n choose k (binomial coefficients):
For any set containing n elements, the number of distinct k-element subsets
Arranging binomial coefficients into rows for successive values of n, and in which k ranges from 0 to n, gives a triangular array called Pascal's triangle.
( n ) ( n-1 ) ( n-1 )
( ) = ( ) + ( ) for 1 <= k <= n - 1
( k ) ( k-1 ) ( k )
( n ) ( n )
( ) = ( ) = 1 for n >= 0
( 0 ) ( n )