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Unistochastic matrix : ウィキペディア英語版
Unistochastic matrix
In mathematics, a unistochastic matrix (also called ''unitary-stochastic'') is a doubly stochastic matrix whose entries are the squares of the absolute values of the entries of some unitary matrix.
A square matrix ''B'' of size ''n'' is doubly stochastic (or ''bistochastic'') if all its entries are non-negative real numbers and each of its rows and columns sum to 1. It is unistochastic if there exists a unitary matrix ''U'' such that
: B_=|U_|^2 \text i,j=1,\dots,n. \,
All 2-by-2 doubly stochastic matrices are unistochastic and orthostochastic, but for larger ''n'' it is not the case.
Already for n=3 there exists a bistochastic matrix B which is not unistochastic:
:
B= \frac
\begin
1 & 1 & 0 \\
0 & 1 & 1 \\
1 & 0 & 1 \end

since any two vectors with moduli equal to the square root of the entries of two columns (rows)
of B cannot be made orthogonal by a suitable choice of phases.
== Properties ==

* the set of unistochastic matrices contains all permutation matrices
* for n \ge 3 this set is not convex
* for n =3 the set of unistochastic matrices is star shaped.
* for n =3 the relative volume of the set of unistochastic matrices with respect to the Birkhoff polytope of bistochastic matrices is 8\pi^2/105 \approx 75.2 \%

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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