Hello,

The Stirling numbers of the second kind count how many ways can a set of $k$ elements be partitioned into $n$ non-empty classes, with $k=n,n+1,\dots$.

My question is: Is there a generalization of these numbers such that the classes are not merely non-empty, but instead occupied to a minimum level with each of the $n$ classes having a minimum of, say, $r$ elements in it? Of course, in this case, $k=rn,rn+1,\dots$.