“…These matrices are formalized as, with an increasing level of complexity, hierarchically off-diagonal low-rank (HODLR) matrices, 55 hierarchically semiseparable (HSS) matrices, 56,57 -matrices, 46,58 and 2 -matrices. 59,60 Fast methods for efficiently constructing and storing approximate inverses of these matrices have been developed in recent years, [61][62][63] mainly for the aforementioned HODLR and HSS matrices. Moreover, once the approximate inverse, which only depends on the geometry, is computed and stored, it can be applied with a minimal amount of CPU time to multiple right-hand sides.…”