ON LIFTING AND MODULARITY OF REDUCIBLE RESIDUALGALOIS REPRESENTATIONS OVER IMAGINARY QUADRATICFIELDSTOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Abstract. In this paper we study deformations of mod p Galois representations τ (over an imaginary quadratic field F ) of dimension 2 whose semisimplification is the direct sum of two characters τ1 and τ2 . As opposed to[BK13] we do not impose any restrictions on the dimension of the crystalline
1 (F, Hom(τ , τ )) ⊂ Ext1 (τ , τ ). We establish that there exSelmer group HΣ
2 1
2 1
1 (F, Hom(τ , τ )) arising from automorphic representations
ists a basis B of HΣ2 1over F (Theorem 8.1). Assuming among other things that the elements of B
admit only finitely many crystalline characteristic 0 deformations we prove a
modularity lifting theorem asserting that if τ itself is modular then so is its
every crystalline characteristic zero deformation (Theorems 8.2 and 8.4).

1. Introduction
Let p be an odd prime. Let F be a number field, Σ a finite set of primes of F(containing all primes p of F lying over p) and GΣ the Galois group of the maximal
extension of F unramified outside Σ. Let E be a finite extension of Qp with ringof integers O and residue field F. Let τ1 , τ2 : GΣ → GLni (F) be two absolutely
irreducible mutually non-isomorphic representations with n1 + n2 = n, which we
assume lift uniquely to crystalline representations τ̃i : GΣ → GLni (O).
The aim of this article is to study deformations of non-semi-simple continuous
crystalline representations τ : GΣ → GLn (F) whose semi-simplification is τ1 ⊕ τ2 inthe case n = 2 and F is an imaginary quadratic field. We analyzed this deformation
problem in [BK13] under the additional assumption that HΣ1 (F, Hom(τ2 , τ1 )) is onedimensional (which is equivalent to saying that there exists only one such τ up toisomorphism). Here HΣ1 denotes the subgroup of H 1 consisting of classes unramified
outside Σ and crystalline at all p | p. In this paper we do not make any assumption
on this dimension. Disposing of the “dim=1” assumption is more than a technicality
as in the general case one can no longer expect to be able to identify the universal
deformation ring with a Hecke algebra.
This question was studied by Skinner and Wiles for n = 2 and totally real fields
F in the seminal paper [SW99]. In that paper the authors analyze primes q of the(ordinary) universal deformation ring Rτ of τ and prove that they are ‘pro-modular’in the sense that the trace of the deformation corresponding to Rτ  Rτ /q occurs
in the Hecke algebra T. In particular no direct identification of Rτ and T is made.
In this article we take a different approach and work with the reduced universal
deformation ring Rτred and its ideal of reducibility. In the “dim=1”-case, the authors
Date: 2 April 2013.
2000 Mathematics Subject Classification. 11F80, 11F55.
Key words and phrases. Galois deformations, automorphic forms.
1

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TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

proved (as a consequence of an R = T -theorem - Theorem 9.14 in [BK13]) that Rτred
is a finitely generated Zp -module. In contrast, if dim HΣ1 (F, Hom(τ2 , τ1 )) > 1, while
there are only finitely many automorphic representations whose associated Galois
representations are deformations of τ , the ring Rτred may potentially be infinite overZp (Remark 2.17). This is a direct consequence of the existence of linearly independent cohomology classes inside the Selmer group which can be used to constructnon-trivial lifts to GL2 (F[[X]]). The resulting (potentially large) characteristic p
components of Rred do not arise from automorphic representations and in this paper we will ignore them by considering the image of Rτred inside Rτred ⊗Zp Qp (which
we will denote by Rτ0 ) instead of Rτred itself. It is however possible that by doing so
we are excluding some characteristic p deformations whose traces may be modular
in the sense of [CMar] (i.e. arise from torsion Betti cohomology classes).
On the other hand, as opposed to the situation studied in [SW99], over animaginary quadratic field there are no reducible deformations to characteristic zero
which in turn is a consequence of the finiteness of the Bloch-Kato Selmer groupHΣ1 (F, Hom(τ̃2 , τ̃1 ) ⊗ Qp /Zp ) (Lemma 2.20), where τ̃1 , τ̃2 are (unique) lifts to characteristic zero of τ1 and τ2 respectively.
While each τ may possess non-modular reducible characteristic p deformations,the situation is complicated further by the fact that in general many τ ’s do not
admit any modular deformations at all (this phenomenon does not arise in the“dim=1” case). Indeed, first note that two extensions in Ext1GΣ (τ2 , τ1 ) define thesame representation of GΣ if and only if they are (non-zero) scalar multiples ofeach other. In particular, if dimF Ext1GΣ (τ2 , τ1 ) = 1, then there is a unique nonsemi-simple representation of GΣ with semi-simplification τ1 ⊕ τ2 . (Similarly, if
dimF HΣ1 (F, Hom(τ2 , τ1 )) = 1 then there exists a unique crystalline such represenm−1
non-isomorphic
tation.) However, if dimF Ext1GΣ (τ2 , τ1 ) = m, then there are qq−1
such representations where q = #F. This demonstrates that in general not all
reducible representations τ can be modular (of a particular level and weight), as
the number of such characteristic zero automorphic forms is fixed (in particular
it is independent of making a residue field extension). Nevertheless, we are able
to prove (see Corollary 4.8) that there exists an F- basis B of HΣ1 (F, Hom(τ2 , τ1 ))arising from modular forms. For this we combine a congruence ideal bound for a
Hecke algebra with the upper bound on the Selmer group of Hom(τ̃2 , τ̃1 ) predictedby the Bloch-Kato conjectures.
Let Rτtr,0 be the image in Rτ0 of the subalgebra generated by traces of Rτred for τ
arising from a modular form. As pointed out we can extend the set consisting of τ
to a modular basis B := {τ 1 = τ, τ 2 , . . . , τ s } of HΣ1 (F, Hom(τ2 , τ1 )). Our ultimate
goal is to show that it is possible to identify Rτtr,0 with the quotient Tτ of a Hecke
algebra T. Here the quotient Tτ corresponds to automorphic forms for which there
exists a lattice in the associated Galois representation with respect to which themod p reduction equals τ .
To prove our main modularity lifting theorem (Theorem 8.2) we work under thefollowing two assumptions. On the one hand we assume that the modular basis
B is unique in the sense that any other such consists of scalar multiples of theelements of B. On the other hand we assume that Rτ0 is a finitely generated Zp module for all τ ∈ B. The first assumption can be replaced with the assumptionthat the Bloch-Kato Selmer group HΣ1 (F, Hom(τ̃2 , τ̃1 ) ⊗ Qp /Zp ) is annihilated by p
(Theorem 8.4). This second result is in a sense ‘orthogonal’ to the main results of

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

3[BK11] and [BK13], where the same Selmer group is assumed to be cyclic, but ofarbitrary finite order.
Our approach relies on simultaneously considering all the deformation problems
for representations τ i (i = 1, 2, . . . , s). As in [BK13] we first study “reducible”
tr,0
deformations via the quotients Rτtr,0
for the reducibility ideal Iτtr,0of the tracei /Iτ ii0of the universal deformation into GL2 (Rτ ) as defined by Bellaı̈che and Chenevier.
These ideals are the analogues of Eisenstein ideals Jτi on the Hecke algebra side.
Q
tr,0
To relate # i Rτtr,0
to the order of a Bloch-Kato Selmer group we make use
i /Iτ iof a lattice construction of Urban (Theorem 1.1 of [Urb01], see Theorem 4.1 inthis paper). In fact it is a repeated application of Urban’s theorem (on the Hecke
side and on the deformation side) that allows us to prove a modularity lifting
theorem. We show that when the upper bound on the Selmer group and the lower
bound on the congruence ideal agree (which in many cases is a consequence of the
Bloch-Kato conjecture), this implies that every reducible deformation which lifts
to characteristic zero of every τ i is modular (cf. section 6). It is here that we
make use of the assumption on the ‘uniqueness’ of B to be able to use a result ofKenneth Kramer and the authors [BKK13] on the distribution of Eisenstein-type
congruences among various residual isomorphism classes of Galois representations(cf. Section 5). Yet another application of Urban’s Theorem allows us to prove theexistence of a deformation to GL2 (Rτtr,0 ) and as a consequence to identify Rτtr,0 withRτ0 (Theorem 6.2). Using the fact that the ideal of reducibility of Rτred and hence
also of Rτ0 is principal (Proposition 7.1) and applying the commutative algebra
criterion (Theorem 4.1 in [BK13]) we are finally able to obtain an isomorphismRτred ∼= Tτ and thus a modularity lifting theorem (Theorems 8.2 and 8.4).
Throughout the paper we work in a slightly greater generality than necessary
for the imaginary quadratic case to stress that our results apply in a more generalcontext if one assumes some standard conjectures. However, in section 8 we gather
all the assumptions in the imaginary quadratic case as well as the statements ofthe main theorems (Theorems 8.1, 8.2 and 8.4) in this context. Hence the reader
may refer directly to that section for the precise (self-contained) statements of themain results of the paper in that case.
2. Deformation rings
Let F be a number field and p > 2 a prime with p - # ClF and p unramified in
F/Q. Let Σ be a finite set of finite places of F containing all the places lying overp. Let GΣ denote the Galois group Gal(FΣ /F ), where FΣ is the maximal extensionof F unramified outside Σ. For every prime q of F we fix compatible embeddings
F ,→ F q ,→ C and write Dq and Iq for the corresponding decomposition and inertia
subgroups of GF (and also their images in GΣ by a slight abuse of notation). Let
E be a (sufficiently large) finite extension of Qp with ring of integers O and residue
field F. We fix a choice of a uniformizer $.
2.1. Deformations. Denote the category of local complete Noetherian O-algebras
with residue field F by LCN(E). Let m be any positive integer. Supposer : GΣ → GLm (F)is a continuous homomorphism.
We recall from [CHT08] p. 35 the definition of a crystalline representation:
Let p | p and A be a complete Noetherian Zp -algebra. A representation ρ : Dp →

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TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

GLn (A) is crystalline if for each Artinian quotient A0 of A, ρ⊗A0 lies in the essential
image of the Fontaine-Lafaille functor G (for its definition see e.g. [BK13] Section
5.2.1). We also call a continuous finite-dimensional GΣ -representation V over Qp(short) crystalline if, for all primes p | p, Fil0 D = D and Filp−1 D = (0) for thefiltered vector space D = (Bcrys ⊗Qp V )Dp defined by Fontaine (for details see[BK13] Section 5.2.1).
A (crystalline) O-deformation of r is a pair consisting of A ∈ LCN(E) and astrict equivalence class of continuous representations r̃ : GΣ → GLm (A) that are
crystalline at the primes dividing p and such that r = r̃ (mod mA ), where mA is the
maximal ideal of A. (So, in particular we do not impose on our lifts any conditionsat primes in Σ \ Σp ). Later we assume that if q ∈ Σ, then #kq 6≡ 1 (mod p),
which means that all deformations we consider will trivially be “Σ-minimal”. As iscustomary we will denote a deformation by a single member of its strict equivalenceclass.
If r has a scalar centralizer then the deformation functor is representable byRr ∈ LCN(E) since crystallinity is a deformation condition in the sense of [Maz97].
We denote the universal crystalline O-deformation by ρr : GΣ → GLm (Rr ). Thenfor every A ∈ LCN(E) there is a one-to-one correspondence between the set of
O-algebra maps Rr → A and the set of crystalline deformations r̃ : GΣ → GLm (A)of r.
For j ∈ {1, 2} let τj : GΣ → GLnj (F) be an absolutely irreducible continuous
representation. Assume that τ1 ∼6 τ2 . Consider the set of isomorphism classes of=n-dimensional residual (crystalline at all primes p | p) representations of the form:


τ1 ∗
(2.1)
τ=
: GΣ → GLn (F),τ2which are non-semi-simple (n = n1 + n2 ).
From now on assume p - n!.
Lemma 2.1. Every representation τ of the form (2.1) has scalar centralizer.
Proof. This is easy.



