# Review of Actual Problems of Nonlinear Dynamics

## Patterns in excitable dynamics driven by additive dichotomic noise

Pattern formation due the presence of additive dichotomous fluctuations is studied an extended system with diffusive coupling and a bistable FitzHugh–Nagumo kinetics. The fluctuations vary in space and/or time being noise or disorder, respectively. Without perturbations the dynamics does not support pattern formation. With proper dichotomous fluctuations, however, the homogeneous steady state is destabilized either via a Turing instability or the fluctuations create spatial nuclei of an inhomogeneous states.

## Spectral problems for the perron–frobenius operator

A method of solving the spectral problem for the Perron–Frobenius operator of onedimensional piecewise linear chaotic maps is demonstrated. The method is based on introducing generating functions for the eigenfunctions of the operator. It is shown that the behavior of autocorrelation functions for chaotic maps depends on eigenvalues of the PerronFrobenius operator.

## Hyperbolic strange attractors of physically realizable systems

A review of studies aimed on revealing or constructing physical systems with hyperbolic strange attractors, like Plykin attractor and Smale–Williams solenoid, is presented. Examples of iterated maps, differential equations, and simple electronic devices with chaotic dynamics associated with such attractors are presented and discussed. A general principle is considered and illustrated basing on manipulation of phases in alternately excited oscillators and timedelay systems. Alternative approaches are reviewed outlined in literature, as well as the prospects of further researches.

## Two-stream instability – linear and nonlinear microwave phenomena

This article is the first of two parts of the review devoted to the phenomenon of two-beam instability in microwave electronics. The main goal is to cover as much as possible the most complete list of papers on various models and methods of analysis of the two-beam instability. The first part contains papers which, in our view, most fully describe the development of ideas of two-stream instability in microwave electronics.

## Two-stream instability – linear and nonlinear microwave phenomena

This article is the second part of the review of works devoted to the phenomenon of two-stream instability in microwave electronics. As it is known, a problem of creating devices operating in the terahertz frequency range is a rather actual today. Although there are many devices that can generate or amplify signals in this range, most of them refer to extremely powerful relativistic devices. At the same time there is a lack of compact medium power devices. In recent years, models based on the interaction of two electron beams appear in the papers of foreign research groups even more often.

## Self-sustained oscillations in quasiharmonic and chaotic oscillators in the presence of fluctuations

The paper presents the results of the classical theory of fluctuations in the quasiharmonic van der Pol oscillator. Stochastic equations for amplitude and phase of selfsustained oscillations are formulated and then their solutions are analyzed. The autocorrelation function and power spectrum of noisy selfsustained oscillations are studied. Then the spectral and correlations characteristics of chaotic selfsustained oscillations are numerically analyzed in spiral chaos oscillators.

## Entropy and forecasting of time series in the theory of dynamical systems

A contemporary consideration of such concepts as dimension and entropy of dynamical systems is given. Description of these characteristics includes into the analysis the other notions and properties related to complicated behavior of nonlinear systems as embedding dimension, prediction horizon etc., which are used in the paper. A question concerning the application of these ideas to real observables of the economical origin, i.e. market prices of the companies Schlumberger, Deutsche Bank, Honda, Toyota, Starbucks, BP is studied.

## Fractional diffusion equation for aging and equilibrated random walks

We consider continuous time random walks and discuss situations pertinent to aging. These correspond to the case when the initial state of the system is known not at preparation (at t = 0) but at the later instant of time t1 > 0 (intermediatetime initial condition).

## Ultrawideband wireless sensor networks based on chaotic radiopulses

Wireless sensor networks that is a fast emerging branch of modern telecommunications are considered in this paper. Particular attention is paid on ultrawideband sensor networks where chaotic radiopulses are used as an information carrier between sensor nodes. Development of such wireless sensor networks became possible after longterm investigations of chaotic oscillations and chaos control.

## Polynomial eigenfuctions of the perron–frobenius operator

In the paper, we reveal the structure of polynomial functions of the eigenfunctions and the kernel of the Perron–Frobenius operator for one-dimensional chaotic maps that iterative functions have the following properties: they are piecewise-linear ones; they have full branches transforming the domain of its definition to the full range of the mapping; the have arbitrary slope of branches; they have not some gaps between the branches.