A Gleason–Kahane–Z˙ elazko theorem for modules and applications to holomorphic function spaces

mashreghi, javad; ransford, thomas;

We generalize the Gleason–Kahane–Z˙ elazko theorem to modules. As an application, we show that every linear functional on a Hardy space that is non-zero on the outer functions is a multiple of a point evaluation. A further consequence is that every linear endomorphism of a Hardy space that maps outer functions to nowhere-zero functions is a weighted composition operator. In neither case is continuity assumed. We also consider some extensions to other function spaces, including the Bergman, Dirichlet and Besov spaces, the little Bloch space and VMOA.