Symmetric Iterative Proportional Fitting

Iterative Proportional Fitting (IPF) generates from an input matrix W a sequence of matrices that converges, under certain conditions, to a specific limit matrix W*. This limit is the relative-entropy nearest solution to W among all matrices of prescribed row marginals r and column marginals c. We prove this known fact by a novel strategy that contributes a pure algorithmic intuition. Then we focus on the symmetric setting: W=W’ and r=c. Since IPF inherently generates non-symmetric matrices, we introduce two symmetrized variants of IPF. We prove convergence for both of them. Further, we give a novel characterization for the existence of W* in terms of expansion properties of the undirected weighted graph represented by W. Finally, we show how our results contribute to recent work in machine learning.