On Representatives of Subsets
P. Hall · Birkhäuser Boston eBooks · 2009
Let a set S of mn things be divided into m classes of n things each in two distinct ways, ( a ) and ( b ); so that there are m ( a )-classes and m ( b )-classes. Then it is always possible to find a set R of m things of S which is at one and the same time a C.S.R. (= complete system of rcpresentatives) for the ( a )-classes, and also a C.S.R. for the ( b )-classes.