## Journals

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### Open Access Journals

DCDS

Yakubovich, Fradkov, Hill and Proskurnikov have used the Yaku-bovich Frequency Theorem to prove that a strictly dissipative linear-quadratic control process with periodic coefficients admits a storage function, and various related results. We extend their analysis to the case when the coefficients are bounded uniformly continuous functions.

DCDS-B

In recent years, the area of nonautonomous dynamical systems has matured into a field with recognizable contours together with well-defined themes and methods. Its development has been strongly stimulated by various problems of applied mathematics, and it has in its turn influenced such areas of applied and pure mathematics as spectral theory, stability theory, bifurcation theory, the theory of bounded/recurrent motions, etc. Much work in this field concerns the asymptotic properties of the solutions of a nonautonomous differential or discrete system. However, that is by no means always the case, and the reader will find papers in this volume which are concerned only at a distance or not at all with asymptotic matters.

There is a close relation between the field of nonautonomous dynamical systems and that of stochastic dynamical systems. They can be distinguished to a certain extent by the observation that a nonautonomous dynamical system often arises from the study of a differential or discrete system whose coefficients depend on time, but in a non-stochastic way. The limiting case is that of periodic coefficients, but one is also interested in equations whose coefficients exhibit weaker recurrence properties; for example almost periodicity, Birkhoff recurrence, Poisson recurrence, etc. A distinction also occurs on the methodological level in that topological methods tend to find more application in the former field as compared to the latter (while analytical and ergodic tools are heavily used in both). In any case, some people use the term “random dynamics” to refer to both fields in a more or less interchangeable way.

For the full preface, please click on the Full Text "PDF" button above.

There is a close relation between the field of nonautonomous dynamical systems and that of stochastic dynamical systems. They can be distinguished to a certain extent by the observation that a nonautonomous dynamical system often arises from the study of a differential or discrete system whose coefficients depend on time, but in a non-stochastic way. The limiting case is that of periodic coefficients, but one is also interested in equations whose coefficients exhibit weaker recurrence properties; for example almost periodicity, Birkhoff recurrence, Poisson recurrence, etc. A distinction also occurs on the methodological level in that topological methods tend to find more application in the former field as compared to the latter (while analytical and ergodic tools are heavily used in both). In any case, some people use the term “random dynamics” to refer to both fields in a more or less interchangeable way.

For the full preface, please click on the Full Text "PDF" button above.

keywords:

DCDS-B

We study the concept of dissipativity in the sense of Willems for
nonautonomous linear-quadratic (LQ) control systems.
A nonautonomous system of Hamiltonian ODEs is associated with such
an LQ system by way of the Pontryagin Maximum Principle.
We relate the concepts of exponential dichotomy and weak disconjugacy
for this Hamiltonian ODE to that of dissipativity for the LQ system.

DCDS-S

Under the assumption of lack of uniform controllability for a family of time-dependent linear control systems, we study the dimension, topological structure and other dynamical properties of the sets of null controllable points and of the sets of reachable points. In particular, when the space of null controllable vectors has constant dimension for all the systems of the family, we find a closed invariant subbundle where the uniform null controllability holds. Finally, we associate a family of linear Hamiltonian systems to the control family and assume that it has an exponential dichotomy in order to relate the space of null controllable vectors to one of the Lagrange planes of the continuous hyperbolic splitting.

DCDS

We study the properties of the error covariance matrix and the asymptotic error covariance
matrix of the Kalman-Bucy filter model with time-varying coefficients. We make use of such
techniques of the theory of nonautonomous differential systems as the exponential dichotomy
concept and the rotation number.

DCDS-S

Generally speaking, the term nonautonomous dynamics refers to the systematic use of dynamical tools to study the solutions of differential or difference equations with time-varying coefficients. The nature of the time variance may range from periodicity at one extreme, through Bohr almost periodicity, Birkhoff recurrence, Poisson recurrence etc. to stochasticity at the other extreme. The ``dynamical tools'' include almost everywhere Lyapunov exponents, exponential splittings, rotation numbers, and the theory of cocycles, but are by no means limited to these. Of course in practise one uses whatever ``works'' in the context of a given problem, so one usually finds dynamical methods used in conjunction with those of numerical analysis, spectral theory, the calculus of variations, and many other fields. The reader will find illustrations of this fact in all the papers of the present volume.

For more information please click the “Full Text” above.

For more information please click the “Full Text” above.

keywords:

DCDS

A detailed dynamical study of the skew-product semiflows induced by
families of AFDEs with infinite delay on a Banach space is
carried over. Applications are given for families of non-autonomous
quasimonotone reaction-diffusion PFDEs with delay in the nonlinear
reaction terms, both with finite and infinite delay. In this
monotone setting, relations among the classical concepts of sub and
super solutions and the dynamical concept of semi-equilibria are
established, and some results on the existence of minimal semiflows
with a particular dynamical structure are derived.

CPAA

This special issue collects eleven papers in the general area of nonautonomous
dynamical systems. They contain a rich selection of new results on pure and applied
aspects of the eld.

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DCDS-B

In the extension of the concepts of saddle-node, transcritical and pitchfork bifurcations to the non-autonomous case, one considers the change in the number and attraction properties of the minimal sets for the skew-product flow determined by the initial one-parametric equation. In this work conditions on the coefficients of the equation ensuring the existence of a global bifurcation phenomenon of each one of the types mentioned are established. Special attention is paid to show the importance of the non-trivial almost automorphic extensions and pinched sets in describing the dynamics at the bifurcation point.

DCDS

Using methods of the theory of nonautonomous linear differential
systems, namely exponential dichotomies
and rotation numbers, we generalize some aspects of Yakubovich's
Frequency Theorem from periodic control
systems to systems with bounded uniformly continuous coefficients.

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