We write Rτred for the quotient of Rτ by its nilradical and ρred
τ for the corresponding universal deformation, i.e. the composite of ρτ with Rτ  Rτred . We further
write Rτtr ⊂ Rτred for the closed O-subalgebra of Rτred generated by the set
S := {tr ρτ (Frobq ) | q 6∈ Σ}.
Lemma 2.2. Write m for the maximal ideal of Rτred . The O-algebra Rτtr is completewith respect to its maximal ideal mtr := m ∩ Rτtr and local. Moreover, Rτtr /mtr = F.
Proof. First note that mtr is a maximal ideal of Rtr . Indeed, as a contraction of aprime ideal it is clearly prime, so Rτtr /mtr is a domain which injects into Rτred /m.
Since the latter is a finite field, Rτtr /mtr must also be a finite field. Moreover, byTheorem 8.1 in [Mat89] we know that Rτtr is complete with respect to mtr . Thus
by [Eis95], p. 183, Rτtr must be local. The last assertion follows from the fact thatRτtr is an O-algebra.

2.2. Pseudo-representations and pseudo-deformations. We next recall the
notion of a pseudo-representation (or pseudo-character) and pseudo-deformations(from [BC09] Section 1.2.1 and [Böc11] Definition 2.2.2).

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

5Definition 2.3. Let A be a topological ring and R and A-algebra. A (continuous) A-valued pseudo-representation on R of dimension d, for some d ∈ N>0 , is acontinuous function T : R → A such that(i) T (1) = d and d! is a non-zero divisor of A;(ii) T is central, i.e. such that T (xy) = T (yx) for all x, y ∈ R;(iii) d is minimal such that Sd+1 (T )(x) = 0, where, for every integer N ≥ 1,
SN (T ) : RN → A is given byX
(σ)T σ (x),SN (T )(x) =σ∈SNwhere for a cycle σ = (j1 , . . . jm ) we define T σ ((x1 , . . . xd+1 )) =Q
T (xj1 · · · xjm ),rand for aQgeneral permutation σ with cycle decomposition i=1 σi we letrσσi
T (x) = i=1 T (x).
In the case when R = A[GΣ ] we will also call the restriction of T to GΣ a pseudorepresentation.
We note that if ρ : A[GΣ ] → Mn (A) is a morphism of A-algebras then tr ρ is a
pseudo-representation of dimension n (see [BC09] Section 1.2.2).
According to [BC09] Section 1.2.1, if T : R → A is a pseudo-representation ofdimension d and A0 an A-algebra, then T ⊗ A0 : R ⊗ A0 → A0 is again a pseudorepresentation of dimension d.
Following [SW99] (see also [Böc11] Section 2.3) we define a pseudo-deformationof tr τ1 + tr τ2 to be a pair (T, A) consisting of A ∈ LCN(E) and a continuous
pseudo-representation T : GΣ → A such that T = tr τ1 + tr τ2 (mod mA ), wheremA is the maximal ideal of A.
By [SW99] Lemma 2.10 (see also [Böc11] Proposition 2.3.1) there exists a universal pseudo-deformation ring Rps ∈ LCN(E) and we write T ps : GΣ → Rps forthe universal pseudo-deformation. For every A ∈ LCN(E) there is a one-to-one
correspondence between the set of O-algebra maps Rps → A (inducing identity onF) and the set of pseudo-deformations T : GΣ → A of tr τ1 +tr τ2 . Any deformationof a representation τ as in (2.1) gives rise (via its trace) to a pseudo-deformation oftr τ1 + tr τ2 , so there exists a unique O-algebra map Rps → Rτ such that the traceof the deformation equals the composition of T ps with Rps → Rτ .
Lemma 2.4. The image of Rps → Rτred is Rτtr .
Proof. This is clear (cf. [CV03] Theorem 3.11) since Rps is topologically generated
by T (Frobp ) (and Rτtr is closed).

Corollary 2.5. The ring Rτtr is an object in the category LCN(E).
Proof. As a quotient of Rps the ring Rτtr is Noetherian. So, the claim follows from
Lemma 2.2.

2.3. Selmer groups. For a p-adic GΣ -module M (finitely generated or cofinitely
generated over O - for precise definitions cf. [BK13], section 5) we define the Selmer
1group HΣ1 (F, M ) to be the subgroup of Hcont(FΣ , M ) consisting of cohomology
classes which are crystalline at all primes p of F dividing p. Note that we place norestrictions at the primes in Σ that do not lie over p. For more details cf. [loc.cit.].
We are now going to state our assumptions. The role of the first one is to rigidify
the problem of deforming the representations τj appearing on the diagonal of the

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TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

residual representations. The role of the second is to rule out characteristic zero
upper triangular deformations.
Assumption 2.6. Assume that Rτj = O and denote by τ̃j the unique lifts of τj
to GLnj (O).
Assumption 2.7 (“Bloch-Kato conjecture”). One has the following bound:#HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O) ≤ #O/L,
for some non-zero L ∈ O.
Remark 2.8. In applications the constant L will be the special L-value at zero of
the Galois representation HomO (τ̃2 , τ̃1 ) divided by an appropriate period.
For the remainder of this section we will work under the above two assumptions.
2.4. Ideal of reducibility. Let A be a Noetherian Henselian local (commutative)
ring with maximal ideal mA and residue field F and let R be an A-algebra. We
recall from [BC09] Proposition 1.5.1 the definition of the ideal of reducibility of a(residually multiplicity free) pseudo-representation T : R → A of dimension n, forwhich we assume thatT = tr τ1 + tr τ2 mod mA
Definition 2.9 ([BC09] Definition 1.5.2). The ideal of reducibility of T is thesmallest ideal I of A such that T mod I is the sum of two pseudo-characters T1 , T2
with Ti = tr τi mod mA . We will denote it by IT .
Definition 2.10. We will write I ps ⊂ Rps for the ideal of reducibility of the universal pseudo-deformation T ps : Rps [GΣ ] → Rps , Iτ ⊂ Rτ for the ideal of reducibility of: Rτred [GΣ ] →tr ρτ : Rτ [GΣ ] → Rτ , Iτred ⊂ Rτred for the ideal of reducibility of tr ρred
τ
tr
tr
red
tr
red
Rτ and Iτ for the ideal of reducibility of tr ρτ : Rτ [GΣ ] → Rτ .
Lemma 2.11. Let I0 be the smallest closed ideal of Rps containing the set{T ps (Frobv ) − tr τ̃1 (Frobv ) − tr τ̃2 (Frobv ) | v 6∈ Σ}.
Then I0 equals the ideal of reducibility I ps ⊂ Rps .
Proof. By the Chebotarev density theorem we get T ps = tr τ̃1 +tr τ̃2 (mod I0 ), hence
I0 ⊃ I ps . Conversely, we know from the definition of the ideal of reducibility thatT ps (mod I ps ) is given by the sum of two pseudo-characters reducing to tr τi . By
assumption 2.6 and Theorem 7.6 of [BK13] (see also [Böc11] Theorem 2.4.1) thesetwo pseudo-characters must equal tr τ̃i mod(I). This shows that I ps ⊃ I0 .
Corollary 2.12. The quotient Rps /I ps is cyclic.

Remark 2.13. Combined with Lemma 7.11 of [BK13] this shows that for any
pseudo-deformation T : A[GΣ ] → A of tr τ1 + tr τ2 with ideal of reducibility IT for
which the corresponding map Rps → A is surjective, the quotient A/IT is cyclic.
Proposition 2.14. The module Rτ /Iτ is a torsion O-module.
Proof. Let R+ ⊂ Rτ /Iτ be the image of the structure map O → Rτ /Iτ . Fix
σ ∈ Z+ and consider the quotient R := Rτ /(Iτ + $σ Rτ ). Suppose that Rτ /Iτ is
not torsion. This implies that R+ = O and A = O/$σ ⊂ R, where A is the imageof the structure map O → R. We begin with the following lemma.

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

7

Lemma 2.15. There exists an O-submodule B ⊂ R such that
R=A⊕B
as O-modules.
Proof. This follows from the following result.
Lemma 2.16 (Lemma 6.8(ii), p.222 in [Hun80]). Let A0 be a module over a PID
R0 such that pn A0 = 0 and pn−1 A0 6= 0 for some prime p ∈ R0 and a positive integer
n. Let a be an element of A0 of order pn . Then there is a submodule C 0 of A0 suchthat A0 = R0 a ⊕ C 0 .
Apply Lemma 2.16 for R0 = O, A0 = R, p = $, n = σ, a = ψ(1). Then
R a = A.
0

We now finish the proof of Proposition 2.14. Let e be an O-module generatorof A. Write ρI : G → GLn (R) for the deformation corresponding to the canonicalmap Rτ  R. Then we can writeτ̃ αe + β
ρI = 1,τ̃2
where α : G → Mn1 ×n2 (O) and β : G → Mn1 ×n2 (B) are maps (here we identify τ̃jwith its composition with O → R). Defineτ̃1 (g) α(g)e+ρI : G → GLn (A) g 7→.
τ̃2 (g)
We must check that ρ+
I is a homomorphism. This follows easily from the fact that
ρI is a homomorphism and the fact that A is a direct summand of R. Moreover,note that the image of α is not contained in Mn1 ×n2 ($O) because ρI reduces to τ
which is not semi-simple.
σ
Note that ρ+
I is an upper-triangular deformation into GLn (O/$ ). Moreover,+1
since ρI reduces to τ , it gives rise to an element in HΣ (F, HomO (τ̃2 , τ̃1 ) ⊗ E/O)
which generates an O-submodule isomorphic to O/$σ . Since σ was arbitrary weconclude that HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗ E/O) must be infinite which contradicts Assumption 2.7. This concludes the proof of Proposition 2.14.

