Continuity of real valued subquadratic functions

The aim of this paper is to prove a regularity theorem for real valued subquadratic mappings that are solutions of the inequality \[ \varphi (x + y) + \varphi(x - y) \leq 2\varphi(x) + 2\varphi(y),\quad x, y \in X, where $X=(X,+)$ is a topological group.