Here it is, from this paper:

Proposition 5.1.1. The number of skyline polyominoes of area $A$ and width $w$ is $\left(\!\binom{w}{A-W}\!\right) = \binom{A-1}{w-1}$.

I'm referring to the first expression, not the binomial coefficient. I think it's a Stirling number of some kind, but I'm not familiar enough with them to be sure.


1 Answer 1


It’s probably a multiset coefficient;


is the number of multisets of cardinality $k$ that can be chosen from a set of $n$ distinct types of object, and the notation has become moderately standard.


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