Admissibility and general dichotomies for evolution families

For an arbitrary noninvertible evolution family on the half-line and for $\rho \colon [0, \infty)\to [0, \infty)$ in a large class of rate functions, we consider the notion of a $\rho$-dichotomy with respect to a family of norms and characterize it in terms of two admissibility conditions. In particular, our results are applicable to exponential as well as polynomial dichotomies with respect to a family of norms. As a nontrivial application of our work, we establish the robustness of general nonuniform dichotomies

Robustness of nonuniform and random exponential dichotomies with applications to differential equations

In this thesis, we study hyperbolicity for deterministic and random nonautonomous dynamical systems and their applications to differential equations. More precisely, we present results in the following topics: nonuniform hyperbolicity for evolution processes and hyperbolicity for nonautonomous random dynamical systems. In the first topic, we study the robustness of the nonuniform exponential dichotomy for continuous and discrete evolution processes. We present an example of an infinitedimensional differential equation that admits a nonuniform exponential dichotomy and apply the robustness result. Moreover, we study the persistence of nonuniform hyperbolic solutions in semilinear differential equations. Furthermore, we introduce a new concept of nonuniform exponential dichotomy, provide examples, and prove a stability result under perturbations for it. In the second topic, we introduce exponential dichotomies for random and nonautonomous dynamical systems. We prove a robustness result for this notion of hyperbolicity and study its applications to random and nonautonomous differential equations. Among these applications, we study the existence and continuity of random hyperbolic solutions and their associated unstable manifolds. As a consequence, we obtain continuity and topological structural stability for nonautonomous random attractors

Safety criteria for aperiodic dynamical systems

The use of dynamical system models is commonplace in many areas of science and engineering. One is often interested in whether the attracting solutions in these models are robust to perturbations of the equations of motion. This question is extremely important in situations where it is undesirable to have a large response to perturbations for reasons of safety. An especially interesting case occurs when the perturbations are aperiodic and their exact form is unknown. Unfortunately, there is a lack of theory in the literature that deals with this situation. It would be extremely useful to have a practical technique that provides an upper bound on the size of the response for an arbitrary perturbation of given size. Estimates of this form would allow the simple determination of safety criteria that guarantee the response falls within some pre-specified safety limits. An excellent area of application for this technique would be engineering systems. Here one is frequently faced with the problem of obtaining safety criteria for systems that in operational use are subject to unknown, aperiodic perturbations. In this thesis I show that such safety criteria are easy to obtain by using the concept of persistence of hyperbolicity. This persistence result is well known in the theory of dynamical systems. The formulation I give is functional analytic in nature and this has the advantage that it is easy to generalise and is especially suited to the problem of unknown, aperiodic perturbations. The proof I give of the persistence theorem provides a technique for obtaining the safety estimates we want and the main part of this thesis is an investigation into how this can be practically done. The usefulness of the technique is illustrated through two example systems, both of which are forced oscillators. Firstly, I consider the case where the unforced oscillator has an asymptotically stable equilibrium. A good application of this is the problem of ship stability. The model is called the escape equation and has been argued to capture the relevant dynamics of a ship at sea. The problem is to find practical criteria that guarantee the ship does not capsize or go through large motions when there are external influences like wind and waves. I show how to provide good criteria which ensure a safe response when the external forcing is an arbitrary, bounded function of time. I also consider in some detail the phased-locked loop. This is a periodically forced oscillator which has an attracting periodic solution that is synchronised (or phase-locked) with the external forcing. It is interesting to consider the effect of small aperiodic variations in the external forcing. For hyperbolic solutions I show that the phase-locking persists and I give a method by which one can find an upperbound on the maximum size of the response

Generalized Trichotomies: robustness and global and local invariant manifolds

In a Banach space, given a differential equation v′(t) = A(t)v(t), with an initial condition v(s) = vs and that admits a generalized trichotomy, we studied which type of conditions we need to impose to the linear perturbations B so that v′(t) = [A(t) + B(t)] v(t) continues to admit a generalized trichotomy, that is, we studied the robustness of generalized trichotomies. In the same way, it was also the aim of our work the study of a differential equation with another type of nonlinear perturbations, v′(t) = A(t)v(t) + f(t, v). We sought conditions to impose on the function f so that the new perturbed equation would admit a global Lipschitz invariant manifold as well as the necessary conditions for the existence of local Lipschitz invariant manifolds.Num espaço de Banach, dada uma equação diferencial v′(t) = A(t)v(t), sujeita a uma condição inicial v(s) = vs e que admite uma tricotomia generalizada, estudámos o tipo de condições a impor às perturbações lineares B de modo que a equação v′(t) = [A(t) + B(t)] v(t) ainda admita uma tricotomia generalizada, ou seja, estudámos a robustez das tricotomias generalizadas. Da mesma forma, foi também objecto deste trabalho, o estudo de uma equação diferencial com outro tipo de perturbações não lineares, v′(t) = A(t)v(t) + f(t, v). Procurámos condições necessárias a impor à função f por forma a que a nova equação perturbada admitisse uma variedade invariante Lipschitz global, bem como as condições necessárias para a existência de variedades invariantes Lipschitz locais