Christoffel-Darboux formula

Etymology
Named after Elwin Bruno Christoffel and Jean Gaston Darboux.

Proper noun

 * 1)  An identity for a sequence of orthogonal polynomials: $$\sum_{j=0}^n \frac{f_j(x) f_j(y)}{h_j} = \frac{k_n}{h_n k_{n+1}} \frac{f_n(y) f_{n+1}(x) - f_{n+1}(y) f_n(x)}{x - y}$$ where fj(x) is the jth term of a set of orthogonal polynomials of squared norm hj and leading coefficient kj.