Abstract
Let Γ ⊂ SL 2(Z) be a congruence subgroup, and λ0 = 0 < λ1 < … be the eigenvalues of the non-euclidean Laplacian on L 2(Γ\H 2). A fundamental conjecture of Selberg ([Se]) asserts that the smallest nonzero eigenvalue λ1(Γ) ≥1/4 = 0.25. In the same paper Selberg proved that λ1(Γ) ≥3/16 = 0.1875. Gelbart and Jacquet ([GJ]), using very different methods, improved this to λ1(Γ) > 3/16. Iwaniec ([I]) showed that for almost all Hecke congruence groups Γ0(p) with a certain multiplier χ p , one has λ1(Γ0(p), χ p ) ≥ 44/225 = 0.19555…. In [I], he also established a density theorem for possible exceptional eigenvalues as above, which while not giving any improvement on 3/16 for an individual Γ is sufficiently strong to substitute for Selberg’s conjecture in many applications to number theory. Selberg’s conjecture is the archimedean analogue of the “Ramanujan Conjectures” on the Fourier coefficients of Maass forms. For these, much progress has been made in improving the relevant estimates, beginning with Serre ([Ser]) and later on Shahidi ([Sh2]) and Bump-Duke-Hoffstein-Iwaniec ([BDHI]). In this paper we restore the balance and establish in part for the archimedean place what is known at the finite places. The method on the face of it is quite different, but the quality of the results coincide (the reason will be made clear later).
Keywords
Congruence Subgroup Dirichlet Character Maass Form Cuspidal Automorphic Representation Archimedean ComponentPreview
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