Monday, 16 April 2007

nt.number theory - Is there an R=T type result for modular forms with additive reduction?

If you fix a prime ellell, and consider the Galois action of the decomposition group DpDp on the (rational) ellell-adic Tate module (here "rational" means tensored with mathbbQellmathbbQell),
then (in a standard way, due to Deligne) you can convert this action into a representation
of the Weil--Deligne group, and so in particular of the Weil group. Restricting to the
inertia group, you get a representation of the inertia group IpIp, known as the inertial type tautau. It is independent of ellell. (The only reason to detour through the Weil--Deligne group is to deal with possibly infinite image of tame inertia; if the elliptic curve has potentially good reduction, then we can skip this step and just take the representation of IpIp on the ellell-adic Tate module, which has finite image and is independent of ellell.)



[Added: In the above, one should insist that ellneqpellneqp. If ell=pell=p, then one can also arrive at a Weil-Deligne representation, and hence
inertial type, which is the same as the one obtained as above for ellneqpellneqp, but to do this one must use Fontaine's theory: one forms the DpstDpst of the rational pp-adic Tate module,
which then can be converted into a Weil--Deligne representation in a standard way,
and hence gives an inertial type.]



Now one can look at the deformation ring R[0,1],taurhoR[0,1],taurho parameterizing lifts
of rhorho of which at pp are of inertial type tautau and Hodge--Tate weights 00 and 11. (See Kisin's recent
JAMS paper about potentially semi-stable deformation rings.)



[Added: Here ell=pell=p,
i.e. we are looking at pp-adic deformations of rhorho which are potentially semi-stable
at pp, and whose inertial type, computed via DpstDpst as in the above added remark,
is equal to tautau. But note that, by the preceding discussion, these deformations do precisely capture the idea of lifts of rhorho having the same "reduction type" as
the original elliptic curve EE.]



Let's suppose that EE really does have potentially good reduction. Then Kisin's "moduli of finite flat group schemes" paper shows that any lift parameterized by R[0,1],tauR[0,1],tau is modular. This shows that R[0,1],tau=mathbbT,R[0,1],tau=mathbbT, for an appropriately chosen
mathbbTmathbbT.



One thing to note: unlike in the original Taylor--Wiles setting, from this statement one doesn't get quantitive information about adjoint Selmer groups, and one doesn't get any simple interpretation of what this R=mathbbTR=mathbbT theorem means on the integral level.
(In other words, Artinian-valued points of R[0,1],tauR[0,1],tau have no simple interpretation in terms of a ramification condition at pp; this is related to the fact that the theory
of DpstDpst only applies rationally, i.e. to mathbbQpmathbbQp-representations, not integrally,
i.e. not to representations over mathbbZpmathbbZp or over Artin rings.)

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