Explicit non-Gorenstein \(R={\mathbb {T}}\) via rank bounds I: deformation theory
摘要
Ribet has proven remarkable results about non-optimal levels of residually reducible Galois representations. We focus on a non-optimal level N that is the product of two distinct primes and where the Galois deformation ring is not expected to be Gorenstein. We prove a Galois-theoretic criterion for the deformation ring to be as small as possible—that is, for there to be a unique newform of level N with reducible residual representation. When this criterion is satisfied, we deduce an