As a follow up also to the previous posts:
- The original arpack is available via: https://www.arpack.org/
- arpack-ng can be used together with CalculiX instead of the original one (@CharlieM I guess you had bad luck trying to access it and I’d put some trust into GitHub that it will be available for some more time)
- There are not many straightforward alternatives to arpack as far as I’m aware (even scipy uses arpack for sparse eigenvalue problems). There is e.g. Spectra and Anasazi, but both have additional dependencies for their data structures and are not the same in terms of the algorithms they provide.