Section: New Results
Complex multiplication and modularity
Participants : Jean-Marc Couveignes, Andreas Enge, Nicolas Mascot, Enea Milio, Aurel Page, Damien Robert.
A. Enge and E. Thomé describe in [20]
a quasi-linear algorithm for computing Igusa class polynomials
of Jacobians of genus 2 curves via complex floating-point approximations
of their roots. After providing an explicit treatment of the computations
in quartic CM fields and their Galois closures, they pursue an approach due
to Dupont for evaluating
In [34] E. Milio explains how to generalise the work of Régis Dupont for computing modular polynomials in dimension 2 to invariants derived from theta constants. Modular polynomials have many applications. In particular, they could speed up the CRT-algorithm to compute class fields of degree 4 CM-fields which would lead to faster algorithms to construct cryptographically secure Jacobians of hyperelliptic curves. They are also used to compute graphs of isogenies. This paper presents how to compute modular polynomials and the polynomials computed and then it proves some of their properties.
With F. Morain, A. Enge has determined exhaustively under which conditions
“generalised Weber functions”, that is, simple quotients of
N. Mascot has continued his work on computing Galois representations attached to Jacobians of modular curves. He has given tables of modular Galois representations in [33] obtained using the algorithm of [39] . He has computed Galois representations modulo primes up to 31 for the first time. In particular, he has computed the representations attached to a newform with non-rational (but of course algebraic) coefficients, which had never been done before. These computations take place in the Jacobians of modular curves of genus up to 26.