Don Bernard Zagier (born 29 June 1951) is an American-German mathematician whose main area of work is number theory.

Stephen Kudla, John Millson and others generalized this result to intersection numbers of algebraic cycles on arithmetic quotients of symmetric spaces.

This theorem has some applications, including implying cases of the Birch and Swinnerton-Dyer conjecture, along with being an ingredient to Dorian Goldfeld's solution of the class number problem.

[7] Zagier later found a formula for traces of singular moduli as Fourier coefficients of a weight 3/2 modular form.

[8] Zagier collaborated with John Harer to calculate the orbifold Euler characteristics of moduli spaces of algebraic curves, relating them to special values of the Riemann zeta function.