## Stirling Number of the Second Kind

The number of ways of partitioning a set of elements into nonempty Sets (i.e., Blocks), also called a Stirling Set Number. For example, the Set can be partitioned into three Subsets in one way: ; into two Subsets in three ways: , , and ; and into one Subset in one way: .

The Stirling numbers of the second kind are denoted , , , or , so the Stirling numbers of the second kind for three elements are

 (1) (2) (3)

Since a set of elements can only be partitioned in a single way into 1 or Subsets,
 (4)

The triangle of Stirling numbers of the second kind is (Sloane's A008277).

The Stirling numbers of the second kind can be computed from the sum

 (5)

with a Binomial Coefficient, or the Generating Functions
 (6)

 (7)

and
 (8)

The following diagrams (Dickau) illustrate the definition of the Stirling numbers of the second kind for and 4.

Stirling numbers of the second kind obey the Recurrence Relations

 (9)

An identity involving Stirling numbers of the second kind is

 (10)

It turns out that can have only 0, 2, or 6 as a last Digit (Riskin 1995).

See also Bell Number, Combination Lock, Lengyel's Constant, Minimal Cover, Stirling Number of the First Kind

References

Abramowitz, M. and Stegun, C. A. (Eds.). Stirling Numbers of the Second Kind.'' §24.1.4 in Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing. New York: Dover, pp. 824-825, 1972.

Comtet, L. Advanced Combinatorics: The Art of Finite and Infinite Expansions, rev. enl. ed. Boston, MA: Reidel, 1974.

Conway, J. H. and Guy, R. K. In The Book of Numbers. New York: Springer-Verlag, pp. 91-92, 1996.

Dickau, R. M. Stirling Numbers of the Second Kind.''

Graham, R. L.; Knuth, D. E.; and Patashnik, O. Concrete Mathematics: A Foundation for Computer Science, 2nd ed. Reading, MA: Addison-Wesley, 1994.

Knuth, D. E. Two Notes on Notation.'' Amer. Math. Monthly 99, 403-422, 1992.

Riordan, J. An Introduction to Combinatorial Analysis. New York: Wiley, 1980.

Riordan, J. Combinatorial Identities. New York: Wiley, 1968.

Riskin, A. Problem 10231.'' Amer. Math. Monthly 102, 175-176, 1995.

Sloane, N. J. A. Sequence A008277 in The On-Line Version of the Encyclopedia of Integer Sequences.'' http://www.research.att.com/~njas/sequences/eisonline.html.

Stanley, R. P. Enumerative Combinatorics, Vol. 1. Cambridge, England: Cambridge University Press, 1997.