Let $X$ be a real vector space. A map $p: X\to\mathbb{R}$ is called \emph{sublinear} if (i) $p(x+y)\le p(x)+p(y)$ for all $x,y\in X$, and (ii) $p(\lambda x)=\lambda\,p(x)$ for all $x\in X$ and all scalars $\lambda\ge 0$.