Conformal Composition for Borderline Fractional Sobolev Spaces

Abstract

We establish a pointwise property for homogeneous fractional Sobolev spaces in domains with non-empty boundary, extending a similar result of Koskela–Yang–Zhou. We use this to show that a conformal map from the unit disk onto a simply connected planar domain induces a bounded composition operator from the borderline homogeneous fractional Sobolev space of the domain into the corresponding space of the unit disk. © Springer-Verlag GmbH Germany & The Editorial Office of AMS 2025.