Polytopes of Stochastic Tensors

Abstract: Considering $n\times n\times n$ stochastic tensors $(a_{ijk})$ (i.e., nonnegative hypermatrices in which every sum over one index $i$, $j$, or $k$, is 1), we study the polytope ($\Omega_{n}$) of all these tensors, the convex set ($L_n$) of all tensors in $\Omega_{n}$ with some positive diagonals, and the polytope ($\Delta_n$) generated by the permutation tensors. We show that $L_n$ is almost the same as $\Omega_{n}$ except for some boundary points. We also present an upper bound for the number of vertices of $\Omega_{n}$.