Sparse simultaneous equations# References# https://en.wikipedia.org/wiki/Gaussian_elimination https://watlab-blog.com/2020/05/01/gaussian-elimination/ http://www.na.scitec.kobe-u.ac.jp/~yamamoto/work/JSCES2007.PDF http://www.young.iis.u-tokyo.ac.jp/personal/morita/misc/direct/20180512_direct_method.pdf