Systems of Inequalities Characterizing Ring Homomorphisms

Abstract

Assume that T : P -> R and U : P -> R are arbitrary mappings between two partially ordered rings P and R. We study a few systems of functional inequalities which characterize ring homomorphisms. For example, we prove that if T and U satisfy T(f+g) >= T(f)+T(g), U(f.g) >= U(f).U(g), for all f,g is an element of P and T >= U, then U = T and thismapping is a ring homomorphism. Moreover, we find two other systems for which we obtain analogous assertions.