Wednesday, 17 February 2010

modular forms - Galois representations attached to newforms

Suppose that f is a weight k newform for Gamma1(N) with attached p-adic Galois representation rhof. Denote by rhof,p the restriction of rhof to a decomposition group at p. When is rhof,p semistable (as a representation of
mathrmGal(overlinemathbfQp/mathbfQp)?



To make things really concrete, I'm happy to assume that k=2 and that the q-expansion of f lies in mathbfZ[[q]].



Certainly if N is prime to p then rhof,p is in fact crystalline, while
if p divides N exactly once then rhof,p is semistable (just thinking about the Shimura construction in weight 2 here, and the corresponding reduction properties of X1(N)
over mathbfQ at p). For N divisible by higher powers of p, we know that these representations are de Rham, hence potentially semistable. Can we say more? For example,
are there conditions on "numerical data" attached to f (e.g. slope, p-adic valuation of N, etc.) which guarantee semistability or crystallinity over a specific
extension? Can we bound the degree and ramification of
the minimal extension over which rhof,p becomes semistable in terms of numerical
data attached to f? Can it happen that N is highly divisible by p and yet rhof,p is semistable over mathbfQp?



I feel like there is probably a local-Langlands way of thinking about/ rephrasing this question, which may be of use...



As a possible example of the sort of thing I have in mind: if N is divisible by p and f is ordinary at p then rhof,p becomes semistable over an abelian extension of
mathbfQp
and even becomes crystalline over such an extension provided that the Hecke eigenvalues
of f for the action of mup1subseteq(mathbfZ/NmathbfZ)times via the diamond operators
are not all 1.

No comments:

Post a Comment