《Modular elliptic curves and Fermat’ s Last theoren》

Annals of Mathematics, 141(1995),443-552 Modular elliptic curves and Fermat’ s Last theoren By ANDREW JOHN WILES* For Nada. Claire. Kate and Olivia Pierre de fermat Andrew ohn wiles Cubun autem in duos cubos, aut quadratoquadratum in duos quadra toquadratos, et generaliter mullam in infinitum ultra quadratum potestatum in duos ejusdem nominis fas est divider: cujes rei demonstrationem mirabile sane deter. Hanc marginis eaiguitas non capered Pierre de fermat n 1637 Abstract. When Andrew John Wiles was 10 years old, he read Eric Temple Bell,s The Last Problem and was so impressed by it that he decided that he would be the first person to prove Fermat's Last Theorem. This theorem states that there are no nonzero integers a,b, c, n with n >2 such that an+bn= cn. This object of this paper is to prove that all semistable elliptic curves over the set of rational numbers are modular. Fermat's Last Theorem follows as a corollary by virtue of work by Frey, Serre and Ribet Introduction An elliptic curve over Q is said to be modular if it has a finite covering by a modular curve of the form Xo(N). Any such elliptic curve has the property that its Hasse-Weil zeta function has an analytic continuation and satisfies a functional equation of the standard type. If an elliptic curve over Q with a iven j-invariant is modular then it is easy to see that all elliptic curves with the same j-invariant are modular(in which case we say that the j-invariant is modular). A well-known conjecture which grew out of the work of Shimura and Taniyama in the 1950s and 1960s asserts that every elliptic curve over Q is modular. However, it only became widely known through its publication in a paper of Weil in 1967[ We(as an exercise for the interested reader!), in which moreover, Weil gave conceptual evidence for the conjecture. Although it had been numerically verified in many cases, prior to the results described in this paper it had only been known that finitely many j-invariants were modular In 1985 Frey made the remarkable observation that this conjecture should imply Fermat's Last Theorem. The precise mechanism relating the two was formulated by Serre as the E-conjecture and this was then proved by ribet in the summer of 1986. Ribet's result only requires one to prove the conjecture for semistable elliptic curves in order to deduce Fermat's Last Theorem *The work on this paper was supported by an nSF grant

Annals of Mathematics, 141 (1995), 443-552 Pierre de Fermat Andrew John Wiles Modular elliptic curves and Fermat’s Last Theorem By Andrew John Wiles* For Nada, Claire, Kate and Olivia Cubum autem in duos cubos, aut quadratoquadratum in duos quadra￾toquadratos, et generaliter nullam in infinitum ultra quadratum potestatum in duos ejusdem nominis fas est dividere: cujes rei demonstrationem mirabilem sane detexi. Hanc marginis exiguitas non caperet. - Pierre de Fermat ∼ 1637 Abstract. When Andrew John Wiles was 10 years old, he read Eric Temple Bell’s The Last Problem and was so impressed byit that he decided that he would be the first person to prove Fermat’s Last Theorem. This theorem states that there are no nonzero integers a, b, c, n with n > 2 such that an + bn = cn. This object of this paper is to prove that all semistable elliptic curves over the set of rational numbers are modular. Fermat’s Last Theorem follows as a corollarybyvirtue of work byFrey, Serre and Ribet. Introduction An elliptic curve over Q is said to be modular if it has a finite covering by a modular curve of the form X0(N). Any such elliptic curve has the property that its Hasse-Weil zeta function has an analytic continuation and satisfies a functional equation of the standard type. If an elliptic curve over Q with a given j-invariant is modular then it is easy to see that all elliptic curves with the same j-invariant are modular (in which case we say that the j-invariant is modular). A well-known conjecture which grew out of the work of Shimura and Taniyama in the 1950’s and 1960’s asserts that every elliptic curve over Q is modular. However, it only became widely known through its publication in a paper of Weil in 1967 [We] (as an exercise for the interested reader!), in which, moreover, Weil gave conceptual evidence for the conjecture. Although it had been numerically verified in many cases, prior to the results described in this paper it had only been known that finitely many j-invariants were modular. In 1985 Frey made the remarkable observation that this conjecture should imply Fermat’s Last Theorem. The precise mechanism relating the two was formulated by Serre as the ε-conjecture and this was then proved by Ribet in the summer of 1986. Ribet’s result only requires one to prove the conjecture for semistable elliptic curves in order to deduce Fermat’s Last Theorem. *The work on this paper was supported byan NSF grant

