On an integral transform by R. S. Phillips

Abstract

Abstract The properties of a transformation $$ f \mapsto \tilde f_h $$ by R.S. Phillips, which transforms an exponentially bounded C 0-semigroup of operators T(t) to a Yosida approximation depending on h, are studied. The set of exponentially bounded, continuous functions f: [0, ∞[→ E with values in a sequentially complete L c-embedded space E is closed under the transformation. It is shown that $$ (\tilde f_h )\widetilde{_k } = \tilde f_{h + k} $$ for certain complex h and k, and that $$ f(t) = \lim _{h \to 0^ + } \tilde f_h (t) $$, where the limit is uniform in t on compact subsets of the positive real line. If f is Hölder-continuous at 0, then the limit is uniform on compact subsets of the non-negative real line. Inversion formulas for this transformation as well as for the Laplace transformation are derived. Transforms of certain semigroups of non-linear operators on a subset X of an L c-embedded space are studied through the C 0-semigroups, which they define by duality on a space of functions on X.