A p-adic algorithm to compute the Hilbert class polynomial

Classicaly, the Hilbert class polynomial P∆ ∈ Z[X] of an imaginary quadratic discriminant ∆ is computed using complex analytic techniques. In 2002, Couveignes and Henocq [5] suggested a p-adic algorithm to compute P∆. Unlike the complex analytic method, it does not suffer from problems caused by rounding errors. In this paper we complete the outline given in [5] and we prove that, if the Generalized Riemann Hypothesis holds true, the expected runtime of the p-adic algorithm is e O(|∆|). We illustrate the algorithm by computing the polynomial P −639 using a 643-adic algorithm.