The nonlinear semigroup generated by the subdifferential of a convex lower semicontinuous
function
has a smoothing effect, discovered by Haïm Brezis, which implies maximal
regularity for the evolution equation. We use this and Schaefer’s fixed
point theorem to solve the evolution equation perturbed by a Nemytskii
operator of sublinear growth. For this, we need that the sublevel sets of
are not only closed, but even compact. We apply our results to the
-Laplacian
and also to the Dirichlet-to-Neumann operator with respect to
-harmonic
functions.