Since, however, the process of exchanging opinions across the ocean is rather slow, I have been using this time to learn something more about the problem from the purely mathematical point of view, which is very narrow, of course. Here I send you a small diagram [...]“. – Auf Briefpapier mit gedr. Briefkopf der Universität Padua; Bl. 1 horizontal durchrissen; mit kleinen Randläsuren.