English

On exceptional eigenvalues of the Laplacian for $\Gamma_0(N)$

Number Theory 2007-05-23 v1

Abstract

An explicit Dirichlet series is obtained, which represents an analytic function of ss in the half-plane s>1/2\Re s>1/2 except for having simple poles at points sjs_j that correspond to exceptional eigenvalues λj\lambda_j of the non-Euclidean Laplacian for Hecke congruence subgroups Γ0(N)\Gamma_0(N) by the relation λj=sj(1sj)\lambda_j=s_j(1-s_j) for j=1,2,...,Sj=1,2,..., S. Coefficients of the Dirichlet series involve all class numbers hdh_d of real quadratic number fields. But, only the terms with hdd1/2ϵh_d\gg d^{1/2-\epsilon} for sufficiently large discriminants dd contribute to the residues mj/2m_j/2 of the Dirichlet series at the poles sjs_j, where mjm_j is the multiplicity of the eigenvalue λj\lambda_j for j=1,2,...,Sj=1,2,..., S. This may indicate (I'm not able to prove yet) that the multiplicity of exceptional eigenvalues can be arbitrarily large. On the other hand, by density theorem [3] the multiplicity of exceptional eigenvalues is bounded above by a constant depending only on NN.

Keywords

Cite

@article{arxiv.math/0610120,
  title  = {On exceptional eigenvalues of the Laplacian for $\Gamma_0(N)$},
  author = {Xian-Jin Li},
  journal= {arXiv preprint arXiv:math/0610120},
  year   = {2007}
}