# Ribet's theorem

(Redirected from Epsilon conjecture)

In mathematics, Ribet's theorem (earlier called the epsilon conjecture or ε-conjecture) is a statement in number theory concerning properties of Galois representations associated with modular forms. It was proposed by Jean-Pierre Serre and proved by Ken Ribet. The proof of epsilon conjecture was a significant step towards the proof of Fermat's Last Theorem. As shown by Serre and Ribet, the Taniyama–Shimura conjecture (whose status was unresolved at the time) and the epsilon conjecture together imply that Fermat's Last Theorem is true.

## Statement

Let E be an elliptic curve with integer coefficients in a Néron minimal model. Suppose that the discriminant Δ of E is written as the product Πpδp of prime powers pδp and similarly the conductor N of E is the product Πpnp of prime powers. Suppose that E is a modular elliptic curve, then we can perform a level descent modulo primes ℓ dividing one of the exponents δp of a prime dividing the discriminant. If pδp is an odd prime power factor of Δ and if p divides N only once (i.e. np=1), then there exists another elliptic curve E' , with conductor N' = N/p, such that the coefficients of the L-series of E are congruent modulo ℓ to the coefficients of the L-series of E' .

The epsilon conjecture is a relative statement: assuming that a given elliptic curve E over Q is modular, it predicts the precise level of E.

## History

In his thesis, Yves Hellegouarch[1] came up with the idea of associating solutions (a,b,c) of Fermat's equation with a completely different mathematical object: an elliptic curve. If ℓ is an odd prime and a, b, and c are positive integers such that

$a^\ell + b^\ell = c^\ell,\$

then a corresponding Frey curve is an algebraic curve given by the equation

$y^2 = x(x - a^\ell)(x + b^\ell).\$

This is a nonsingular algebraic curve of genus one defined over Q, and its projective completion is an elliptic curve over Q.

In 1982[2] Gerhard Frey called attention to the unusual properties of the same curve as Hellegouarch, now called a Frey curve. This provided a bridge between Fermat and Taniyama by showing that a counterexample to Fermat's Last Theorem would create such a curve that would not be modular. The conjecture attracted considerable interest when Frey (1986)[3] suggested that the Taniyama–Shimura–Weil conjecture implies Fermat's Last Theorem. However, his argument was not complete. In 1985[4][5] Jean-Pierre Serre proposed that a Frey curve could not be modular and provided a partial proof of this. This showed that a proof of the semistable case of the Taniyama-Shimura conjecture would imply Fermat's Last Theorem. Serre did not provide a complete proof and what was missing became known as the epsilon conjecture or ε-conjecture. In the summer of 1986, Ribet (1990)[6] proved the epsilon conjecture, thereby proving that the Taniyama–Shimura–Weil conjecture implied Fermat's Last Theorem.

## Implication of Fermat's Last Theorem

Suppose that the Fermat equation with exponent ℓ ≥ 3 had a solution in non-zero integers a, b, c. Let us form the corresponding Frey curve E. It is an elliptic curve and one can show that its discriminant Δ is equal to 16 (abc)2ℓ and its conductor N is the radical of abc, i.e. the product of all distinct primes dividing abc. By the Taniyama–Shimura conjecture, E is a modular elliptic curve. Since N is square-free, by the epsilon conjecture one can perform level descent modulo ℓ. Repeating this procedure, we will eliminate all odd primes from the conductor and reach the modular curve X0(2) of level 2. However, this curve is not an elliptic curve since it has genus zero, resulting in a contradiction.