On the pro-modularity in the residually reducible case for some totally real fields

Abstract

Let $p$ be an odd prime and $F$ an abelian totally real field of even degree in which $p$ splits completely. We study the fixed-determinant pseudo-deformation ring of $1+\bar{\chi }$. Under local hypotheses and a degree bound in terms of the tame auxiliary places, we prove that every irreducible component of at least the expected dimension $1+2[F:\mathbb{Q}]$ is pro-modular---its generic pseudo-representation occurs in a big Hecke algebra---and has exactly this dimension. Under a further cohomological bound, the deformation ring of every non-split residual extension is a local complete intersection and all its primes are pro-modular. After a finite abelian totally real base change, this componentwise result may be viewed as a potential big $R=\mathbb{T}$ theorem at the level of irreducible components. The proof uses a specialization argument that constructs a one-dimensional patching prime while preserving, for a single stable lattice, both non-split residual reduction and the prescribed local conditions.

arXiv comment40 pages