I guess you mean this one:
https://projects.archlinux.de/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/imath
Call for an update here: https://forum.manjaro.org/t/update-requests-for-packages-imported-from-arch-linux
I can’t find this package either, only Manjaro - Branch Compare shows me, that it should be there.