# Stability problems in non autonomous linear differential equations in infinite dimensions

## TL;DR

This paper investigates the robustness of stability in nonautonomous linear differential equations within infinite-dimensional Banach spaces, introducing generalized almost periodic functions and demonstrating stabilization techniques through small perturbations.

## Contribution

It extends stability robustness results to infinite dimensions, introduces generalized almost periodic functions, and provides new stabilization methods using small perturbations.

## Key findings

- Stability is robust under integrally small perturbations in infinite dimensions.
- Generalized almost periodic functions effectively handle oscillatory perturbations.
- Unstable systems can be stabilized with large period, small mean value perturbations.

## Abstract

One goal of this paper is to study robustness of stability of nonautonomous linear ordinary differential equations under integrally small perturbations in an infinite dimensional Banach space. Some applications are obtained to the case of rapid oscillatory perturbations, with arbitrary small periods, showing that even in this case the stability is robust. These results extend to infinite dimensions some results given in Coppel (1970). Based in Rodrigues (1970) and in Kloeden and Rodrigues (2011) we introduce a class of functions that we call Generalized Almost Periodic Functions that extend the usual almost periodic functions and are suitable to deal with oscillatory perturbations. We also present an infinite dimensional example of the previous results. We show in another example that it is possible to stabilize an unstable system using a perturbation with large period and small mean value. Finally, we give an example where we stabilize an unstable linear ODE with small perturbation in infinite dimensions using some ideas developed in Rodrigues andSol\`a-Morales (2019)} and in an example of Kakutani, see Rickard (1960).