Remark 2.17. If dimF HΣ1 (F, Hom(τ2 , τ1 )) = 1 then Rτ /Iτ and Rτred /Iτred are
cyclic O-modules by Corollary 7.12 in [BK13] which combined with Proposition 2.14
implies finiteness of Rτ /Iτ and Rτred /Iτred . On the other hand if dimF HΣ1 (F, Hom(τ2 , τ1 )) >1 it is easy to construct an upper-triangular (not necessarily crystalline) lift of τ
to F[[X]] which would suggest that in general Rτ /Iτ , and even Rτred /Iτred (since
F[[X]] is reduced), may have positive Krull dimension. Indeed, to see this, let f
be a cohomology class corresponding to τ and let g be a cohomology class linearly
independent from f . Then the representation
τ1 τ2 (f + gX)ρ=0
τ2
is a non-trivial lift of τ to GLn (F[[X]]). In particular there is no guarantee thatRτred is a finitely generated O-module. Since our method of proving modularity
relies on that property we will restrict in the following section to the ‘characteristic
zero’ part of Rτred of which we will demand that it is finite over O.

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TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

2.5. The ring Rτ0 . Write Rτ0 for the image of Rτ inside
Q Rτ ⊗O E. Equivalently(since E is “sufficiently large”), Rτ0 is the image of Rτ in p∈P(τ ) O, where P(τ ) :={p ∈ Spec(Rτ ) | Rτ /p = O}. It is clear that Rτ0 is an object in LCN(E). Notethat the canonical surjection Rτ  Rτ0 factors through Rτred . Write ρ0τ for thecomposition of ρτ with the map ϕτ : Rτ  Rτ0 . Write Iτ0 for the ideal of reducibilityof tr ρ0τ . By [BK13], Lemma 7.11, we have ϕτ (Iτ ) ⊂ Iτ0 (in fact equality holds since
the opposite inclusion is obvious) and thus ϕτ induces a surjection Rτ /Iτ  Rτ0 /Iτ0 .
Lemma 2.18. If Rτ0 is finitely generated as an O-module, then Rτ0 /Iτ0 is finite.
Proof. This follows immediately from Proposition 2.14 and the surjectivity of Rτ /Iτ →
Rτ0 /Iτ0 .
Define Rτtr,0 ⊂ Rτ0 to be the closed O-subalgebra generated by the set
S := {tr ρ0τ (Frobq ) | q 6∈ Σ}.
Lemma 2.19. Write m for the maximal ideal of Rτ0 . The ring Rτtr,0 is completewith respect to its maximal ideal m0 := m ∩ Rτtr,0 and local. Moreover, one hasRτtr,0 = ϕτ (Rτtr ). Thus Rτtr,0 is an object in the category LCN(E).
Proof. The first part and the fact that the residue field of Rτtr,0 is F is proved exactlyas Lemma 2.2. For the second part, note that it is clear that Rτtr,0 ⊂ ϕτ (Rτtr ). On
the other hand S ⊂ ϕτ (Rτtr ), so the equality holds because S is dense in Rτtr,0 . 
We will write Iτtr,0 ⊂ Rτtr,0 for the ideal of reducibility of tr ρ0τ . By Lemma 2.19
and Lemma 7.11 in [BK13] we get that ϕ(Iτtr ) ⊂ Iτtr,0 (in fact equality holds) andthus ϕ induces a surjection Rτtr /Iτtr  Rτtr,0 /Iτtr,0 . By Remark 2.13 the quotientRτtr,0 /Iτtr,0 is a cyclic O-module.
2.6. Generic irreducibility of ρ0τ .
Lemma 2.20. For any τ as in (2.1) if Rτ0 is finitely generated as a module
Q overO, then ρ0τ ⊗Rτ0 F is irreducible. Here F is any of the fields Fs in Fτ0 = s Fs ,where Fτ0 is the total ring of fractions of Rτ0 .
Proof. First note that since Rτ0 is a finitely generated O-module and since E isassumed to be sufficiently large we may assume that all of the fieldss are equal
LFsto E. If any of the representations ρ0τ ⊗Rτ0 F is reducible write ρ = j=1 ρj for its
semi-simplification with each ρj irreducible, j = 1, 2, . . . , s. Then by compactnessof GΣ for each 1 ≤ j ≤ s there exists a GΣ -stable O-lattice inside the representation
space of ρj . This implies that tr ρj (σ) ∈ O for all σ ∈ GΣ and all 1 ≤ j ≤ s. Hencetr ρ splits over O into the sum of traces of ρj . Since ρ0τ is a deformation of τ weeasily conclude that ρ = ρ1 ⊕ ρ2 with ρj (with respect to some lattice) being a
deformation of τj (j = 1, 2). Using the fact that ρ0τ is a deformation of τ we now
deduce that there is an O-lattice inside the space of ρ0τ ⊗Rτ0 F with respect to whichρ0τ ⊗Rτ0 F is block-upper-triangular (with correct dimensions) and non-semi-simple.
When we reduce it modulo $m , the upper-right shoulder will give rise to an elementof order $m in HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O). Since m is arbitrary this contradicts
Assumption 2.7.


ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

9

3. The rings Tτ
Let us now define the rings Tτ that will correspond to Rτ0 on the Hecke side.
Proposition 3.1. If ρ : GΣ → GLn (E) is irreducible and satisfies∼ τ1 ⊕ τ2
(3.1)
ρss =
then there exists a lattice inside E n so that with respect to that lattice the mod $reduction ρ of ρ has the form


τ1 ∗
ρ=
0 τ2
and is non-semi-simple.
Proof. This is a special case of [Urb01], Theorem 1.1, where the ring B in [loc.cit.]
is a discrete valuation ring = O.
For each representation τ as in (2.1) let Φτ be the set of (inequivalent) characteristic zero deformations of τ , i.e. crystalline at p | p Galois representations
ρ : GΣ → GLn (O) whose reduction equals τ . Also, let Φτ,E be the set of (inequivalent) crystalline at p | p Galois representations ρ : GΣ → GLn (E) such that there
exists a GΣ -stable lattice L in the space of ρ so that the mod $-reduction of ρL
equals τ .
The following is a higher-dimensional analogue of Lemma 2.13(ii) from [SW99]:
Proposition 3.2. One has Φτ,E ∩ Φτ 0 ,E = ∅ if τ ∼6 τ 0.
=
Proof. Let ρ : GΣ → GLn (E) be a representation such that ρss = τ1 ⊕ τ2 andlet T equal its trace. Suppose there exist two lattices Li in the representation
space of ρ such that the reductions of the corresponding representations ρLi are
given by τ and τ 0 with τ 6∼
= τ 0 as in (2.1). We now consider the classes cLi ofthe cocycles corresponding to ρLi in Ext1F[GΣ ]/ ker T (τ2 , τ1 ). Using Assumption 2.7
above and Corollary 7.8 in [BK13] we conclude that the quotient O/IT is finite.
Thus arguing as in the proof of Proposition 1.7.4 in [BC09] but using Proposition
3.1 in [BK13] instead of generic irreducibility of T to conclude that ker T = ker ρ(see [BC09], Proof of Proposition 1.7.2, on how this equality - which follows from
Proposition 1.6.4 in [loc.cit.] in the generically irreducible case - is used) we obtainthat the existence of ρLi with trace T and non-split reduction as in (2.1) implies
that Ext1X (τ2 , τ1 ) is 1-dimensional, where X := (O[GΣ ]/ ker T )/$(O[GΣ ]/ ker T ).
First note that X = O[GΣ ]/($O[GΣ ] + ker T ). Secondly one clearly has thatker(O[GΣ ]  F[GΣ ]) = $O[GΣ ]. These two facts imply that the map O[GΣ ]  X
factors through O[GΣ ]  F[GΣ ] and that the kernel of the resulting surjection
F[GΣ ]  X equals (ker T )F[GΣ ]. Thus we have X = F[GΣ ]/(ker T )F[GΣ ], so by
the above we conclude that Ext1F[GΣ ]/ ker T (τ2 , τ1 ) is one-dimensional. This means
the corresponding representations of F[GΣ ]/ ker T are isomorphic. Since ker T =ker ρ (as noted above) the reductions both factor through this quotient of F[GΣ ],and so they are isomorphic as representation of F[GΣ ], in contradiction to ourassumption.

The following notation will remain in force throughout the paper.
Notation 3.3. Write T for the S
set of isomorphism classes of residual representations of the form (2.1). Set Φ = τ ∈T Φτ .

10

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Remark 3.4. The condition that Rτ0 be a finitely generated O-module is equivalent
to assuming that the set Φτ is a finite set.
We now fix subsets Πτ ⊂ Φτ and Π ⊂ Φ of deformations. In our later applicationthese will be taken to correspond to all the modular deformations corresponding tocuspforms of a particular weight and level which are congruent to a fixed Eisensteinseries. In particular Πτ may be empty.
Q
Whenever Πτ 6= ∅ we obtain an O-algebra map Rτ → ρ∈Πτ O. This induces a
map from
Y(3.2)
Rτtr →
O.
ρ∈Πτ

Definition 3.5. We (suggestively) write Tτ for the image of the map (3.2) - notethat this also depends on the choice of the set Πτ - and denote the resulting surjective O-algebra
map Rτtr  Tτ by φτ . Also we will write T for the image ofQps
φ : R → ρ∈Π O, where φ is induced from the traces of the deformations ρπ . Finally we will write Jτ ⊂ Tτ for the ideal of reducibility of the pseudo-representationTτ ⊗Rτtr ,φτ tr ρτ : Tτ [GΣ ] → Tτ and J ⊂ T for the ideal of reducibility of thepseudo-representation T ps ⊗Rps ,φ T : T[GΣ ] → T.
Lemma 3.6. The maps Rτtr  Tτ and Rps  T factor through Rτtr,0 and theimage of Rps inside Rps ⊗O E respectively.
Proof. Let P(τ )Q= {p ∈ Spec(RτT) | Rτ /p = O} be as before. Thenclearly theQkernel of Rτ → ρ∈Πτ O contains p∈P(τ ) p. Thus the map Rτ → ρ∈Πτ O factors

through Rτ0 . Then the claim follows since ϕτ (Rτtr ) = Rτtr,0 by Lemma 2.19.
Lemma 3.7. The quotient Tτ /Jτ is cyclic and one has Jτ = φτ (Iτtr ).
Proof. The first part is a consequence of Lemma 2.11 and was already mentioned
in Remark 2.13.
By Lemma 7.11 in [BK13] we know that Jτ ⊃ φτ (Iτtr ). For the opposite inclusion
we argue as follows. We need to show that φτ ◦ tr ρτ ≡ Ψ01 + Ψ02 mod φτ (Iτtr ) forΨ01 , Ψ02 pseudorepresentation.
Put B = T, A = Rτtr and write ϕ for φτ : Rτtr → T and TB for Tτ ⊗Rτtr ,φτ tr ρτ .
Let x ∈ B[GΣ ]. Since ϕ is surjective there exists y ∈ A[GΣ ] such that ϕ(y) = x.
Then by definition of TB we have TB (x) = ϕ ◦ T (y) = ϕ(Ψ1 (y) + Ψ2 (y) + i) forsome pseudo-representations Ψ1 , Ψ2 and i ∈ Iτtr . Now set Ψ0j (x) := ϕ ◦ Ψj (y) for
j = 1, 2.

