so that took me a bit longer to finally do the upload, but kwant and tinyarray can now be found again on the AUR [1,2]. Since the URL did not change, even the links on the Kwant website  work again. I'd be grateful if you could change that paragraph to
Arch install scripts for Kwant are kindly provided by Jörg Behrmann (formerly by Max Schlemmer). To install, follow the Arch User Repository installation instructions. Note that for checking the validity of the package you need to add the key used for signing to your user's keyring via
gpg --keyserver pool.sks-keyservers.net --recv-key C3F147F5980F3535
The fingerprint of the key is 5229 9057 FAD7 9965 3C4F 088A C3F1 47F5 980F 3535.
To answer the rest:
On Fri, Sep 11, 2015 at 03:35:26PM +0200, Christoph Groth wrote:
Jörg Behrmann wrote: How can I actually test whether Kwant uses mumps?
Try to “import kwant.solvers.mumps”. If this fails, the Kwant MUMPS solver does not work.
Ah, that's good to know. So from this I now get an ImportError: /usr/lib/libscotch.so: undefined symbol: gzclose when building kwant with mumps on Arch. I guess my libscotch is borked for some reason. I'll find out later why that is.
At least I can use this import to test whether the mumps configuration works and can later add a test to the PKGBUILD.
If not a runtime warning about a missing mumps might be a good idea.
You might be right. MUMPS is not necessary for Kwant to work, and the performance of umfpack used to be quite OK for many applications. Now that umfpack is no longer included in SciPy, without MUMPS Kwant is indeed very slow.
I just build the git head and the current warning is good.