Maximization of the Stability Radius of an Infinite Dimensional System Subjected to Stochastic Unbounded Structured Multi-perturbations With Unbounded Input Operator

In this paper we consider infinite dimensional systems subjected to stochastic structured multiperturbations. We address the problem of robustness optimization with respect to state feedback but allow both unbounded input and perturbations. Conditions are derived for the existence of a stabilizing controller ensuring that the norm of the closed loop operator below a prespecified bound. Such controllers will be called suboptimal controllers. The suboptimality conditions are obtained in terms of a Riccati equation which satisfies an operator inequality. Finally, we give a lower bound for the supremal achievable stability radius via the Riccati equation.