THE CHARACTERIZATION OF GENERALIZED CONVEX FUNCTIONS
Frank F. Bonsall · The Quarterly Journal of Mathematics · 1953
The assertion of Theorem 3 Corollary (ii), repeated in Theorem 4, that f has a second derivative p.p. is faLse. In fact Corollary (ii) does not follow from Corollary (i), since the coi.intable set where the first derivative does not exist may have a derived set with positive measure. This error does not affect any other result in the paper.