Corollary 3.8. One has Jτ = φτ (Iτtr,0 ).
Proof. By Lemma 3.6 the map φτ factors through Rτtr,0 . By abuse of notation wewill denote the induced map also by φτ as in the statement of the Corollary. Thensince ϕτ (Iτtr ) = Iτtr,0 we get the corollary.

4. The lattice L and modular extensions
We will make a frequent use of the following result that is due to Urban [Urb01].
Let B be a Henselian and reduced local commutative algebra that is a finitely

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

11

generated O-module. Since O is assumed to be sufficiently large and B is reduced
we havess
YY
B ⊂ B̂ =O⊂
E = FB ,i=1

i=1

where B̂ stands for the normalization of B and FB for its total ring of fractions.
Write mB for the maximal ideal of B. For any finitely generated free FB -module
M , any B-submodule N ⊂ M which is finitely generated as a B-module and has
the property that N ⊗B FB = M will be called a B-lattice.
Theorem 4.1 ([Urb01] Theorem 1.1). Let R be a B-algebra, and let ρ be an absolutely irreducible representation of R on FBn such that there exist two representations
ρi for i = 1, 2 in Mni (B) and I a proper ideal of B such that(i) the coefficients of the characteristic polynomial of ρ belong to B;(ii) the characteristic polynomials of ρ and ρ1 ⊕ ρ2 are congruent modulo I;(iii) ρ1 := ρ1 mod mB and ρ2 := ρ2 mod mB are absolutely irreducible;(iv) ρ1 ∼6 ρ2 .
=
Then there exist an R-stable B-lattice L in FBn and a B-lattice T of FB such that
we have the following exact sequence of R-modules:
0 → ρ1 ⊗B T /IT → L ⊗B B/I → ρ2 ⊗B B/I → 0
which splits as a sequence of B-modules. Moreover, L has no quotient isomorphic
to ρ1 .
Since we will not only use Theorem 4.1 itself but also the construction of thelattice L let us briefly summarize how L is built (for details cf. [loc.cit.], p. 490491). Let ρi be the composition of the representation ρ with the projection B  O
onto the ith component of B̂. Urban shows that we can always conjugate ρi (overE) so that the mod $ -reduction of (the conjugate of ρi which we will from now
on denote by) ρi has the form


ρ1 ∗
.
(4.1)
0 ρ2


In1 0
Set ρB̂ := (ρi )i . It is also shown in [loc.cit.] that the matrices
and
0 0


0 0
are in the image of ρB̂ . One then defines the lattice L to be the B0 I n2
submoduleof B̂ n generated by ρ(r)t 0, 0, . . . , 0, 1 , where r runs over R and set

I
0
0 0
L1 := n1
L and L2 :=
L.
0 0
0 In2
Let T be as in
Q Notation 3.3. Let T, Π ⊂ Φ and Πτ ⊂ Φτ be as in section
3. WriteT̃=
for the normalization of T. Let ρ in Theorem 4.1 beπ∈Π O Q
Q
Q
ρΠ = τ ∈T ρπ ∈Πτ ρπ = π∈Π ρπ and ρi = τ̃i , i = 1, 2, where τ̃i : GΣ → GLni (O)is a fixed crystalline deformation of τi which we from now on assume exists. (Ifone works under Assumption 2.6, then the τ̃i ’s are unique, but we do not need thisuniqueness for the arguments of this section.) Note that the reduction of ρπ alreadyhas the form (4.1), so we can take ρB̂ = ρΠ and define lattices L, L1 and L2 asabove (with B = T, R = T[GΣ ]). The GΣ -action on L is then via restriction ofρΠ to L. Write m = mT for the maximal ideal of the local ring T and let J as insection 3 be its reducibility ideal.

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

12

By Theorem 4.1 (and Lemmas 1.1 and 1.5(ii) in [Urb01]) there exists a finitely
generated free T-module T and a short exact sequence of T[GΣ ]-modules (which
splits as a sequence of T-modules):(4.2)

0 → L1 ⊗T T/J → L ⊗T T/J → L2 ⊗T T/J → 0with
L1 ⊗T T/J ∼= τ̃1 ⊗T T /JT and L2 ⊗T T/J ∼= τ̃2 ⊗T T/J.
Note that we have the following identification
(4.3)

∼

HomO (τ̃2 , τ̃1 ) ⊗T T /JT −→ HomT/J (L2 ⊗T T/J, L1 ⊗T T/J).

Let s : L2 ⊗T T/J → L ⊗T T/J be a section of T/J-modules of (4.2). Using (4.3)
as in [Klo09], p.159-160, we define a cohomology class c ∈ H 1 (FΣ , HomO (τ̃2 , τ̃1 ) ⊗T
T /JT ) by
g 7→ (λ2 ⊗ t 7→ s(λ2 ⊗ t) − g · s(g −1 · λ2 ⊗ t)).
We also define a map
ιJ : HomO (T /JT , E/O) → H 1 (FΣ , HomO (τ̃2 , τ̃1 ) ⊗O E/O),

f 7→ (1 ⊗ f )(c).

Let us just briefly remark that ιJ is independent of the choice of the section s.
From now on we will make the following assumption on the quotient T/J.
Assumption 4.2. One has#T/J ≥ #O/Lwith L as in Assumption 2.7.
Remark 4.3. In Section 7 we will describe a particular setup for n = 2 and F
an imaginary quadratic field under which Assumptions 2.7 and 4.2 are satisfied.
However, we expect that these conditions hold also for other CM fields (for n = 2),
and have therefore presented the results of this and the following sections underthese two general assumptions.
Lemma 4.4. If Assumptions 2.7 and 4.2 hold, then the map
ιJ : HomO (T /JT , E/O) → H 1 (FΣ , HomO (τ̃2 , τ̃1 ) ⊗O E/O)
is injective and its image equals HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O)).
Proof. For the injectivity of ιJ one follows the strategy in [Ber05], p.119-120 which
was later spelled out in a higher dimensional case in [Klo09], Lemmas 9.25 and 9.26.
Using [Klo09], Lemma 9.21 (which is just a slightly expanded version of Theorem
4.1) we get FittT T = 0. Then arguing as in [Klo09], the last four lines before theproof of Lemma 9.25 on p. 161 we get that
(4.4)

valp (#T /JT ) ≥ valp (#T/J)(see also the arguments on p. 120 in [Ber05]). This, combined with Assumption 4.2
and Assumption 2.7 implies that ιJ must in fact surject onto the Selmer group. 
Since (4.2) splits as a sequence of T/J-modules we can tensor it with ⊗T/J F
and obtain an exact sequence of F[GΣ ]-modules
0 → L1 ⊗T F → L ⊗T F → L2 ⊗T F → 0
with
L 1 ⊗T F ∼= T ⊗ T τ1and L2 ⊗T F ∼= τ2 .

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

13

Arguing as above (with mT instead of J) we again obtain an injective map
ι : HomO (T /JT , F) → H 1 (FΣ , HomF (τ2 , τ1 )).
Lemma 4.5. Suppose that Assumptions 2.7 and 4.2 hold. The map ι : HomO (T /JT , F) →H 1 (FΣ , HomF (τ2 , τ1 )) is injective and its image equals HΣ1 (F, HomF (τ2 , τ1 )).
Proof. We have the following commutative diagram
HomO (T /JT , E/O)O
HomO (T /JT , F)ιJ

/ H 1 (FΣ , HomO (τ̃2 , τ̃1 ) ⊗O E/O)Oι

/ H 1 (FΣ , HomF (τ2 , τ1 ))Denote the right vertical arrow by f . Lemma 4.4 implies that the image of f ◦ι is contained in the $-torsion of HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O). Moreover, by
Proposition 5.8 in [BK13], we know that the $-torsion in HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O
E/O) coincides with f (HΣ1 (F, HomF (τ2 , τ1 ))). Since f is injective, this implies thatthe image of ι is contained in the Selmer group. Hence it remains to show that ιJ
is an isomorphism on $-torsion. But this is clear since ιJ is an isomorphism byLemma 4.4.
Q
Lemma 4.6. Suppose that Assumptions 2.7 and 4.2 hold. Write ρΠ for ρπ ∈Π ρπ .
Q
The F[GΣ ]-module L ⊗T F coincides with the F-subspace of ρπ ∈Π Fn generatedby ρΠ (r)en , where r runs over F[GΣ ], en is a column matrix in Fn whose last entry
is 1 and all the other ones are zero.
P
Proof. By definition of L, every element of L⊗T F can be written as i ti ρΠ (gi )en ⊗image of ti under theai with ti ∈ T, ai ∈ F and gi ∈ GΣ . Writing ti for thePcanonical map T  F we can re-write the above sum as i ρΠ (gi )en ⊗ ai ti andai ti ∈ F. It suffices to show now that for every g ∈ GΣ we get ρΠ (g)en ⊗ 1 =ρΠ (g)en ⊗ 1. Write
a (g) + a011 (g) a12 (g) + a012 (g)ρΠ (g) = 11 0,
a21 (g)a22 (g) + a022 (g)where a11 , a011 are (n − 1) × (n − 1)-matrices, a22 , a022 are scalars and the othermatrices have sizes determined by these two and the entries of a0ij (g) lie in $O ⊕$O ⊕ · · · ⊕ $O Thus,




 0

a12 (g) + a012 (g)a12 (g)a12 (g)/$ρΠ (g)en ⊗ 1 =
⊗1=
⊗1+ 0
⊗ $,
a22 (g) + a022 (g)a22 (g)a22 (g)/$and the latter tensor is zero. This proves the lemma.

Let us now turn to the 2-dimensional situation, where every τ is (up to a twist)of the form


1 ∗
τ=
0 χ
for a Galois character χ. Note that HomO (T /JT , F) = HomO (T ⊗T T/J, F) =HomO (T ⊗T T/m, F) = HomO (T ⊗T F, F).
Proposition 4.7. Suppose that Assumptions 2.7 and 4.2 hold. The image of ι :Hom(T ⊗T F, F) ,→ HΣ1 (F, χ−1 ) is spanned by extensions τ such that Πτ 6= ∅.

