Derandomization Beyond Connectivity: Undirected Laplacian Systems in Nearly Logarithmic Space

We give a deterministic \tilde{O}(\log n)-space algorithm for approximately solving linear systems given by Laplacians of undirected graphs, and consequently also approximating hitting times, commute times, and escape probabilities for undirected graphs. Previously, such systems were known to be solvable by randomized algorithms using O(\log n) space (Doron, Le Gall, and Ta-Shma, 2017) and hence by deterministic algorithms using O(\log^{3/2} n) space (Saks and Zhou, FOCS 1995 and JCSS 1999).Our algorithm combines ideas from time-efficient Laplacian solvers (Spielman and Teng, STOC 04; Peng and Spielman, STOC 14) with ideas used to show that Undirected S-T Connectivity is in deterministic logspace (Reingold, STOC 05 and JACM 08; Rozenman and Vadhan, RANDOM 05).