The space of all real $(m \times n)$ matrices $A=\left\{a_{i j}\right\}$ can be considered to be isomorphic to the $m n$-dimensional Euclidean space $\mathbf{R}^{m n}$. Let $S_{m n}$ denote the set of all real ( $m \times n$ ) matrices which, considered as the pay-off matrix of a zero-sum game, admit pure strategy saddle-point equilibria. Then $S_{m n}$ is isomorphic to a subset of $\mathbf{R}^{m n}$, to be denoted by $S^{m, n}$.
(i) Is $S^{m, n}$ a subspace?
(ii) Is $S^{m, n}$ closed, convex?
(iii) Describe $S^{2,3}$ explicitly.