# Galois representations with open image

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1 Galois representations with open image Ralph Greenberg University of Washington Seattle, Washington, USA May 7th, 2011

2 Introduction This talk will be about representations of the absolute Galois group of Q: G Q = Gal(Q/Q). We will consider continuous representations ρ : G Q GL n (Z p ) where n 3 and Z p denotes the ring of p-adic integers. We will be interested in constructing such representations so that the index [GL n (Z p ) : Im(ρ)] is finite. Equivalently, this means that Im(ρ) is an open subgroup of GL n (Z p ).

3 The case n = 1 For n = 1, for any prime p, and for any t 0, the group of p t -th roots of unity in Q is isomorphic to Z/p t Z. We denote this group by µ p t. The group G Q acts on µ p t. One can consider the inverse limit of the µ p t s as t. This is isomorphic to Z p and has a continuous action of G Q. This gives a continuous representation The map χ p is surjective. χ p : G Q GL 1 (Z p ).

4 The case n = 2 For n = 2, One obtains examples from the theory of elliptic curves. Suppose that E is an elliptic curve defined over Q. Suppose that p is any prime. For any t 0, the p t -torsion in the abelian group E(Q) is isomorphic to Z/p t Z Z/p t Z. It is denoted by E[p t ]. The Galois group G Q acts on E[p t ]. The p-adic Tate module for E is defined to be the inverse limit of the groups E[p t ] as t. It is a free Z p -module of rank 2. Thus, we get a continuous representation ρ E,p : G Q GL 2 (Z p ).

5 Serre s Theorem There is a famous theorem of Serre which states that if End C (E) = Z, then Im(ρ E,p ) has finite index in GL 2 (Z p ). Furthermore, under the same assumption, ρ E,p is surjective for all but finitely many primes p (depending on E). The assumption that End C (E) = Z means that E does not have complex multiplications. The entire first page of Cremona s table of elliptic curves are such non-cm elliptic curves.

6 Some known results when n = 3 Representations ρ : G Q GL 3 (Z p ) with open image have been constructed by Spencer Hamblen when p 8 (mod 21). The approach uses deformation theory. Surjective representations ρ : G F GL 3 (Z p ), where F = Q( 3), and for all but finitely many primes p 1 (mod 3), have been constructed by Margaret Upton. The construction is based on the action of G F on the p-adic Tate module of certain abelian varieties whose endomorphism ring contains the integers of F.

7 More results for n 3. I have managed to construct such representations ρ in the following cases: 1. p is an odd, regular prime and [ ] n 2 p 1 4 In particular, the construction works if n = 3 and p is a regular prime 5. Definition: Recall that p is a regular prime if p doesn t divide the class number of Q(µ p ). Here µ p denotes the p-th roots of unity. All primes p < 100 are regular, except for p = 37, 59, and 67.

8 More results. 2. n = 3, p 1 (mod 4) and p < 10, 000 (and even p < if p 1 or 4 (mod 5)). 3. p = 3, 4 n 29; p = 5, 4 n 13. In principle, the construction should work for every pair (p, n) where p is odd and n 3, except for (p, n) = (3, 3). It depends on finding an abelian extension K of Q with certain properties.

9 Our approach The approach that we will describe here is an algebraic number theory approach and involves the structure of the Galois groups of certain infinite extensions. The approach also involves some observations about the structure of a Sylow pro-p subgroup of SL n (Z p ). A Sylow p-subgroup of SL n (F p ) is the subgroup U n of upper triangular, unipotent matrices. A Sylow pro-p subgroup of SL n (Z p ) is the subgroup of matrices whose image under reduction modulo p is in U n. We denote this subgroup by P n. Let D n denote the subgroup of the diagonal matrices in GL n (Z p ) whose entries are (p 1)-st root of unity (in Z p.) Thus, D n is a finite subgroup of GL n (Z p ) of order (p 1) n. The group D n acts (as a group of automorphisms) on P n by conjugation.

10 More about the structure of P n The Sylow pro-p subgroup P n of SL n (Z p ) can be topologically generated by the following set of n elements: T n = { I n + E 12,..., I n + E (n 1)n } { In + pe n1 }. In contrast, the congruence subgroup I n + pm n (Z p ) requires n 2 topological generators. Its intersection with SL n (Z p ) requires n 2 1 generators.

