Pascal's rule

In mathematics, Pascal's rule (or Pascal's formula) is a combinatorial identity about binomial coefficients. It states that for positive natural numbers n and k, In mathematics, Pascal's rule (or Pascal's formula) is a combinatorial identity about binomial coefficients. It states that for positive natural numbers n and k, where ( n k ) {displaystyle {n choose k}} is a binomial coefficient; one interpretation of which is the coefficient of the xk term in the expansion of (1 + x)n. There is no restriction on the relative sizes of n and k, since, if n < k the value of the binomial coefficient is zero and the identity remains valid. Pascal's rule can also be generalized to apply to multinomial coefficients. Pascal's rule has an intuitive combinatorial meaning, that is clearly expressed in this counting proof. Proof. Recall that ( n k ) {displaystyle {n choose k}} equals the number of subsets with k elements from a set with n elements. Suppose one particular element is uniquely labeled X in a set with n elements. To construct a subset of k elements containing X, choose X and k-1 elements from the remaining n-1 elements in the set. There are ( n − 1 k − 1 ) {displaystyle {n-1 choose k-1}} such subsets. To construct a subset of k elements not containing X, choose k elements from the remaining n-1 elements in the set. There are ( n − 1 k ) {displaystyle {n-1 choose k}} such subsets. Every subset of k elements either contains X or not. The total number of subsets with k elements in a set of n elements is the sum of the number of subsets containing X and the number of subsets that do not contain X, ( n − 1 k − 1 ) + ( n − 1 k ) {displaystyle {n-1 choose k-1}+{n-1 choose k}} . This equals ( n k ) {displaystyle {n choose k}} ; therefore, ( n k ) = ( n − 1 k − 1 ) + ( n − 1 k ) {displaystyle {n choose k}={n-1 choose k-1}+{n-1 choose k}} .