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Minimally ramified deformations when
$\ell \neq p$
- Part of
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- Journal:
- Compositio Mathematica / Volume 155 / Issue 1 / January 2019
- Published online by Cambridge University Press:
- 12 November 2018, pp. 1-37
- Print publication:
- January 2019
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- Article
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Let
$p$ and 
$\ell$ be distinct primes, and let 
$\overline{\unicode[STIX]{x1D70C}}$ be an orthogonal or symplectic representation of the absolute Galois group of an 
$\ell$-adic field over a finite field of characteristic 
$p$. We define and study a liftable deformation condition of lifts of 
$\overline{\unicode[STIX]{x1D70C}}$ ‘ramified no worse than 
$\overline{\unicode[STIX]{x1D70C}}$’, generalizing the minimally ramified deformation condition for 
$\operatorname{GL}_{n}$ studied in Clozel et al. [Automorphy for some
$l$-adic lifts of automorphic mod
$l$Galois representations, Publ. Math. Inst. Hautes Études Sci. 108 (2008), 1–181; MR 2470687 (2010j:11082)]. The key insight is to restrict to deformations where an associated unipotent element does not change type when deforming. This requires an understanding of nilpotent orbits and centralizers of nilpotent elements in the relative situation, not just over fields.
