## Computing Igusa class polynomials.(English)Zbl 1322.11066

Summary: We bound the running time of an algorithm that computes the genus-two class polynomials of a primitive quartic CM-field $$K$$. This is in fact the first running time bound and even the first proof of correctness of any algorithm that computes these polynomials. Essential to bounding the running time is our bound on the height of the polynomials, which is a combination of denominator bounds of E. Z. Goren and K. E. Lauter [Ann. Inst. Fourier 57, No. 2, 457–480 (2007; Zbl 1172.11018)] and our own absolute value bounds. The absolute value bounds are obtained by combining Dupont’s estimates of theta constants [R. Dupont, Math. Comput. 80, No. 275, 1823–1847 (2011; Zbl 1221.65075)] with an analysis of the shape of CM period lattices (Section 8). The algorithm is basically the complex analytic method of Anne-Monika Spallek [Kurven vom Geschlecht 2 und ihre Anwendung in Public-Key-Kryptosystemen. Ph.D. thesis, Institut für Experimentelle Mathematik, Universität GH Essen (1994)] and P. van Wamelen [Math. Comput. 68, 307–320 (1999; Zbl 0906.14025)], and we show that it finishes in time $$\widetilde {O}(\Delta ^{7/2})$$, where $$\Delta$$ is the discriminant of $$K$$. We give a complete running time analysis of all parts of the algorithm, and a proof of correctness including a rounding error analysis. We also provide various improvements along the way.

### MSC:

 11G15 Complex multiplication and moduli of abelian varieties 14K22 Complex multiplication and abelian varieties 11Y40 Algebraic number theory computations

### Citations:

Zbl 1172.11018; Zbl 1221.65075; Zbl 0906.14025

### Software:

Echidna; SageMath
Full Text:

### References:

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