Combination

The number of ways of picking $r$ unordered outcomes from $n$ possibilities. Also known as the Binomial Coefficient or Choice Number and read ``$n$ choose $r$.''

$${}_nC_r \equiv {n\choose r} \equiv {n!\over r!(n-r)!},$$

where $n!$ is a Factorial.

See also Binomial Coefficient, Derangement, Factorial, Permutation, Subfactorial

References

Conway, J. H. and Guy, R. K. ``Choice Numbers.'' In The Book of Numbers. New York: Springer-Verlag, pp. 67-68, 1996.

Ruskey, F. ``Information on Combinations of a Set.'' http://sue.csc.uvic.ca/~cos/inf/comb/CombinationsInfo.html.