BIBO stability in infinite dimensions: extensions and limitations of finite-dimensional theory

The study of bounded-input bounded-output (BIBO) stability of infinite-dimensional systems gives rise to difficulties that do not occur in the classical finite-dimensional setting. In this paper, we resolve several questions concerning the definition and characterisation of BIBO stability in the framework of system nodes. In particular, we show that ${\mathrm{C}}^{\mathrm{\infty }}$-BIBO stability need not imply $L^{\mathrm{\infty }}$-BIBO stability in general, but that the two notions are equivalent when the output space is reflexive. Furthermore, we investigate to what extent classical results extend to the infinite-dimensional setting, including the relation between BIBO stability and exponential stability under controllability and observability assumptions.