English

Genus formulas for families of modular curves

Number Theory 2025-01-22 v1

Abstract

For each open subgroup HGL2(Z^)H\leq \operatorname{GL}_2(\widehat{\mathbb{Z}}), there is a modular curve XHX_H, defined as a quotient of the full modular curve X(N)X(N), where NN is the level of HH. The genus formula of a modular curve is well known for X0(N)X_0(N), X1(N)X_1(N), X(N)X(N), Xsp(N)X_{\mathrm{sp}}(N), Xns(N)X_{\mathrm{ns}}(N), and XS4(p)X_{S_4}(p) for pp prime. We explicitly work out the invariants of the genus formulas for Xsp+(N)X_{\mathrm{sp}}^+(N), Xns+(N)X_{\mathrm{ns}}^+(N), and Xarith,1(M,MN)X_{\text{arith},1}(M,MN). In Table 11, we provide the invariants of the genus formulas for all of the modular curves listed.

Keywords

Cite

@article{arxiv.2501.10883,
  title  = {Genus formulas for families of modular curves},
  author = {Asimina S. Hamakiotes and Jun Bo Lau},
  journal= {arXiv preprint arXiv:2501.10883},
  year   = {2025}
}