This came out in today's Proceedings of the (US) National Academy of Science.
formatting link
It's not on open access, but I've got a copy of the paper as a .pdf file.
Apparently somebody worked how to get a useful upper limit back in 1928.
It the kind of odd mathematical insight that looks to be of only theoretical interest, until somebody runs into the right practical problem (but if they have it isn't mentioned in the paper).