Properties of extremal families of MN-convex (MN-concave) functions

In this paper MN-convex and MN-concave functions are examined and compared for particular cases of M and N. Characterization of pairs M,N for the family of MN-convex (MN-concave) functions consisting of all functions and partial characterization of pairs M,N for the family of MN-convex functions consisting of all constant functions will be presented. The condition of MN-convexity was proposed by Aumann in 1933. It was convexity with respect to arbitrary binary means M and N (abbreviated to MN-convexity). Recently many authors have considered this notion with suitable pairs of means. Here aggregation functions M,N will be used for this type of convexity and concavity.