Functions on a convex set which are both $ \omega $-semiconvex and $ \omega $-semiconcave II

Abstract

In a recent article (2022) we proved with L. Zaj\'i\v{c}ek that if $ G\subset\R^n $ is an unbounded open convex set that does not contain a translation of a convex cone with non-empty interior, then there exist $ f:G\to\R $ and a concave modulus $ \omega $ such that $ \lim_{t\to\infty}\omega(t)=\infty $, $ f $ is both semiconvex and semiconcave with modulus $ \omega $ and $ f\notin C^{1,\omega}(G) $. Here we improve the previous result as follows: If $ G $ is as above and $ \omega(t)=t^{\alpha} $ for some $ \alpha\in(0,1) $, then there exists $ f:G\to\R $ that is both semiconvex and semiconcave with modulus $ \omega $ and $ f\notin C^{1,\alpha}(G) $. This result has immediate consequences concerning a first-order quantitative converse Taylor theorem and the problem whether $ f\in C^{1,\alpha}(G) $ whenever $ f $ is smooth in a corresponding sense on all lines.