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arXiv · 2605.26771

Towards a generalized Maeda conjecture for modular forms with quadratic nebentypus

Debargha Banerjee·Dhrubajyoti Das·Srijan Das·Tathagata Mandal·Sudipa Mondal
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Abstract

We establish a lower bound for the number of non-CM Galois orbits of newforms in $S_k(N,\Psi)$ with non-trivial quadratic nebentypus $\Psi$ for sufficiently large weights. Extending the work of Dieulefait, Pacetti, and Tsaknias in the trivial nebentypus setting, we analyze the restrictions imposed by the quadratic character on local inertial types and determine the number of admissible Galois orbits of such types. We further prove that Atkin-Li pseudo-eigenvalues are Galois equivariant and hence, up to a natural equivalence relation, define a global Galois invariant. Together with existence results for newforms having prescribed local behavior, these invariants yield a lower bound for the number of non-CM Galois orbits by counting compatible pairs of local-global invariants. Finally, computations in small weights show that this lower bound is not always attained, indicating that certain local equivalences are not realized globally by Galois conjugation over the coefficient field of the modular form.

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Debargha Banerjee, Dhrubajyoti Das, Srijan Das, Tathagata Mandal, Sudipa Mondal. 2026-05-26. Towards a generalized Maeda conjecture for modular forms with quadratic nebentypus. https://arxiv.org/abs/2605.26771

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