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Abstract The main topic of the paper is spectral theory for noncongruence subgroups of the modular group. We have studied a selection of the main conjectures in the area: the Roelcke–Selberg and Phillips–Sarnak conjectures and Selberg's conjecture on exceptional eigenvalues. The first two concern the existence and nonexistence of an infinite discrete spectrum for certain types of Fuchsian groups and last states that there are no exceptional eigenvalues for congruence subgroups, or in other words, there is a specific spectral gap in the cuspidal spectrum. Our main theoretical result states that if the corresponding surface has a reflectional symmetry which preserves the cusps then the Laplacian on this surface has an infinite number of "new" discrete eigenvalues. We define old and new spaces of Maass cusp forms for noncongruence subgroups in a way that provides a natural generalization of the usual definition from congruence subgroups and give a method for determining the structure of the old space algorithmically. In addition to the theoretical result we also present computational data, including a table of subgroups of the modular group and eigenvalues of Maass forms for noncongruence subgroups. We also present, for the first time, numerical examples of both exceptional and (non-trivial) residual eigenvalues. To be able to (even heuristically) certify lists of computed eigenvalues we also proved an explicit average Weyl's law in this setting. [ABSTRACT FROM AUTHOR]

The goal of this paper was to explain certain experimentally observed properties of the (cuspidal) spectrum and its associated automorphic forms (Maass waveforms) on the congruence subgroup Γ0(9). The first property is that the spectrum possesses multiplicities in the so-called new part, where it was previously believed to be simple. The second property is that the spectrum does not contain any ‘genuinely new’ eigenvalues, in the sense that all eigenvalues of Γ0(9) appear in the spectrum of some congruence subgroup of lower level.The main theorem in this paper gives a precise decomposition of the spectrum of Γ0(9) and in particular, we show that the genuinely new part is empty. We also prove that there exist an infinite number of eigenvalues of Γ0(9) where the corresponding eigenspace is of dimension at least two and has a basis of pairs of Hecke-Maass newforms which are related to each other by a character twist. These forms are non-holomorphic analogues of modular forms with inner twists and also provide explicit (affirmative) examples of a conjecture stating that if the Hecke eigenvalues of two ‘generic’ Maass newforms coincide on a set of primes of density ½ then they have to be related by a character twist. [ABSTRACT FROM PUBLISHER]