On Removable Sets For Convex Functions

Abstract

In the present article we provide a sufficient condition for a closed set F in R^d to have the following property which we call c-removability: Whenever a function f:R^d->R is locally convex on the complement of F, it is convex on the whole R^d. We also prove that no generalized rectangle of positive Lebesgue measure in R^2 is c-removable. Our results also answer the following question asked in an article by Jacek Tabor and Jozef Tabor [J. Math. Anal. Appl. 365 (2010)]: Assume the closed set F in R^d is such that any locally convex function defined on R^d\F has a unique convex extension on R^d. Is F necessarily intervally thin (a notion of smallness of sets defined by their "essential transparency" in every direction)? We prove the answer is negative by finding a counterexample in R^2.