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

14

Proof. Let Π be as above and Π be a subset Π consisting of representatives of
distinct isomorphism classes of residual representations (i.e., one element from every
non-empty Πτ ). By Lemma 4.6 the lattice L ⊗T F is generated by vectors (1, 1, . . . , 1)
χ(g)α(g)0x=.
(0, 0, . . . , 0) (χ(g), χ(g), . . . , χ(g)) 1
Let us explain the notation:
There are r elements in Π (which we will denote byPrτ 1 , τ 2 , . . . , τ r ), σ := i=1 si elements in Π. Moreover, α is a σ-tuple of functions
such that α(g) equals(α1,1 f1 (g), . . . , α1,s1 f1 (g), α2,1 f2 (g), . . . , α2,s2 f2 (g), . . . , αr,1 fr (g), . . . αr,sr fr (g)) ∈ Fσ ,where the αi,j are elements of F× . We get that x equals(α1,1 f1 (g), . . . , α1,s1 f1 (g), α2,1 f2 (g), . . . , α2,s2 f2 (g), . . . , αr,1 fr (g), . . . αr,sr fr (g))χ(g).
(1, 1, . . . , 1)
Let αj be the jth entry of α. Then we conclude that L ⊗T F ∼= T ⊗T V = V , where
V is the subspace of (F ⊕ F)#Π spanned over F by the set vectors of the form

 



χ(g)α1 (g)χ(g)α2 (g)χ(g)ασ (g)
,
,...,
.
χ(g)
χ(g)
χ(g)
For j ∈ {1, 2, . . . , σ} define integers n(j) ∈ {1, 2, . . . , r} and m(j) ∈ {1, 2, . . . , sn(j) }by the equality
αj (g) = αn(j),m(j) fn(j) (g).
χ(g)αj (g)via the n(j)thThe GΣ -action on V is via ρΠ , hence h ∈ GΣ acts on
χ(g)residual representation in Π, i.e., by multiplication by τ n(j) (h). In particular all
the vectors in V have the form   
 
a1
a
a
(4.5)
v=
, 2 ,..., σ.
aaa
By definition we have
(4.6)


0
L ⊗T F =
0
2



0
0
L ⊗T F =
1
0


0
V ∼
= F(χ)
1

as F[GΣ ]-modules, where we write F(χ) for the one-dimensional F-vector space on
which GΣ acts via χ. The surjective F[GΣ ]-module map V  F(χ) is given by0
sending a vector v asin (4.5)
 to a. Write V for the kernel of this map. Identifying
1 0
1 0
V 0 with L1 ⊗T F =L ⊗T F =V provides us with a splitting (only
0 0
0 0
as F-vector spaces) of the short exact sequence of F[GΣ ]-modules
0 → V 0 → V → F(χ) → 0.
Since the GΣ -action on L1 ⊗T F is trivial, we have V 0 = L1 ⊗T F = T ⊗T F.
Clearly, we may assume that the vectors in V 0 all have the form   
 
a1
a
a
(4.7)
v0 =
, 2 ,..., σ.
000
Let φj ∈ HomF (V 0 , F) = Hom(T ⊗T F, F) be the homomorphism sending v0 as in(4.7) to aj .

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

15

Then the map ι sends φj tothe cocycle αn(j),m(j) fn(j) , i.e, to the residual repre1 χαn(j),m(j) fn(j)sentation, which is isomorphic to the residual representation0χ
of the n(j)th element of Π. So, this proves that the image of ι is spanned bymodular extensions.

Corollary 4.8. If Assumptions 2.7 and 4.2 are satisfied then the space HΣ1 (F, χ−1 )has a basis consisting of extensions τ such that Πτ 6= ∅.
Proof. This follows from Lemma 4.5 and Proposition 4.7.



Remark 4.9. Corollary 4.8 does not imply that Πτ 6= ∅ for all isomorphism classes
τ ∈ T. In fact, if we replace F by its finite extension F0 of degree m, then the orderof T increases (since it is given by #HΣ1 (F, Hom(τ2 , τ1 ))/(q −1), P
where q is the orderof the residue field), while the number of modular forms, i.e., τ ∈T #Πτ remains
the same.
5. Bounding the size ofQ

i Ti /φ(Ii )In this section we keep in force Assumptions 2.7 and 4.2. Moreover, we workin the two-dimensional setup and we set τ1 = 1 and τ2 = χ (which can always beachieved by twisting by a Galois character). Let B := {e1 , . . . , es } be a basis ofHΣ1 (F, Hom(τ2 , τ1 )) = HΣ1 (F, χ−1 ) consisting of ‘modular’ extensions, i.e., extensions τ such that Πτ 6= ∅ (cf. Corollary 4.8) and write τ i for the corresponding
residual representations. Let us write Ti for Tτ i . Similarly let us write Ji andWrite pi : T  Ti for the canonical projection.
Πi for Jτ i and Πτ i respectively.
Qs
Consider the map T → i=1 Ti . Let J ⊂ T be as in section 3. Set Ji = pi (J).
Note that Ji is an ideal because pi is surjective.
Let us begin with a trivial observation that one expects that the canonical map
T/J →

s
Y

Ti /Jii=1

is neither injective nor surjective, because the source is cyclic, while the target isnot. However, as we shall see below the orders of both sides are equals provided
that the basis B is unique up to scaling and that all of the ideals Ji are principal.
Qs
Proposition 5.1. # i=1 Ti /Ji ≤ #T/J.
Proof. In this proof we follow mostly the notation
Q of [Klo09], section 9. Let Πi be
as before. As in section 4 we get a lattice Li ⊂ π∈Πi ρπ and a finitely generated
Ti -module Ti such that the following sequence of Ti /Ji [GΣ ]-modules is exact:(5.1)

0 → (Ti /Ji Ti ) ⊗ τ̃1 → Li /Ji → (Ti /Ji ) ⊗ τ̃2 → 0.

Fix i. As before we obtain an injective map(5.2)

ιi : Hom(Ti /Ji Ti , E/O) ,→ HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O).

By the structure theorem of finitely generated modules over PID we get that
0

(5.3)

HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O)) ∼
=

s
Y
j=1

O/$ri O.

16

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Write cj ∈ HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O) for a generator of the jth factor in (5.3).
Since
HΣ1 (F, HomO (τ̃2 , τ̃1 ) ⊗O E/O)[$] = HΣ1 (F, HomF (τ2 , τ1 ))by Proposition 5.8 in [BK13], we conclude that s0 = s and that we can choose thecj s so that each cj corresponds to ej under the canonical projection
0

s
Y
j=1

O/$ri O sY

F = HΣ1 (F, HomF (τ2 , τ1 )).

j=1

Lemma 5.2. For every i = 1, 2, . . . , s one has Im(ιi ) ⊂ hci i.
Proof. Let f ∈ Hom(Ti /Ji Ti , E/O). It is enough to show that the image of f
in Hom(Ti /Ji Ti , F) is sent by ιi ⊗ F to a scalar multiple of ei ∈ HΣ1 (F, χ−1 ).
Arguing exactly as in the proof of Proposition 4.7 we see that the image of ιi ⊗ F
is one-dimensional and is spanned by the cohomology classes corresponding to theisomorphism class of τ i .

Lemma 5.2 implies that Hom(Ti /Ji Ti , E/O) and hence Ti /Ji Ti is a cyclic Omodule, and hence a cyclic Ti /Ji (= O/$di )-module. Arguing as in section 4 weobtain an analog of inequality (4.4):
valp (#Ti /Ji ) ≤ valp (#Ti /Ji Ti ).
This combined with the fact that Ti /Ji Ti is a cyclic Ti /Ji -module implies that
Ti /Ji ∼= Ti /Ji Ti . In particular this implies that the lattice Li /Ji ∼= (Ti /Ji )2 as
Ti -modules.
Let ρ̃i : G → GL2 (Ti /Ji ) be the representation given by the short exact sequence
0 → (Ti /Ji ) ⊗ τ̃1 → Li /Ji → (Ti /Ji ) ⊗ τ̃2 → 0 (coming from the sequence (5.1)
and the fact that Ti /Ji Ti ∼= Ti /Ji ). One has Ti /Ji = O/$di and since ρ̃i reducesito τ P
we mustPhave di ≤ ri , where Oci ∼= O/$ri O. So, in particular we getthat i di ≤ri . Combining Assumptions 2.7 with 4.2 we obtain the claim of
Proposition 5.1.

Our goal is now to prove the opposite inequality, which under some additional
assumption will follow from a more general commutative algebra result which wasproved by the authors and Kenneth Kramer in [BKK13] and which we will now
present.
n =1 , n2 , . . . , ns } be a set of s positive
PsLet s ∈ Z+ and let n{nQsintegers. Set
n
i
n
.
Let
A
=
O
with
i
∈
{1,
2,
.
.
.
,
s}.
Set
A
=
A
=
O
.
Let
i
i=1 i
i=1 i
ϕi : A  Ai be the canonical projection. Let T ⊂ A be a (local complete) Osubalgebra which is of full rank as an O-submodule and let J ⊂ T be an ideal of
finite index. Set Ti = ϕi (T ) and Ji = ϕi (J). Note that each Ti is also a (local
complete) O-subalgebra and the projections ϕi |T are local homomorphisms. ThenJi is also an ideal of finite index in Ti .
×
TheoremQs 5.3 ([BKK13], Theorem 2.1). If #F ≥ s − 1 and each Ji is principal,
then # i=1 Ti /Ji ≥ #T /J.

Let V be a vector space and write P1 (V ) for the set of all lines in V passing
through the origin. There is a canonical map V \ {0} → P1 (V ) sending a vector v
to the line spanned by v.
Let S be the set of all modular bases of HΣ1 (F, Hom(τ2 , τ1 )), i.e., the set of bases0
B = {e01 , e02 , . . . , e0s } having the property that Πτi0 6= ∅, where τi0 is the residualON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

17

representation corresponding to the extension represented by e0i . The set S is nonempty as B ∈ S.
Definition 5.4. We will say that HΣ1 (F, Hom(τ2 , τ1 )) has a projectively uniquemodular basis if the images of all the elements of S in P1 (HΣ1 (F, Hom(τ2 , τ1 )) agree.
In the case when HΣ1 (F, Hom(τ2 , τ1 )) has this property we will refer to any elementof S as the projectively unique modular basis.
Note that it is possible to find i0 ∈ {1, 2, . . . , s} such that the set B 0 := B ∪ {eτ } \{ei0 } is still a basis of HΣ1 (F, Hom(τ2 , τ1 )) (and one still has that Πτ 0 6= ∅ for allτ 0 ∈ B 0 ). Hence we can assume without loss of generality that B = {e1 , e2 , . . . , es }with τ 1 = τ . In fact, if HΣ1 (F, Hom(τ2 , τ1 )) has a projectively unique modular basis,
it follows that if B 0 is another modular basis, then the isomorphism classes of theresidual representations corresponding to the elements of B 0 are the same as theisomorphism classes of the residual representations corresponding to the elementsof B.
Proposition 5.5. If HΣ1 (F, Hom(τ2 , τ1 )) hasQ
a projectively unique modular basissand each Ji is a principal ideal of Ti , then # i=1 Ti /Ji ≥ #T/J.
Proof. First note that our assumption that E be sufficiently large allows us toassume that #F× satisfies the inequality in Theorem 5.3. Since T is a free Omodule of finite rank we set n to be that rank and define ni to be the rank of
Ti . The assumption that B be projectively unique guarantees that every O-algebra
homomorphism has a corresponding
residual Galois representation isomorphic toPs
τ i for some i. Hence n = i=1 ni . The Proposition now follows from Theorem 5.3
by taking T = T.
Remark 5.6. Theorem 5.3 also has consequences for congruences between modular
forms. Suppose that T = TΣ is the cuspidal Hecke algebra (acting on the space ofautomorphic forms over imaginary quadratic fields of weight 2 right invariant undera certain compact subgroup Kf ) localized at a maximal ideal corresponding to an
Eisenstein series, say E. Let J = JΣ be the Eisenstein ideal corresponding to E (seesection 7.3 for the details). Let N be the set of O-algebra homomorphisms T → O,i.e. to cuspidal Hecke eigencharacters congruent to the eigencharacter λ0 of E mod$. For λ ∈ N write mλ for the largest positive integer such that λ(T ) ≡ λ0 (T ) mod$mλ for all T ∈ T. Let e be the ramification index of E over Qp . As a consequenceof Theorem 5.3 we get the following inequality (cf. Proposition 4.3 in [BKK13]
1 X
mλ ≥ val$ (#T/J).
eλ∈N

For many more applications of Theorem 5.3 see [loc.cit.].
Corollary 5.7. Suppose that Assumptions 2.7 and 4.2 hold. If the modular basis
B is projectively unique and each Ji is a principal ideal of Ti , then
#

s
Y
i=1

Ti /Ji = #T/J ≥ #O/L.

