Stochastic Matrix-Free Equilibration

We present a novel method for approximately equilibrating a matrix using only multiplication by the matrix and its transpose. Our method is based on convex optimization and projected stochastic gradient descent, using an unbiased estimate of a gradient obtained by a randomized method. Our method provably converges in expectation and empirically gets good results with a small number of iterations. We show how the method can be applied as a preconditioner for matrix-free iterative algorithms, substantially reducing the iterations required to reach a given level of precision. We also derive a novel connection between equilibration and condition number, showing that equilibrationminimizes an upper bound on the condition number over all choices of row and column scalings.