Comparison of integral structures on spaces of modular forms of weight two, and computation of spaces of forms mod 2 of weight one, with appendices by JeanFrancois Mestre and Gabor Wiese
Abstract
Two integral structures on the Qvector space of modular forms of weight two on X_0(N) are compared at primes p exactly dividing N. When p=2 and N is divisible by a prime that is 3 mod 4, this comparison leads to an algorithm for computing the space of weight one forms mod 2 on X_0(N/2). For p arbitrary and N>4 prime to p, a way to compute the Hecke algebra of mod p modular forms of weight one on Gamma_1(N) is presented, using forms of weight p, and, for p=2, parabolic group cohomology with mod 2 coefficients. Appendix A is a letter from Mestre to Serre, of October 1987, where he reports on computations of weight one forms mod 2 of prime level. Appendix B reports on an implementation for p=2 in Magma, using Stein's modular symbols package, with which Mestre's computations are redone and slightly extended.
 Publication:

arXiv Mathematics eprints
 Pub Date:
 December 2003
 arXiv:
 arXiv:math/0312019
 Bibcode:
 2003math.....12019E
 Keywords:

 Mathematics  Number Theory;
 Mathematics  Algebraic Geometry;
 14G35 (Primary) 11F11;
 11Y40 (Secondary)
 EPrint:
 39 pages, Latex