Contains the full SSAI algorithm Run file test_SSAI.m to run SSAI with PCG on an SPD square matrix and test_SSAI_rect on a rectangular matrix. SSAI_PCG.m is a function that solves an SPD system with SSAI + PCG spcgls.m is the same for a rectangular matrix and PCGLS entire_r_sparse_inverse.m calculates a right approximate inverse to a matrix