Yang Liu 0179 Pieter Ghysels Lisa Claus Xiaoye Sherry Li Sparse Approximate Multifrontal Factorization with Butterfly Compression for High-Frequency Wave Equations. S367-S391 2021 43 SIAM J. Sci. Comput. 5 https://doi.org/10.1137/20M1349667 db/journals/siamsc/siamsc43.html#LiuGCL21