Random Osborne: a simple, practical algorithm for Matrix Balancing in near-linear time
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We revisit Matrix Balancing, a pre-conditioning task used ubiquitously for computing eigenvalues and matrix exponentials. Since 1960, Osborne's algorithm has been the practitioners' algorithm of choice, and is now implemented in most numerical software packages. However, the theoretical properties of Osborne's algorithm are not well understood. Here, we show that a simple random variant of Osborne's algorithm converges in near-linear time in the input sparsity. Specifically, it balances K∈R_≥ 0^n× n after O(mϵ^-2logκ) arithmetic operations, where m is the number of nonzeros in K, ϵ is the ℓ_1 accuracy, and κ=∑_ijK_ij/(min_ij:K_ij≠ 0K_ij) measures the conditioning of K. Previous work had established near-linear runtimes either only for ℓ_2 accuracy (a weaker criterion which is less relevant for applications), or through an entirely different algorithm based on (currently) impractical Laplacian solvers. We further show that if the graph with adjacency matrix K is moderately connected–e.g., if K has at least one positive row/column pair–then Osborne's algorithm initially converges exponentially fast, yielding an improved runtime O(mϵ^-1logκ). We also address numerical precision issues by showing that these runtime bounds still hold when using O(log(nκ/ϵ))-bit numbers. Our results are established through a potential argument that leverages a convex optimization perspective of Osborne's algorithm, and relates the per-iteration progress to the current imbalance as measured in Hellinger distance. Unlike previous analyses, we critically exploit log-convexity of the potential. Our analysis extends to other variants of Osborne's algorithm: along the way, we establish significantly improved runtime bounds for cyclic, greedy, and parallelized variants.
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