18

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

6. Urban’s method applied to Rtr,0In this section we again set n = 2, τ1 = 1 and τ2 = χ and we fix a residual
representation
1 χf
τ : GΣ → GL2 (F), τ =.
0 χLet Πτ be as in section 3. From now on we will assume that Πτ is non-empty.
The surjection φ : Rτtr,0  Tτ (cf. Definition 3.5 and Lemma 3.6) descends to asurjection
Rτtr,0 /Iτtr,0  Tτ /Jτ .

(6.1)(since by Lemma 3.8 φ(Iτtr,0 ) = Jτ ). The main goal of this section is to provethat under certain assumptions the map in (6.1) is an isomorphism (Theorem 6.3).
Before we state the theorem let us demonstrate several properties of Rτtr,0 . In
particular we will show that Rτtr,0 ∼= Rτ0 (Theorem 6.2). In this section we assume
that Assumptions 2.6, 2.7 and 4.2 are satisfied.
Lemma 6.1. The O-rank of Rτ0 equals the O-rank of Rτtr,0 . In particular thenormalizations (and the total rings of fractions) of Rτ0 and Rτtr,0 coincide.
Proof. Write ρ for ρ0τ , i.e., ρ : GΣ → GL2(Rτ0 ). We claim that we can conjugate ρa bso that for every g ∈ GΣ we have ρ(g) =with a, c, d ∈ Rτtr,0 . Indeed, sincec d
the characters on the diagonal of τ are distinct mod $, we can find σ ∈ GΣ onwhich they differ, so that the eigenvalues of τ (σ) lift by Hensel’s lemma to distinct
eigenvalues of ρ(σ) in Rτ0 , and we can conjugate (over Rτ0 ) to have ρ(σ) be diagonalwith these lifted eigenvalues. For a general element g ∈ GΣ , we then comparetr ρ(g) with tr ρ(σg) and use that the eigenvalues are distinct mod $ to see thatthe two diagonal entries of ρ(g) lie in Rτtr,0 . Similarly we show (cf. the proof of
Lemma 3.27 in [DDT97]) that the lower-left entry also lies in Rτtr,0 .
Note that since Rτ0 is a finitely generated O-module (and O is assumed to be
sufficiently large) we get a canonical embeddingRτ0 ,→ R̂τ0 ∼
=

k
Y

O,

i=1

where R̂τ0 is the normalization of Rτ0 . For i = 1, . . . , k, write ρi for the compositionof ρ with the projection onto the ith component of R̂τ0 . Suppose that the O-rankof Rτtr,0 is strictly smaller than the O-rank of Rτ0 . Then there exist two minimal
primes (after possibly renumbering the minimal primes we will call them p1 , p2 ) ofRτ0 which contract to the same minimal prime p of Rτtr,0 . Hence we get the following
commutative diagram:/ Rτ0Rτtr,0Rτ0 /p1:Rτtr,0 /p

∼

∼

$$
0
5 Rτ /p2

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

19



a b1
This implies that the corresponding two deformations (to O) ρ1 = 1
and
c1 d1


a b2
ρ2 = 2(since their a-, c- and d-entries (as functions) factor through Rτtr,0 /p)
c2 d2
must satisfy a1 = a2 =: a, c1 = c2 =: c and d1 = d2 =: d. In particular their traces
are equal. Using Lemma 2.20 we see that both ρ1 ⊗O E and ρ2 ⊗O E are irreducible
and thus by the Brauer-Nesbitt Theorem we conclude that ρ1 ⊗O E ∼= ρ2 ⊗O E.
A B
Let M =∈ GL2 (E) be such that
C D
(6.2)

M ρ1 = ρ2 M.

Then an easy matrix calculation shows that
(6.3)

Aa + Bc = aA + b2 C

and
(6.4)

Cb1 + Dd = cB + dD

from which we get that Cb1 = Cb2 . Suppose for the moment that there existsg ∈ GΣ such that b1 (g) 6= b2 (g). Since O is a domain we conclude that C = 0.
Since the representations ρ1 and ρ2 are irreducible over E, the function c cannot
be identically zero. Using (6.4) we conclude that B = 0. Finally, computing thelower-left entries on both sides of (6.2) we get Dc = cA, so again using the factc is not identically zero and that O is a domain we get that A = D. Thus, M isa non-zero scalar matrix. Hence we get a contradiction to our assumption on theexistence of g and we conclude that b1 = b2 . In particular ρ1 and ρ2 are identicaldeformations of τ which correspond to distinct minimal primes of Rτ0 . Hence ρ1and ρ2 give rise to two different homomorphisms from Rτred to O. This contradicts
the bijectivity of the correspondence
HomO−alg (Rτred , O) ↔ {deformations of τ into O}/equivalence.

Theorem 6.2. Suppose that Rτ0 is a finitely generated O-module. There exists a: GΣ → GL2 (Rτtr,0 ) of τ . The resulting canonical map Rτred →deformation ρtr,0τ
tr,0Rτ factors through Rτ0 and induces an isomorphism Rτ0 ∼= Rτtr,0 .
Proof. We will (once again) apply Theorem 4.1 (due to Urban). In the notationof section 4 we will write FB = F to be total ring of fractions of B = Rτtr,0 ⊂ F.
Note that by Lemma 6.1, F is also the total ring of fractions of Rτ0 . Moreover,
we take R = Rτtr,0 [GΣ ], ρ = ρτ ⊗B F : GΣ → GL2 (F) which induces a morphismρ : Rτtr,0 [GΣ ] → M2 (F) of Rτtr,0 -algebras. As before, the representations denoted in
Theorem 4.1 by ρ1 and ρ2 are our unique lifts τ̃1 , τ̃2 : GΣ → GL2 (O) ,→ GL2 (Rτtr,0 )and we set I = Iτtr,0 . Note that conditions (i) and (ii) of Theorem 4.1 are satisfied
respectively by the definition of Rτtr,0 and of Iτtr,0 and conditions (iii) and (iv) aresatisfied by our assumptions on τ1 and τ2 . Finally, the condition of irreducibilityof ρ is satisfied by Lemma 2.20.
Hence we conclude from Theorem 4.1 that there exists an Rτtr,0 [GΣ ]-stable lattice
L ⊂ F 2 and a finitely generated Rτtr,0 -module Tτ ⊂ F such that we have an exact
sequence of Rτtr,0 [GΣ ]-modules:(6.5)

0 → τ̃1 ⊗ Tτ /Iτtr,0 Tτ → L ⊗ Rτtr,0 /Iτtr,0 → τ̃2 ⊗ Rτtr,0 /Iτtr,0 → 0.

20

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

It follows from [Urb01] Lemmas 1.1 and 1.5 that L = Tτ ⊕ Rτtr,0 as Rτtr,0 -modules.
We will now show that Tτ ∼= Rτtr,0 . Indeed, the lattice L is defined as in section 4,
but since we only work with
Q a fixed residual representation τ , the representation ρ in
Theorem 4.1 equals the ρπ ∈Φτ ρπ . Using Lemma 4.6 for this representation (i.e.,
when we replace ρΠ with ρ as above), we conclude that the GΣ -module L ⊗Rτtr,0 F isQ
the subspace of ρπ ∈Φτ F2 generated by ρ(r)e2 (with notation as in that lemma).
This subspace is clearly isomorphic to τ as a GΣ -module. So, the middle term in(6.5) after tensoring with F is two-dimensional, hence we must have Tτ /Iτtr,0 Tτ ⊗Rτtr,0
F = Tτ /mTτ = F, where m is the maximal ideal of Rτtr,0 . Thus, by Nakayama’s
Lemma, we see that Tτ is generated over Rτtr,0 by one element, say x. Consider thesurjective map φ : Rτtr,0 → Tτ given by r 7→ rx. Let a ∈ ker φ. Then a annihilatesTτ . However, by definition of Tτ and the fact that Rτtr,0
Q embeds into its ring of
fractions F we can consider x and a as elements of F = E. If a = (a1 , a2 , . . . , as )and x = (x1 , x2 , . . . , xs ) and aj 6= 0, then xj = 0. However, this implies thatT ⊗ F 6= F, which contradicts the fact that Tτ is a lattice in F (cf. Theorem4.1). Hence φ is injective and we indeed have Tτ = Rτtr,0 . Thus L = (Rτtr,0 )2 as anRτtr,0 -module and ρ gives rise to a representation ρτtr,0 : GΣ → GL2 (Rτtr,0 ).
By the above it is clear that ρtr,0
reduces to τ . Let us make sure that theτresulting representation is crystalline. Indeed, the lattice L lives inside the finite
direct product of the representations ρπ for π ∈ Φτ and each of the ρπ is crystalline.
Hence as a submodule of a finite direct sum of crystalline representations ρtr,0isτ
crystalline. This proves the first assertion.
By universality of Rτred we obtain an O-algebra map φ : Rτred → Rτtr,0 . Since
tr,0Rτ is a subring of the direct product of finitely many copies of O, the map φclearly factors through Rτ0 . Let us denote the induced map Rτ0 → Rτtr,0 also by φ.
We claim that φ is surjective. Indeed, by its definition Rtr,0 is generated by tracesof ρ0τ . So, it is enough to show that the traces of ρ0τ and ρτtr,0 = φ◦ρ0τ coincide. This
follows from the construction of the lattice L which is a GΣ -stable lattice inside therepresentation ρ0τ ⊗ F.
are isomorphic after tensoring
In particular, both representations ρ0τ and ρtr,0τ
with F, hence they must have equal traces. Since both Rτ0 and Rτtr,0 are finitely
generated O-modules with the same rings of fractions (Lemma 6.1), the kernel ofφ must be a Q
torsion O-module. This implies that the kernel must be zero (as Rτ0embeds into E). This proves the second assertion.
Suppose that τ is such that Πτ 6= ∅. Recall that by Corollary 4.8 there exists
a basis B = {e1 , e2 , . . . , es } of HΣ1 (F, Hom(τ2 , τ1 )) such that Πτ i 6= ∅ for all i =1, 2, . . . , s, where τ i denotes the representation corresponding to the extension ei .
It is possible to find i0 ∈ {1, 2, . . . , s} such that the set B 0 := B ∪ {eτ } \ {ei0 } isstill a basis (and one still has that Πτ 0 6= ∅ for all τ 0 ∈ B 0 ). Hence we can assume
without loss of generality that B = {e1 , e2 , . . . , es } with τ 1 = τ .
Theorem 6.3. Suppose Assumptions 2.6, 2.7 and 4.2 are satisfied. Suppose moreover that the modular basis B is projectively unique and that for each i = 1, 2, . . . , s
the corresponding ideal Jτ i is principal. If Rτ0 is a finitely generated O-module thenthe map
Rτtr,0 /Iτtr,0  Tτ /Jτ
is an isomorphism.