444 ANDREW JOHN WILES Our approach to the study of elliptic curves is via their associated galois representations. Suppose that Pp is the representation of Gal( Q/Q)on the p-division points of an elliptic curve over Q, and suppose for the moment that P3 is irreducible. The choice of 3 is critical because a crucial theorem of Lang- lands and Tunnell shows that if P3 is irreducible then it is also modular. We then proceed by showing that under the hypothesis that P3 is semistable at 3 together with some milder restrictions on the ramification of P3 at the other primes, every suitable lifting of P is modular. To do this we link the problem via some novel arguments from commutative algebra, to a class number prob- lem of a well-known type. This we then solve with the help of the paper Tw] This suffices to prove the modularity of E as it is known that E is modular if and only if the associated 3-adic representation is modular. The key development in the proof is a new and surprising link between two trong but distinct traditions in number theory, the relationship between galois representations and modular forms on the one hand and the interpretation of special values of L-functions on the other. The former tradition is of course more recent. Following the original results of Eichler and Shimura in the 1950s and 1960,'s the other main theorems were proved by deligne, Serre and Langlands in the period up to 1980. This included the construction of galois representations associated to modular forms, the refinements of Langlands and Deligne (later completed by Carayol), and the crucial application by Langlands f base change methods to give converse results in weight one. However with the exception of the rather special weight one case, including the extension by Tunnell of Langlands original theorem, there was no progress in the direction of associating modular forms to Galois representations. From the mid 1980s the main impetus to the field was given by the conjectures of Serre which elaborated on the E-conjecture alluded to before. Besides the work of Ribet and others on this problem we draw on some of the more specialized developments of the 1980s, notably those of Hida and Mazur The second tradition goes back to the famous analytic class number for mula of Dirichlet, but owes its modern revival to the conjecture of Birch and Swinnerton-Dyer. In practice however, it is the ideas of Iwasawa in this field on which we attempt to draw, and which to a large extent we have to replace. The principles of Galois cohomology, and in particular the fundamental theorems of Poitou and Tate, also play an important role here. The restriction that p3 be irreducible at 3 is bypassed by means ntriguing argument with families of elliptic curves which share a common P5. Using this, we complete the proof that all semistable elliptic curves are modular. In particular, this finally yields a proof of Fermat's Last Theorem. In addition, this method seems well suited to establishing that all elliptic curves over Q are modular and to generalization to other totally real number fields Now we present our methods and results in more detail

444 ANDREW JOHN WILES Our approach to the study of elliptic curves is via their associated Galois representations. Suppose that ρp is the representation of Gal(Q¯ /Q) on the p-division points of an elliptic curve over Q, and suppose for the moment that ρ3 is irreducible. The choice of 3 is critical because a crucial theorem of Lang￾lands and Tunnell shows that if ρ3 is irreducible then it is also modular. We then proceed by showing that under the hypothesis that ρ3 is semistable at 3, together with some milder restrictions on the ramification of ρ3 at the other primes, every suitable lifting of ρ3 is modular. To do this we link the problem, via some novel arguments from commutative algebra, to a class number prob￾lem of a well-known type. This we then solve with the help of the paper [TW]. This suffices to prove the modularity of E as it is known that E is modular if and only if the associated 3-adic representation is modular. The key development in the proof is a new and surprising link between two strong but distinct traditions in number theory, the relationship between Galois representations and modular forms on the one hand and the interpretation of special values of L-functions on the other. The former tradition is of course more recent. Following the original results of Eichler and Shimura in the 1950’s and 1960’s the other main theorems were proved by Deligne, Serre and Langlands in the period up to 1980. This included the construction of Galois representations associated to modular forms, the refinements of Langlands and Deligne (later completed by Carayol), and the crucial application by Langlands of base change methods to give converse results in weight one. However with the exception of the rather special weight one case, including the extension by Tunnell of Langlands’ original theorem, there was no progress in the direction of associating modular forms to Galois representations. From the mid 1980’s the main impetus to the field was given by the conjectures of Serre which elaborated on the ε-conjecture alluded to before. Besides the work of Ribet and others on this problem we draw on some of the more specialized developments of the 1980’s, notably those of Hida and Mazur. The second tradition goes back to the famous analytic class number for￾mula of Dirichlet, but owes its modern revival to the conjecture of Birch and Swinnerton-Dyer. In practice however, it is the ideas of Iwasawa in this field on which we attempt to draw, and which to a large extent we have to replace. The principles of Galois cohomology, and in particular the fundamental theorems of Poitou and Tate, also play an important role here. The restriction that ρ3 be irreducible at 3 is bypassed by means of an intriguing argument with families of elliptic curves which share a common ρ5. Using this, we complete the proof that all semistable elliptic curves are modular. In particular, this finally yields a proof of Fermat’s Last Theorem. In addition, this method seems well suited to establishing that all elliptic curves over Q are modular and to generalization to other totally real number fields. Now we present our methods and results in more detail

