Binomial Coefficient -- from Wolfram MathWorld

The binomial coefficient (n; k) is the number of ways of picking k unordered outcomes from n possibilities, also known as a combination or combinatorial number. The symbols _nC_k and (n; k) are used to denote a binomial coefficient, and are sometimes read as "n choose k." (n; k) therefore gives the number of k-subsets possible out of a set of n distinct items. For example, The 2-subsets of {1,2,3,4} are the six pairs {1,2}, {1,3}, {1,4}, {2,3}, {2,4}, and {3,4}, so (4; 2)=6. In...