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

21

Proof. In the proof of Theorem 6.2 we showed the existence of an Rτtr,0 -algebramorphism ρ : Rτtr,0 [GΣ ] → M2 (F) and an Rτtr,0 [GΣ ]-stable lattice L ⊂ F 2 together
with a finitely generated Rτtr,0 -submodule Tτ ⊂ F such that we have an exact
sequence of Rτtr,0 [GΣ ]-modules:
0 → τ̃1 ⊗ Tτ /Iτtr,0 Tτ → L ⊗ Rτtr,0 /Iτtr,0 → τ̃2 ⊗ Rτtr,0 /Iτtr,0 → 0(6.6)

which splits as a sequence of Rτtr,0 /Iτtr,0 -modules. As in section 4 we get a map:
ι : HomO (Tτ /Iτtr,0 Tτ , E/O) → H 1 (FΣ , Hom(τ̃2 , τ̃1 ) ⊗O E/O).
Lemma 6.4. The map ι is injective and its image is contained in the submodule ofHΣ1 (F, Hom(τ̃2 , τ̃1 )⊗O E/O) generated by a cohomology class which reduces (mod $)to the cohomology class corresponding to τ (i.e., the image is cyclic in particular).
Proof. The fact that ι is injective and that its image is contained in the Selmer
group is proved in the same way as Lemma 4.4. The fact that the image is cyclic
and that the generator of the image has the correct reduction is proved as Lemma
5.2.

As in section 4, we obtain an analog of equation (4.4):
valp (#Tτ /Iτtr,0 Tτ ) ≥ valp (#Rτtr,0 /Iτtr,0 ).

(6.7)

Using the above arguments for the rings Ritr,0 corresponding to the residual
representation arising from ei , and putting them together we obtain
(6.8)
s
s
Y
Y
Ti /Iitr,0 Ti ) ≤ valp (#HΣ1 (F, Hom(τ̃2 , τ̃1 ) ⊗ E/O))
Ritr,0 /Iitr,0 ) ≤ valp (#valp (#
i=1

i=1(the first inequality comes from (6.7) while the second one from Lemma 6.4). Combining Assumptions 2.7, 4.2 with Corollary 5.7 we obtain that
s
Y
(6.9)
valp (#HΣ1 (F, Hom(τ̃2 , τ̃1 ) ⊗ E/O)) ≤ valp (#Ti /Ji ).
i=1

Combining (6.8) with (6.9) we conclude that the maps
Ritr,0 /Iitr,0  Ti /Jimust be isomorphisms for every i = 1, 2, . . . , s. This completes the proof of Theorem
6.3.

7. Imaginary quadratic case
For the remainder of the article we specialize to the case n = 2, F imaginary
quadratic, τ1 = 1 and τ2 = χ : GΣ → F× an anticyclotomic character (i.e., weassume that χ(cgc) = χ(g −1 ) for all g ∈ GΣ and c ∈ GQ a complex conjugation).
1−1
7.1. Principality of the ideal of reducibility. Let f ∈ HΣ (F, χ ) be non-zero.
1 χf
Set τ =
with universal deformation ring Rτ .
χ

Proposition 7.1. Suppose Assumption 2.6 is satisfied and write Ψ for τ̃2 . If Rτ0
is a finitely generated O-module, then the ideal of reducibility Iτ0 of Rτ0 is principal.
We begin with the following lemma.

22

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Lemma 7.2. There exists ρ0,opp: G → GL2 (Rτ0 ) such that ρ0,opp
=
τ
τ


χ


∗
and is1

non-split.
Proof. Let c be the complex conjugation. Define ρ0τ byρ0τ (g) = ρ0τ (cgc).
Then

 

χ(cgc)f (cgc)
1 χ−1 (g)f (cgc)=.
χ(cgc)
χ−1 (g)If this is split, then f 0 ∈ HΣ1 (F, χ) defined by f 0 (g) = f (cgc) is the zero cohomology
class. However, a 7→ ac gives an isomorphism of HΣ1 (F, χ−1 ) onto HΣ1 (F, χ) (cf.
Lemma 7.1.5 in [Ber05]), so f 0 6= 0 since f 6= 0. Now set ρ0,opp= ρ0τ ⊗ Ψ.
τ
ρ0τ =


1

Proof of Proposition 7.1. Lemma 7.2 combined with Proposition 1.7.5 and 1.7.4 of[BC09] proves the proposition (note that we can use the above propositions because
by Proposition 3.1 in [BK13] we have that ker ρ = ker T for reduced rings under
the condition that Rτ0 /Iτ0 is finite (cf. Lemma 2.18) and this input can replace thegeneric irreducibility assumption present in [BC09]).

Corollary 7.3. Suppose Assumptions 2.6, 2.7 and 4.2 are satisfied. If Rτ0 is a
finitely generated O-module, then the ideal Jτ of Tτ is principal.
Proof. Let φτ : Rτtr,0  Tτ be the canonical surjection. By Corollary 3.8 we knowthat φ(Iτtr,0 ) = Jτ . The claim follows from combining Theorem 6.2 with Proposition
7.1.

Remark 7.4. For other fields F (e.g. CM fields), principality of the ideal of
reducibility would hold for conjugate self-dual representations (see Theorem 2.11of [BK13]).
7.2. Selmer groups. In this subsection we discuss Assumptions 2.6 and 2.7 forcertain characters χ, for which we will prove our main results.
Example 7.5. Let τ2 = χ be a p-adic Galois character of the following form: Let
φ1 , φ2 be two Hecke characters of infinity types z and z −1 respectively, and set
φ = φ1 /φ2 . Let φp denote the p-adic Galois character corresponding to φ. Set
Ψ := φp , where  is the p-adic cyclotomic character, and χ = Ψ. Assume thatΣ contains the set Sφ of primes dividing M1 M2 Mc1 Mc2 dF p, where Mi denotes the
conductor of φi and that if q ∈ Σ, then #kq 6≡ 1 (mod p). Under this latterassumption Assumption 2.6 will be satisfied by Corollary 9.7 in [BK13].
Let Lint (0, φ) be the special L-value attached to φ as in [BK09]. Write W for
HomO (Ψ, 1) ⊗ E/O. In this case we adapt Assumption 2.7 as follows:
Conjecture 7.6. #Hf1 (F, W ) ≤ #O/$m , where m = val$ (Lint (0, φ)).
Note that this conjecture implies Assumption 2.7 for Σ = Σp = {p | p}. However,
our conclusions hold for all sets Σ ⊃ Σp for which HΣ1 = Hf1 (see Lemma 5.6 of[BK13]).
Remark 7.7. Conjecture 7.6 can in many cases be deduced from the Main conjecture proven by Rubin [Rub91]. If φ−1 = ψ 2 for ψ a Hecke character associated to a
CM elliptic curve, then one can argue as follows. By Proposition 4.4.3 in [Dee99] andusing that Hf1 (F, W ) ∼= Hf1 (F, W c ), we have #Hf1 (F, W ) = #Hf1 (F, E/O(φ−1p )).

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

23

Thus we can use Corollary 4.3.4 in [Dee99] which together with the functional
equation satisfied by L(0, φ) implies the desired inequality.
7.3. Link of rings Tτ to an actual Hecke algebra. In this section we recall
from [Ber09] an Eisenstein ideal bound for a Hecke algebra T(Σ) acting on cuspidal
automorphic forms. We also recall results about Galois representations associatedto such forms and use this to relate T(Σ) to the ring T defined in Section 3.
Assume φ = φ1 /φ2 is unramified. For an ideal N in OF and a finite place q of
F put Nq = NOF,q . We define
U 1 (Nq ) = {k ∈ GL2 (OF,q ) | det(k) ≡ 1

mod Nq }.

Now put
(7.1)

Kf :=

Y

U 1 (M1,q ) ⊂ ResF/Q GL2 (Af ).

q|M1

Denote by S2 (Kf ) the space of cuspidal automorphic forms of ResF/Q GL2 (A) of
weight 2, right-invariant under Kf (for more details see Section 3.1 of [Urb95]).
Put γ = φ1 φ2 and write S2 (Kf , γ) for the forms with central character γ.
From now on, let Σ be a finite set of places of F containing
Sφ := {q | M1 Mc1 } ∪ {q | pdF }.
We denote by T(Σ) the O-subalgebra of EndO (S2 (Kf , γ)) generated by the
Hecke operators Tq for all places q 6∈ Σ.
Let J(Σ) ⊂ T(Σ) be the ideal generated by{Tq − φ1 ($q ) · #kq − φ2 ($q ) | q 6∈ Σ}.
Definition 7.8. Denote by m(Σ) a maximal ideal of T(Σ) containing the image ofJ(Σ). We set TΣ := T(Σ)m(Σ) . Moreover, set JΣ := J(Σ)TΣ . We refer to JΣ as
the Eisenstein ideal of TΣ .
Theorem 7.9 ([Ber09] Theorem 14). Let p > 3 and assume ` 6≡ ±1 mod p for` | dF . Let φ be an unramified Hecke character of infinity type φ(∞) (z) = z 2 . Thereexist Hecke characters φ1 , φ2 with φ1 /φ2 = φ such that their conductors are divisible
only by ramified primes or inert primes not congruent to ±1 mod p and such that#(TΣ /JΣ ) ≥ #(O/(Lint (0, φ))).
L Kf
The space S2 (Kf , ψ) is isomorphic as a G(Af )-module toπf for automorphic
representations π of a certain infinity type (see Theorem 7.10 below) with central
Kcharacter ψ. Here πf denotes the restriction of π to GL2 (Af ) and πf f stands for
the Kf -invariants.
For g ∈ G(Af ) we have the usual Hecke action of [Kf gKf ] on S2 (Kf ) andS2 (Kf , ψ). For primes q such that the vth component of Kf is GL2 (OF,v ) we$qdefine Tq = [KfKf ].
1
Combining the work of Taylor, Harris, and Soudry with results of FriedbergHoffstein and Laumon/Weissauer, one can show the following (see [BH07] for general case of cuspforms of weight k and forthcoming work for general CM fields by
C. P. Mok (with a similar assumption on the central character) and Harris-TaylorThorne-Lan (without such an assumption)):

