The Bell numbers are given by
where are Stirling numbers (cf. Combinatorial analysis) of the second kind, so that is the total number of partitions of an -set.
They are equal to .
The name honours E.T. Bell.
|[a1]||L. Comtet, "Advanced combinatorics" , Reidel (1974)|
Bell numbers. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Bell_numbers&oldid=14335