On a regularity problem occurring in connection with Anosov diffeomorphisms (Q1082674)

Let \({\mathcal M}\) be a \(C^{\infty}\)-manifold and \({\mathcal F}_ s\) and \({\mathcal F}_ u\) be two Hölder foliations, transverse, and with uniformly \(C^{\infty}\) leaves. If a function f is uniformly \(C^{\infty}\) along the leaves of the two foliations, then it is \(C^{\infty}\) on \({\mathcal M}\). The proof is elementary.