24

TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Theorem 7.10 ([BH07] Theorem 1.1). Given a cuspidal automorphic representation π of GL2 (AF ) with π∞ isomorphic to the principal series representation
corresponding to





|t2 |
t1
t1 ∗
7→
t2
|t1 |
t2
and cyclotomic central character ψ (i.e., ψ c = ψ), let Σπ denote the set consistingof the places of F lying above p, the primes where π or π c is ramified, and theprimes ramified in F/Q.
Then there exists a finite extension E of Fp and a Galois representationρπ : GΣπ → GL2 (E)such that if q 6∈ Σπ , then ρπ is unramified at q and the characteristic polynomialof ρπ (Frobq ) is x2 − aq (π)x + ψ($q )(#kq ), where aq (π) is the Hecke eigenvalue
corresponding to Tq . Moreover, ρπ is absolutely irreducible.
Regarding the crystallinity of the representations ρπ we make the following conjecture (see Section 2.1 for the definition of a short crystalline Galois representation,and note that we assume p > 3):
Conjecture 7.11. If π is unramified at q | p then ρπ |Gq is crystalline and short.
This has now been proven in many cases by A. Jorza [Jor10]. Note that for the
choice of characters φi as in Theorem 7.9 the cuspforms occurring in S2 (Kf , ψ) areunramified at q | p.
Definition 7.12. Let χ be the mod $ reduction of φp . We now define thesubsets Π as the set of (strict equivalence classes of Galois) deformations of residual
representations of the form (2.1), one for each ρπ associated to an automorphic
representation π occurring in S2 (Kf , γ)m(Σ) and define Πτ to be the subset with1 ∗residual representation isomorphic to (a twist by φ2,p of) τ =. (Note that0 χΠτ ∩ Πτ 0 = ∅ for τ 6∼
= τ 0 by Proposition 3.2).
For every τ one has the natural surjective map T  Tτ resulting from thesurjections Rps  Rτtr .
Lemma 7.13.
If Π is the set of modular deformations defined above then theQring T ⊂ ρπ ∈Π O defined in the previous section can be identified with the Hecke
algebra TΣ . Furthermore, Tτ agrees with the quotient of TΣ acting on the subspaceof automorphic forms spanned by eigenforms π such that ρπ ∈ Πτ .
Proof. We will just prove the first part (concerning T and TΣ - the proof for Tτ
being analogous). We define the following O-algebra map:Yf : TΣ → T ⊂O : Tq 7→ (aq (π))ρπ ∈Π ,ρπ ∈Πwhere we use that aq (π) = tr ρπ (Frobq ) and therefore (aq (π))ρπ ∈Π = φ ◦ T (Frobq ).
K
We check that this map f is injective: By definition, TΣ ,→ ⊕ρπ ∈Π EndO (Vπ f ),where we denote by Vπ the representation space of π. Since Tv acts on π by av (π),
the image in each summand is given by the O-algebra generated by the av (π)’s.
Hence injectivity follows.
For surjectivity first note that f (TΣ ) ⊃ S := {T (Frobq ) | q 6∈ Σ}. Since f
is injective let us identify TΣ with f (TΣ ). Clearly TΣ is local, complete with

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

25

respect to its maximal ideal mΣ := m(Σ)TΣ and T has the same properties derived
from the properties of Rps . Moreover, looking at the residue fields we see thatmT ∩ TΣ = mΣ , so the mΣ -adic topology on TΣ is the subspace topology induced
from the mT -adic topology on T. Then using Theorem 8.1 in [Mat89] again we seethat the closure of TΣ in T equals the completion of TΣ . But since TΣ is alreadycomplete and T is topologically generated by S (i.e., the closure of S in T equals
T), we conclude that TΣ = T, hence we are done.

8. Main result
In this section we will state our main theorems (Theorems 8.1, 8.2 and 8.4) for thetwo-dimensional Galois representations over imaginary quadratic fields consideredin section 7. In this case many of the assumptions introduced throughout the
paper can be proven to hold. However, we would like to stress that the conclusions
are still valid if instead one assumes Assumptions 2.6, 2.7 and 4.2. To make thissection self-contained we will repeat all the assumptions in the case of an imaginary
quadratic field which were made in section 7.
Let F be an imaginary quadratic field, p > 3, p - #ClF dF , and assume ` 6≡
±1 mod p for ` | dF . Let φ be an unramified Hecke character of infinity typeφ(∞) (z) = z 2 and write χ for the mod $-reduction of φp . Furthermore, assume
that Σ contains the set of places Sφ (containing the primes dividing the conductorsof the two auxiliary characters φi from Theorem 7.9).
Let τ : GΣ → GL2 (F) be a non-semi-simple representation of the form


1 ∗
τ=
.
0 χ
Theorem 8.1. Suppose Assumption 7.6 is satisfied (see Remark 7.7). There exists
an F-basis B of HΣ1 (F, χ−1 ) which is modular, i.e., such that if b ∈ B and f : GΣ →F(χ−1 ) is a cocycle representing b, then the residual representation
1 f (g)χ(g)ρf : GΣ → GL2 (F), ρf (g) =0χ(g)is (up to a twist) the reduction (mod $) of a representation ρπ : GΣ → GL2 (O)attached to an automorphic representation π of GL2 (AF ).
Proof. This follows from Corollary 4.8. Note that Assumption 4.2 is satisfied by
Theorem 7.9 and Assumption 7.6 replaces Assumption 2.7 in Corollary 4.8.
From now on assume that Assumption 7.6 is satisfied. Then by Theorem 8.1 theSelmer group HΣ1 (F, χ−1 ) has a modular basis B. From now on assume also that B
is indeed projectively unique. Then, as discussed in section 5, the set of modular
1 ∗residual representations (i.e., those residual representations τ 0 =for which0 χΠτ 0 6= ∅) is in one-to-one correspondence with the elements of B given by sending
a modular extension to the corresponding residual representation. In particular,
the extension corresponding to τ must be (up to scaling) among the elements of B.
Hence, by rescaling one of the elements of B and reordering we may assume that
B = {e1 = eτ , e2 , . . . , es }.
We assume further now that if q ∈ Σ, then #kq 6≡ 1 (mod p). Using the principality of the ideal of reducibility (see Proposition 7.1) we can prove a modularity
result for residually reducible representations:

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TOBIAS BERGER1 AND KRZYSZTOF KLOSIN2

Theorem 8.2. Assume that the Galois representations ρπ for π occurring in
S2 (Kf , Ψ)m(Σ) (for notation please see Section 7.3) are crystalline at v | p. Also
suppose that #Hf1 (F, E/O((φp ))−1 ) ≤ #O/$m , where m = val$ (Lint (0, φ)).
Let ρ : GΣ → GL2 (E) be a continuous, irreducible representation which is crystalline at p | p and write τ : GΣ → GL2 (F) for its mod $ reduction with respect tosome lattice in E 2 . Suppose that τ ss ∼= 1 ⊕ χ. Assume that the sets Φτ 0 for τ 0 ∈ Bare finite. Then ρ is modular, i.e., there exists an automorphic representation π 0of GL2 (AF ) such that ρ ∼= ρπ0 .
Remark 8.3. As discussed in Remark 7.7 and the paragraph following Conjecture
7.11 the first two assumptions should be satisfied in the majority of cases by workof Jorza and Rubin.
Proof. This is a summary of the arguments carried out so far. As in the proofof Theorem 8.1 we note that Assumption 4.2 is satisfied. Also Assumption 2.6
is satisfied by Corollary 9.7 in [BK13] (see also discussion in Example 7.5) Let
Ritr,0 , Ti be as before the O-subalgebra of Ri0 (defined as the image of Rτred insideRτred ⊗O E) generated by traces and the Hecke algebra (respectively) corresponding
to ei . We denote the corresponding ideals of reducibility and the Eisenstein ideal
by Iitr,0 and Ji respectively. We get for every i = 1, 2, . . . , s a commutative diagramwith surjective arrows:(8.1)

Ritr,0
Ritr,0 /Iitr,0

φi

/ Ti

/ Ti /Ji .

By Theorem 6.3 and Corollary 7.3 the bottom map is an isomorphism for every
i = 1, 2, . . . , s.
By Theorem 6.2 we obtain a canonical map Rτred → Rτtr,0 which factors throughan isomorphism Rτ0 ∼= Rτtr,0 . We know by Proposition 7.1 that Iτ0 is principal. Using
this and diagram (8.1) for i = τ (and our conclusion that the bottom map in (8.1)
is an isomorphism) we can apply Theorem 4.1 from [BK13] to conclude that φτ is
an isomorphism. Then any O-algebra map Rτ  O factors through Rτ0 ∼= Tτ , so
any deformation to O is modular.
Let Ψ be as in example 7.5. The property of HΣ1 (F, χ−1 ) of having a projectively
unique modular basis can be replaced by the assumption that HΣ1 (F, Ψ−1 ⊗ E/O)
is an F-vector space.
Theorem 8.4. Let the notation and assumptions be the same as in Theorem 8.2
except that we do not demand that HΣ1 (F, χ−1 ) has a projectively unique modular
basis, but instead assume that $HΣ1 (F, Ψ−1 ⊗ E/O) = 0. Then, as before, ρ ismodular.
Proof. We need to reprove Theorem 6.3. Our assumption that $ annihilates the
Selmer group along with injectivity of ι in the proof of Theorem 6.3 implies that$ annihilates Tτ /Iτtr,0 Tτ . We showed in the proof of Theorem 6.2 that Tτ ∼= Rτtr,0 .
tr,0
tr,0
Hence we conclude that $ annihilates Rτ /Iτ . As discussed towards the endof section 2.5, the module Rτtr,0 /Iτtr,0 is a cyclic O-module, hence we must have

ON MODULARITY OF REDUCIBLE RESIDUAL GALOIS REPRESENTATIONS

27

Rτtr,0 /Iτtr,0 ∼= F. Since Tτ /Jτ is a non-zero O-module, this implies that the map
Rτtr,0 /Iτtr,0  Tτ /Jτ
must be injective.


9. Acknowledgements

We would like to thank Gebhard Böckle and Jack Thorne for helpful comments
and conversations related to the contents of this article.
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1 School of Mathematics and Statistics, University of Sheffield, Hicks Building,Hounsfield Road, Sheffield S3 7RH, UK.
2 Department of Mathematics, Queens College, City University of New York, 65-30
Kissena Blvd, Queens, NY 11367, USA