AUTOMORPHIC GALOIS REPRESENTATIONS 3

One classical example is the group of Q-defined -torsion points of an elliptic curve

E defined over Q. We will treat this example in Section 2. In this survey we

will be interested in (compatible systems of) Galois representations arising from

automorphic representations. In Section 4 we will describe Galois representations

attached to an automorphic representation π which satisfies several technical con-

ditions. The statements of the most recent results (to the best of my knowledge)

on the inverse Galois problem obtained by means of compatible systems of Galois

representations attached to automorphic representations can be found in Section 5,

together with some ideas about their proofs.

A remarkable feature of this method is that, in addition, one obtains some

control of the ramification of the Galois extension that is produced. Namely, it will

only be ramified at the residual characteristic and at a finite set of auxiliary primes,

that usually one is allowed to choose (inside some positive density set of primes).

This will be highlighted in the statements below.

2. Some classical cases

In this section we revisit some classical examples of Galois representations at-

tached to geometric objects. We begin with the Galois representations attached to

the torsion points of elliptic curves, and later we will see them as a particular case

of Galois representations attached to modular forms.

2.1. Elliptic curves. An elliptic curve is a genus one curve, endowed with a

distinguished base point. Every elliptic curve E can be described by means of a

Weierstrass equation, that is, an aﬃne equation of the form

y2

+ a1xy + a3y =

x3

+

a2x2

+ a4x + a6

where the coeﬃcients a1,...,a6 lie in some field K. The most significant property of

elliptic curves is that the set of points of E (defined over some field extension L/K)

can be endowed with a commutative group structure, where the neutral element is

the distinguished base point.

Let E/Q be an elliptic curve and let be a prime number. We can consider the

subgroup E[ ](Q) of E(Q) consisting of -torsion points. This group is isomorphic

to the product of two copies of F . Moreover, since the elliptic curve is defined over

Q, the absolute Galois group GQ acts naturally on the set of Q-defined points of

E, and this action restricts to E[ ](Q). We obtain thus a Galois representation

ρE, : GQ → Aut(E[ ](Q)) GL2(F ).

As explained in the introduction, the image of ρE, can be realised as a Galois

group over Q. This brings forward the question of determining the image of such a

Galois representation. In this context, there is a classical result by J. P. Serre from

the seventies ([29], Th´ eor` eme 2).

Theorem 2.1 (Serre). Let E/Q be an elliptic curve without complex multi-

plication over Q. Then the representation ρE, is surjective for all except finitely

many primes .

We can immediately conclude that GL2(F ) can be realised as a Galois group

over Q for all except finitely many primes . However, we can do even better by

picking a particular elliptic curve and analysing the Galois representations attached

to it.