Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Oops, yes, wrong paper. It is Theorem 1 in the paper "Some Generalizations of the Problem of Distinct Representatives" same authors, same volume, pages 230-241. doi.org/10.4153/CJM-1958-027-8 .
