Orthostochastic matrix

In mathematics, an orthostochastic matrix is a doubly stochastic matrix whose entries are the square of the absolute value of some orthogonal matrix.

The detailed definition is as follows. An square matrix B of size n is doubly stochastic (or bistochastic) if all its rows and columns sum to 1 and all its entries are nonnegative real numbers, each of whose rows and columns sums to 1. It is orthostochastic if there exists an orthogonal matrix O such that

$B_{ij}=O_{ij}^{2}{\text{ for }}i,j=1,\dots ,n.\,$

All 2-by-2 doubly stochastic matrices are orthostochastic (and also unistochastic) since for any

$B={\begin{bmatrix}a&1-a\\1-a&a\end{bmatrix}}$

we find the corresponding orthogonal matrix

$O={\begin{bmatrix}\cos \phi &\sin \phi \\-\sin \phi &\cos \phi \end{bmatrix}},$

with $\cos ^{2}\phi =a,$ such that $B_{ij}=O_{ij}^{2}.$

For larger n the sets of bistochastic matrices includes the set of unistochastic matrices, which includes the set of orthostochastic matrices and these inclusion relations are proper.