In mathematics, the Hurwitz class numberH(N), introduced by Adolf Hurwitz, is a modification of the class number of positive definite binary quadratic forms of discriminant–N, where forms are weighted by 2/g for g the order of their automorphism group, and where H(0)=–1/12.
Zagier, Don (1975), "Nombres de classes et formes modulaires de poids 3/2", Comptes Rendus de l'Académie des Sciences, Série A et B, 281 (21): Ai, A883–A886, ISSN0151-0509, MR0429750
