The Lanczos method can be used to obtain extremal eigenpairs of sparse symmetric or hermitian matrices. I know there are several implementations of the Lanczos method (as well as Arnoldi, Davidson, and others) out there, but I am specifically interested in one that returns the tridiagonal matrix, not just the eigenvalues and eigenvectors. The way I am doing this now is using my own Lanczos implementation in Fortran. Is there a maintained library that does the same efficiently, preferably in python?
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$\begingroup$ It appears that SLEPc will soon provide this functionality: gitlab.com/slepc/slepc/-/merge_requests/33 $\endgroup$ – delete000 Jun 23 at 18:21
