Past exam of the mathematics course of the University of Cambridge / 2015 / iii / Paper 38 / 5 / b

The preceding perfect matching gives a permutation matrix $P$ supported on the positive entries of $A$. LetIf $\alpha =1$, every row's selected entry is one and all other entries vanish, so $A=P$. Otherwiseis a doubly stochastic matrix with strictly fewer positive entries. Induct on the number of positive entries: the base case has exactly $n$ positive entries and is a permutation matrix. By induction, write $A^{\prime }={\sum }_i{\beta }_i{P}_i$ as a convex combination. Thenis another convex combination, with nonnegative coefficients summing to one. ThusThis constructive proof of the Birkhoff-von Neumann theorem is Birkhoff decomposition by support matchings.