Existence of $p$-adic representations and flatness of even deformation rings in balanced global settings of rank one (2309.01871v1)

Abstract: The main goal of this paper is to prove existence of $p$-adic lifts of even Galois representations in balanced global settings with dual Selmer groups of rank one, and to construct examples of even residual representations in characteristic $p=3$ for which the theorem yields existence of $3$-adic lifts. In the proof of the main theorem, we show that if the global deformation rings are not flat over $\mathbb{Z}p$ at the minimal level, then after allowing ramification at one auxiliary prime, the new parts of the global rings enjoy the flatness property. The proof is independent of the parity of the representations. Previously, flatness was only known in established cases of Langlands reciprocity in the odd parity, while it is new for even deformation rings in balanced global settings of rank one. As a corollary, we show that by choosing the auxiliary prime appropriately, we can control the rank mod $p$ of the new part of the flat ring. The examples of $3$-adic representations and flat, even deformation rings are built by applying techniques from global class field theory to a family of totally real fields studied by D. Shanks. In the process of constructing the even residual representations, we show that there exist cuspidal automorphic representations $\pi$ and $\pi'$ of $\operatorname{GL}(2,\mathbb{A}{\mathbb{Q}})$ attached to classical Maass wave forms of level $\ell=20887$ whose mod 3 reductions are distinct. The proof applies a theorem of R. P. Langlands from his theory of base change.