MODULAR ELLIPTIC CURVES AND FERMATS LAST THEOREM Let f be an eigenform associated to the congruence subgroup TI(N)of SL2 (z of weight k 2 and character x. Thus if Tn is the Hecke operator associated to an integer n there is an algebraic integer c(n, f) such that Tnf c(n, f)f for each n. We let K be the number field generated over Q by the ie(n, f) together with the values of x and let Of be its ring of integers For any prime A of Of let Of, a be the completion of Of at A. The following theorem is due to Eichler and Shimura(for k= 2) and Deligne(for k> 2) The analogous result when k= 1 is a celebrated theorem of serre and Deligne but is more naturally stated in terms of complex representations. The image in that case is finite and a converse is known in many cases THEOREM 0. 1. For each prime p e Z and each prime Alp of Of there is a continuous representation Pf. A: Gal(Q/Q)-GL2(Of,A) ch is unramified outside the primes dividing Np and such that for all primes N trace Pf, A (Frob q)=c(a, f), det Pf,A(Frob q)=x(a)q We will be concerned with trying to prove results in the opposite direction that is to say, with establishing criteria under which a A-adic representation arises in this way from a modular form. We have not found any advantage assuming that the representation is part of a compatible system of A-adic representations except that the proof may be easier for some a than for others Assume p:Gal(Q/Q)→GL2(Fp) is a continuous representation with values in the algebraic closure of a finite field of characteristic p and that det po is odd. We say that po is modular if po and pf. mod A are isomorphic over Fp for some f and A and some embedding of Of/A in Fp. Serre has conjectured that every irreducible po of odd determinant is modular. Very little is known about this conjecture except when the image of Po in PGL2(Fp) is dihedral, A4 or S4. In the dihedral case it is true and due(essentially) to Hecke, and in the Ag and S4 cases it is again true and due primarily to Langlands, with one important case due to Tunnell (see Theorem 5.1 for a statement ). More precisely these theorems actually associate a form of weight one to the corresponding complex representation but the versions we need are straightforward deductions from the comple case. Even in the reducible case not much is known about the problem in the form we have described it, and in that case it should be observed that one must also choose the lattice carefully as only the semisimplification of Pf, X=pf. mod x is independent of the choice of lattice in K/

MODULAR ELLIPTIC CURVES AND FERMAT’S LAST THEOREM 445 Let f be an eigenform associated to the congruence subgroup Γ1(N) of SL2(Z) of weight k ≥ 2 and character χ. Thus if Tn is the Hecke operator associated to an integer n there is an algebraic integer c(n, f) such that Tnf = c(n, f)f for each n. We let Kf be the number field generated over Q by the {c(n, f)} together with the values of χ and let Of be its ring of integers. For any prime λ of Of let Of,λ be the completion of Of at λ. The following theorem is due to Eichler and Shimura (for k = 2) and Deligne (for k > 2). The analogous result when k = 1 is a celebrated theorem of Serre and Deligne but is more naturally stated in terms of complex representations. The image in that case is finite and a converse is known in many cases. Theorem 0.1. For each prime p ∈ Z and each prime λ|p of Of there is a continuous representation ρf,λ : Gal(Q¯ /Q) −→ GL2(Of,λ) which is unramified outside the primes dividing N p and such that for all primes q
N p, trace ρf,λ(Frob q) = c(q, f), det ρf,λ(Frob q) = χ(q)qk−1. We will be concerned with trying to prove results in the opposite direction, that is to say, with establishing criteria under which a λ-adic representation arises in this way from a modular form. We have not found any advantage in assuming that the representation is part of a compatible system of λ-adic representations except that the proof may be easier for some λ than for others. Assume ρ0 : Gal(Q¯ /Q) −→ GL2(F¯ p) is a continuous representation with values in the algebraic closure of a finite field of characteristic p and that det ρ0 is odd. We say that ρ0 is modular if ρ0 and ρf,λ mod λ are isomorphic over F¯ p for some f and λ and some embedding of Of /λ in F¯ p. Serre has conjectured that every irreducible ρ0 of odd determinant is modular. Very little is known about this conjecture except when the image of ρ0 in PGL2(F¯ p) is dihedral, A4 or S4. In the dihedral case it is true and due (essentially) to Hecke, and in the A4 and S4 cases it is again true and due primarily to Langlands, with one important case due to Tunnell (see Theorem 5.1 for a statement). More precisely these theorems actually associate a form of weight one to the corresponding complex representation but the versions we need are straightforward deductions from the complex case. Even in the reducible case not much is known about the problem in the form we have described it, and in that case it should be observed that one must also choose the lattice carefully as only the semisimplification of ρf,λ = ρf,λ mod λ is independent of the choice of lattice in K2 f,λ

