatorics in Dynamics, Stochastics and Control

This Markov assumption is not strictly valid in any physical setting, where the immediate history will play a role in the future evolution see Gardiner ; Risken Nevertheless, the mathematical idealization that is the Markov process is useful for describing reality. The differential form of the Chapman-Kolmogorov equation can also studied.

The process then has continuous paths. Noise-induced states are a nontrivial effect of noise. Their study requires the prior definition of the notion of a "state" in a stochastic sense, distinct from that of a "state variable". A stochastic state is the analogue of an attractor in a deterministic dynamical system. Specifically, it is the value of the dynamical variable for which the stationary probability distribution is a maximum. There can be more than one such state.

### Dynamics of some stochastic chemostat models with multiplicative noise

These states may be the same as in the noiseless case. However, the positions and even the number of stochastic states may differ from the deterministic case. In the case where the number of stochastic states is larger than the number of stable deterministic fixed points , one speaks of the creation of stochastic states by noise. Examples across a variety of disciplines in the natural sciences can be found in the book by Horsthemke and Lefever , and in Schimansky-Geier et al. Generally noise reveals the presence of nearby bifurcations by producing behavior that is stereotyped for a given bifurcation Wiesenfeld It can produce stochastic versions of various deterministic phenomena such as phase locking Longtin and Chialvo, as in neurons and other excitable systems, in which also can create very long time scales, e.

## Application Development

As mentioned in the introduction, noise can have high-dimensional deterministic origins. In fact, a pseudo-random number generator is one such system operating in discrete time, i. SDMs generate a distribution of dynamic states, which we argue represent ideal candidates for modeling how the brain represents states of the world.

When augmented with variational methods for model inversion, SDMs represent a powerful means of inferring neuronal dynamics from functional neuroimaging data in health and disease. Together with deeper theoretical considerations, this work suggests that SDMs will play a unique and influential role in computational psychiatry, unifying empirical observations with models of perception and behavior.

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