The Birkhoff’s theorem states that any doubly stochastic matrix
lies inside a convex polytope with the permutation matrices
at the corners. We prove that any unitary matrix with
equal line sums can also be written as a sum of permutation
matrices (with sum of weights equal 1). Furthermore, when
the matrix dimension is prime, we prove that the unitary matrix
lies inside a convex complex Birkhoff polytope.
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