Hurwitz class number

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Hurwitz class number H(N) is a variant of the usual class number for positive definite binary quadratic forms with discriminant -N. Each form is weighted by 2/g, where g is the number of symmetries (automorphisms) of the form. They define H(0) = -1/12. In 1975, Zagier showed that the Hurwitz class numbers appear as the coefficients of a mock modular form of weight 3/2.