Randomized Sparse Direct Solvers

We propose randomized direct solvers for large sparse linear systems, which integrate randomization into rank structured multifrontal methods. The use of randomization highly simplifies various essential steps in structured solutions, where fast operations on skinny matrix-vector products replace traditional complex ones on dense or structured matrices. The new methods thus significantly enhance the flexibility and efficiency of using structured methods in sparse solutions. We also consider a variety of techniques, such as some graph methods, the inclusion of additional structures, the concept of reduced matrices, information reuse, and adaptive schemes. The methods are applicable to various sparse matrices with certain rank structures. Particularly, for discretized matrices whose factorizations yield dense fill-in with some off-diagonal rank patterns, the factorizations cost about $O(n)$ flops in two dimensions (2D), and about $O(n)$ to O(n4/3) flops in three dimensions (3D). The solution costs and memory sizes are nearly $O(n)$ in both 2D and 3D. These counts are obtained based on two optimization strategies and a sparse rank relaxation idea. The methods are especially useful for approximate solutions and preconditioning. Numerical tests on both discretized PDEs and more general problems are used to demonstrate the efficiency and accuracy. The ideas here also have the potential to be generalized to matrix-free sparse direct solvers based on matrix-vector multiplications in future developments.