I was wondering if it would be possible to enable arm64 in some of the PPA's, such as rrutter4.0. As far as I can tell, currently only i686 and amd64 are provided. It would be great to have arm64 builds for the ubuntu-lts backports of r-base. You can enable other platforms in the PPA settings. Go to "Change details" on your PPA and look under "Processors".