Sincov's inequalities on topological spaces

Abstract

Assume that $X$ is a non-empty set and $T$ and $S$ are real or complex mappings defined on the product $X \times X$. Additive and multiplicative Sincov's equations are: $$T(x,z) = T(x, y ) + T(y, z)$$ and $$S(x,z) = S(x, y ) \cdot S(y, z),$$ respectively. Both equations play important roles in many areas of mathematics. In the present paper we study related inequalities. We deal with functional inequality $$ G(x,z) \leq G(x, y ) \cdot G(y, z), \quad x , y, z \in X $$ and we assume that $X$ is a topological space and $G\colon X \times X \to \mathbb{R}$ is a continuous mapping. In some our statements a considerably weaker regularity than continuity of $G$ is needed. We also study the reverse inequality: $$F(x,z) \geq F(x, y ) \cdot F(y, z), \quad x , y, z \in X $$ and the additive inequality (the triangle inequality): $$H(x,z) \leq H(x,y) + H(y,z), \quad x, y , z \in X.$$ A corollary for generalized (non-symmetric) metric is derived.