Covering a set by subsets

R.J. Clarke
1990 Discrete Mathematics  
We give formulae for determining the number of ways of writing a finite set as the union of a given number of subsets, in such a way that none of the subsets may be omitted. In particular, we consider the case in which the elements of the set are identical.
doi:10.1016/0012-365x(90)90146-9 fatcat:3mlgqzw42rag3kwrfmhmjelv6y