Franck HOUSSEN

Changing dependencies could be costly-to-endless (not really motivating...) : CMake _may_ be easy, autotools likely not... You added SYSTEM_BLAS and SYSTEM_LAPACK "only" for building without BLAS/LAPACK ? > I know...

I suppose what you want is not possible : you should try to check your environment or to set hints (from or outside your external project) for FindBLAS() to find...

Not clear enough to me. Your problem looks really like a pure cmake problem, not an arpack-ng problem. Seems you need : - Either to `set` hints and/or relevant environment...

> Please believe me I believe no one but _the code_. Arpack's API are the `*[ae]upd`, `git grep external SRC/*[ae]upd.f` shows already lots of BLAS/LAPACK hits (*ger, *lacpy, ...): arpack...

There's no right or wrong ?!... CMake evolves to "modern CMake" as it learns from it's errors : CMake fix _same_ rules from _anybody_ https://levelup.gitconnected.com/simple-modern-cmake-tutorial-b0c1c362cd2c

Looks like a cmake problem, not an arpack-ng problem: check `*-devel` are installed and your environment is OK.

Do you set `rvec` to `.false.` ? https://github.com/opencollab/arpack-ng/blob/e3bdd3b8db727a10551c06959e45e866a6b8d8fc/SRC/dseupd.f#L329

> No, I set rvec to true because I want to get the eigenvectors back Sounds like a limitation of arpack, check out : https://www.caam.rice.edu/software/ARPACK/ (dseupd from arpack96.tar.gz)

The error was originally returned by the reference code (as a quick return) : this likely means this case can't be supported unless the doc says so (as it's maybe...

> Well, what do you call the "reference code" ? Here https://www.caam.rice.edu/software/ARPACK/ is the "real / original " code which is no more maintained (= often a pain to compile...