# Projection onto Birkhoff Polytope

Suppose we would like to compute the Euclidean projection of an arbitrary matrix $A$ onto the Birkhoff polytope, the set of doubly-stochastic matrices.

1. Under some conditions on $A$, Sinkhorn's algorithm returns two diagonal matrices $D_1,D_2$ such that $D_1 A D_2$ is a doubly-stochastic matrix (which is unique). How does this matrix compare to the Euclidean projection, and if not, does this solution correspond to the projection under any particular metric?
2. Are there known methods for solving this efficiently besides just formulating the problem as a quadratic program and applying a generic QP algorithm?

## 1 Answer

It is known that symmetric Sinkhorn algorithm in fact minimizes KL divergence [2,3]. In 1, authors present a method to minimize the Euclidean distance. This is called BBS (Bregmanian Bi-Stochastication). It is reported that BBS algorithm using the Euclidian distance has noticeably better potential of producing good clustering results than the SK algorithm, while Sinkhorn performs better in terms of doubly stochasticity.

[1]. Learning a Bi-Stochastic Data Similarity Matrix, Fei Lang, Ping Li, Arnd Christian Konig

[2] J. N. Darroch and D. Ratcliff. Generalized iterative scaling for log-linear models. The Annals of Mathematical Statistics, 43(5):1470–1480, 1972.

[3] G. W. Soules. The rate of convergence of Sinkhorn balancing. Linear Algebra and its Applications, 150:3 – 40, 1991.