On irreducibility of six-dimensional compatible systems of $\mathbb{Q}$
Boyi Dai
TL;DR
The paper proves that a 6-dimensional $E$-rational regular strictly compatible system over $ ext{Q}$ cannot have widespread reducibility: if one member is irreducible, then all but finitely many are irreducible. It distinguishes between non–Lie-irreducible and Lie-irreducible cases, using induction/restriction arguments, density results, and potential automorphy (BLGGT14) to propagate irreducibility and preserve semisimple types across $ ext{λ}$. For pure essentially self-dual regular systems, the authors show a finite decomposition into components that can be extended to strictly compatible subsystems, aligning with conjectures about big image and automorphy. The approach relies on $ ext{λ}$-independence of monodromy groups, semisimple reductions, and automorphy lifting to establish structural rigidity across the whole family, with implications for understanding the arithmetic of high-dimensional Galois representations.
Abstract
We study irreducibility of 6-dimensional strictly compatible systems of $\mathbb{Q}$ with distinct Hodge-Tate weights. We prove if one of the representations is irreducible, then all but finitely many of them are irreducible.
