Abstract

Let E be a separable Banach space with separable dual. We show that the operation of subdifferentiation and the inverse operation are Borel from the convex functions on E into the monotone operators on E for the Effros–Borel structures.We also prove that the set of derivatives of differentiable convex functions is coanalytic non-Borel, by using the already known fact that the set of differentiable convex functions is itself coanalytic non-Borel, as proved in Bossard et al. 142).At last, we give a new proof of this latter fact, for reflexive E's, by giving a coanalytic rank on those sets and constructing functions of “high ranks”. This approach, based on an ordinal rank which follows from a construction of trees, is quite different — not so general but actually more constructive — from the previous results of this kind, in Bossard et al. 142) and Godefroy et al. , based on reductions of arbitrary coanalytic or difference of analytic sets to the studied sets