# Modularity theorem

In mathematics, the modularity theorem (formerly called the Taniyama–Shimura–Weil conjecture and several related names) states that elliptic curves over the field of rational numbers are related to modular forms. Andrew Wiles proved the modularity theorem for semistable elliptic curves, which was enough to imply Fermat's last theorem. Later, Christophe Breuil, Brian Conrad, Fred Diamond, and Richard Taylor extended Wiles' techniques to prove the full modularity theorem in 2001. The modularity theorem is a special case of more general conjectures due to Robert Langlands. The Langlands program seeks to attach an automorphic form or automorphic representation (a suitable generalization of a modular form) to more general objects of arithmetic algebraic geometry, such as to every elliptic curve over a number field. Most cases of these extended conjectures have not yet been proved, however, Freitas, Hung & Siksek (2015) proved that elliptic curves defined over real quadratic fields are modular.

## Statement

The theorem states that any elliptic curve over Q can be obtained via a rational map with integer coefficients from the classical modular curve $X_0(N)$ for some integer N; this is a curve with integer coefficients with an explicit definition. This mapping is called a modular parametrization of level N. If N is the smallest integer for which such a parametrization can be found (which by the modularity theorem itself is now known to be a number called the conductor), then the parametrization may be defined in terms of a mapping generated by a particular kind of modular form of weight two and level N, a normalized newform with integer q-expansion, followed if need be by an isogeny.

The modularity theorem implies a closely related analytic statement: to an elliptic curve E over Q we may attach a corresponding L-series. The L-series is a Dirichlet series, commonly written

$L(E, s) = \sum_{n=1}^\infty \frac{a_n}{n^s}.$

The generating function of the coefficients $a_n$ is then

$f(E, q) = \sum_{n=1}^\infty a_n q^n.$

If we make the substitution

$q = e^{2 \pi i \tau}\$

we see that we have written the Fourier expansion of a function $f(E, \tau)$ of the complex variable τ, so the coefficients of the q-series are also thought of as the Fourier coefficients of $f$. The function obtained in this way is, remarkably, a cusp form of weight two and level N and is also an eigenform (an eigenvector of all Hecke operators); this is the Hasse–Weil conjecture, which follows from the modularity theorem.

Some modular forms of weight two, in turn, correspond to holomorphic differentials for an elliptic curve. The Jacobian of the modular curve can (up to isogeny) be written as a product of irreducible Abelian varieties, corresponding to Hecke eigenforms of weight 2. The 1-dimensional factors are elliptic curves (there can also be higher-dimensional factors, so not all Hecke eigenforms correspond to rational elliptic curves). The curve obtained by finding the corresponding cusp form, and then constructing a curve from it, is isogenous to the original curve (but not, in general, isomorphic to it).

## History

Taniyama (1956) stated a preliminary (slightly incorrect) version of the conjecture at the 1955 international symposium on algebraic number theory in Tokyo and Nikko. Goro Shimura and Taniyama worked on improving its rigor until 1957. Weil (1967) rediscovered the conjecture, and showed that it would follow from the (conjectured) functional equations for some twisted L-series of the elliptic curve; this was the first serious evidence that the conjecture might be true. Weil also showed that the conductor of the elliptic curve should be the level of the corresponding modular form. The "astounding"[1]:211 conjecture (at the time known as the Taniyama–Shimura-Weil conjecture) became a part of the Langlands program, a list of important conjectures needing proof or disproof.[1]:211–215

The conjecture attracted considerable interest when Frey (1986) suggested that the Taniyama–Shimura–Weil conjecture implies Fermat's Last Theorem. He did this by attempting to show that any counterexample to Fermat's Last Theorem would imply the existence of at least one non-modular elliptic curve. This argument was completed when Serre (1987) identified a missing link (now known as the epsilon conjecture or Ribet's theorem) in Frey's original work, followed two years later by Ribet (1990)'s completion of a proof of the epsilon conjecture.

Even after gaining serious attention, the Taniyama–Shimura-Weil conjecture was seen by contemporary mathematicians as extraordinarily difficult or perhaps inaccessible to prove.[1]:203–205, 223, 226 For example, Wiles' ex-supervisor John Coates states that it seemed "impossible to actually prove",[1]:226 and Ken Ribet considered himself "one of the vast majority of people who believed [it] was completely inaccessible".[1]:223

In 1995 Wiles (1995), with some help from Richard Taylor, proved the Taniyama–Shimura–Weil conjecture for all semistable elliptic curves, which he used to prove Fermat's Last Theorem, and the full Taniyama–Shimura–Weil conjecture was finally proved by Diamond (1996), Conrad, Diamond & Taylor (1999), and Breuil et al. (2001) who, building on Wiles' work, incrementally chipped away at the remaining cases until the full result was proved.

Once fully proven, the conjecture became known as the modularity theorem.

Several theorems in number theory[which?] similar to Fermat's Last Theorem follow from the modularity theorem. For example: no cube can be written as a sum of two coprime n-th powers, n ≥ 3. (The case n = 3 was already known by Euler.)