Let $C \subseteq \mathbb{R}^D$ be a D5623: Convex euclidean real set.
A D4364: Real function $f : C \to \mathbb{R}$ is subconvex if and only if
\begin{equation}
\forall \, N \in 1, 2, 3, \ldots :
\forall \, x_1, \ldots, x_N \in C :
\forall \, r_1, \ldots, r_N \in \mathbb{R}
\left[ r_1, \dots, r_N \geq 0 \text{ and } \sum_{n = 1}^N r_n = 1 \quad \implies \quad f \left( \sum_{n = 1}^N r_n x_n \right) \leq \sum_{n = 1}^N r_n f(x_n) \right]
\end{equation}