11 Why does T n generate P n? A key lemma in proving that P n is generated topologically by T n is the following. We let M n (F p ) (0) denote the matrices of trace 0. Lemma: Let U n act on M n (F p ) (0) by conjugation. Then M n (F p ) (0) is a cyclic F p [U n ]-module generated by E n1. One applies this lemma to the successive quotients ( In + p t M n (Z p ) )/( I n + p t+1 M n (Z p ) ) for t 1, all of which can be identified with M n (F p ) by the maps I n + p t A A (mod pm n (Z p )).

12 These facts about P n and the action of D n on that group is a The action of D n on the elements of T n The above topological generating set T n for P n has an additional property. Each element generates a subgroup (topologically) which is fixed by the action of D n. If one conjugates by an element d of D n, with entries d 1,...,d n along the diagonal, then d ( I n + E ij ) d 1 = ( I n + E ij ) di d 1 j. In particular, since (I n + pe n1 ) = (I n + E n1 ) p, one has d ( I n + pe n1 ) d 1 = ( I n + pe n1 ) dnd 1 1.

13 A refinement of the Burnside Basis Theorem Suppose that Π is a pro-p group and that is a finite abelian group such that every element of has order dividing p 1. Suppose that acts on Π. Let Π denote the Frattini quotient of Π, the maximal abelian quotient of Π which has exponent p. Then Π is an F p -vector space with a linear action of. Assume it is finite dimensional. Lemma: If v Π and acts on v by a character χ : F p, then there exists an element π Π which is mapped to v by the map Π Π and such that δ(π) = π χ(δ).

14 p-rational number fields Shafarevich proved the following theorem in the 1960s. Theorem Let K = Q(µ p ). Assume that p is an odd, regular prime. Let M be the compositum of all finite p-extensions of K which are unramified except at the prime above p. Let Π = Gal(M/K). Then Π is a free pro-p group on p+1 2 generators. The field M is very big in general. It contains K(µ p t) for all t 1. It contains the field generated by the p t -th roots of all units in that field. It contains the field generated by all the p t -th roots of all units in all of those new fields. Etc. In general, a number field K is said to be p-rational if Π = Gal(M/K) is a free pro-p group.

15 The basic idea of the construction Consider K = Q(µ p ) and assume that p 3 and is a regular prime. Let = Gal(K/Q). 1. If p+1 2 n, then one can construct a surjective homomorphism σ 0 : Π P n. 2. If σ 0 is chosen carefully, then one can extend σ 0 to a homomorphism σ : Gal(M/Q) D n P n. 3. Define ρ = σ χ p. Then the image of ρ is an open subgroup of GL n (Z p ).

16 The structure of Gal(M/Q) Let K = Q(µ p ). Recall the notation = Gal(K/Q) and Π = Gal(M/K). Let G = Gal(M/Q). We have an exact sequence 1 Π G 1 This sequence turns out to split and so we can identify with a subgroup of G. Then G is a semidirect product and acts on Π by conjugation.

17 The structure of Gal(M/Q) If p is regular, then the Frattini quotient Π can be identified with Gal(L/K), where L = K( {p th roots of units in K} ). The action of on Π is determined by the action of on the units of K. The characters of which occur in its action on Π are all with multiplicity 1. odd {χ 0 }, Thus, one can choose a topological generating set for Π so that acts on the generators by the above characters.

18 The construction of σ acts on Π. Under the assumption that p is a regular prime, the -type for this action is odd {χ 0 }. If we choose a homomorphism ε : D n, then we have an action of on P n. The -type of P n is a set of n elements of, depending on ε. If one can arrange to have type of P n type of Π, then we can define a surjective, -equivariant homomorphism σ 0 : Π P n. We can then extend σ 0 to the semidirect product G = Π: σ : G D n P n.

19 The choice of ε If [ ] n 2 p 1 4, then one can choose χ 1,..., χ n odd {χ o } so that they are distinct and χ 1...χ n = χ 0. One can then define ε : D n by using characters ε 1,..., ε n chosen so that ε 1 /ε 2 = χ 1,..., ε n /ε 1 = χ n.

20 The other cases 2. n = 3, p 1 (mod 4) and p < 10, 000 (and even p < if p 1 or 4 (mod 5)). One applies the idea to K = Q(µ 5 ). For p 5, it turns out that Π = Gal(M/K) is a free pro-p group (on 3 generators) if and only if is not a p-th power in the completion of K at the prime(s) above p. This is true for all the primes that we ve check (myself and Rob Pollack).

21 3. p = 3, 4 n 29; One applies the idea to K = Q( 1, 13, 145, 209, 269, 373) which turns out to be p-rational for p = 3. This example was found by Robert Bradshaw.

22 Thank you!

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