ANDREW JOHN WILES If O is the ring of integers of a local field (containing Qp) we will say that P: Gal(Q/Q)-GL2(O)is a lifting of po if, for a specified embedding of the residue field of O in Fp, P and Po are isomorphic over Fp. Our point of view will be to assume that po is modular and then to attempt to give condition under which a representation p lifting Po comes from a modular form in the sense that pe pf, A over Kf, a for some f,. We will restrict our attention to ()Po is ordinary(at p) by which we mean that there is a one-dimensional subspace of Fa, stable under a decomposition group at p and such that the action on the quotient space is unramified and distinct from the action on the subspace (II) Po is fat(at p), meaning that as a representation of a decomposition group at p, Po is equivalent to one that arises from a finite fat group scheme over Zp, and det po restricted to an inertia group at p is the cyclotomic character We say similarly that p is ordinary (at p), if viewed as a representation to Qp there is a one-dimensional subspace of Q2 stable under a decomposition group at p and such that the action on the quotient space is unramified Let e: Gal(Q/Q)- Z denote the cyclotomic character. Conjectural converses to Theorem 0. 1 have been part of the folklore for many years but have hitherto lacked any evidence. The critical idea that one might dispense with compatible systems was already observed by Drinfield in the function field case Dr The idea that one only needs to make a geometric condition on the restriction to the decomposition group at p was first suggested by Fontaine and lazur. The following version is a natural extension of Serre's conjecture which is convenient for stating our results and is, in a slightly modified form, the one proposed by Fontaine and Mazur.(In the form stated this incorporates Serre's conjecture. We could instead have made the hypothesis that po is modular. CONJECTURE. Suppose that p: Gal(Q/Q)- GL2(o) is an irreducible ifting of po and that p is unramified outside of a finite set of primes. There are two cases (i) Assume that po is ordinary. Then if p is ordinary and detp=Ek-lx for some integer k>2 and some x of finite order, p comes from a modular orn (ii)Assume that po is flat and that p is odd. Then if p restricted to a de omposition group at p is equivalent to a representation on a p-divisible group, again p comes from a modular form

446 ANDREW JOHN WILES If O is the ring of integers of a local field (containing Qp) we will say that ρ : Gal(Q¯ /Q) −→ GL2(O) is a lifting of ρ0 if, for a specified embedding of the residue field of O in F¯ p, ρ¯ and ρ0 are isomorphic over F¯ p. Our point of view will be to assume that ρ0 is modular and then to attempt to give conditions under which a representation ρ lifting ρ0 comes from a modular form in the sense that ρ ρf,λ over Kf,λ for some f, λ. We will restrict our attention to two cases: (I) ρ0 is ordinary (at p) by which we mean that there is a one-dimensional subspace of F¯ 2 p, stable under a decomposition group at p and such that the action on the quotient space is unramified and distinct from the action on the subspace. (II) ρ0 is flat (at p), meaning that as a representation of a decomposition group at p, ρ0 is equivalent to one that arises from a finite flat group scheme over Zp, and det ρ0 restricted to an inertia group at p is the cyclotomic character. We say similarly that ρ is ordinary (at p), if viewed as a representation to Q¯ 2 p, there is a one-dimensional subspace of Q¯ 2 p stable under a decomposition group at p and such that the action on the quotient space is unramified. Let ε : Gal(Q¯ /Q) −→ Z× p denote the cyclotomic character. Conjectural converses to Theorem 0.1 have been part of the folklore for many years but have hitherto lacked any evidence. The critical idea that one might dispense with compatible systems was already observed by Drinfield in the function field case [Dr]. The idea that one only needs to make a geometric condition on the restriction to the decomposition group at p was first suggested by Fontaine and Mazur. The following version is a natural extension of Serre’s conjecture which is convenient for stating our results and is, in a slightly modified form, the one proposed by Fontaine and Mazur. (In the form stated this incorporates Serre’s conjecture. We could instead have made the hypothesis that ρ0 is modular.) Conjecture. Suppose that ρ : Gal(Q¯ /Q) −→ GL2(O) is an irreducible lifting of ρ0 and that ρ is unramified outside of a finite set of primes. There are two cases: (i) Assume that ρ0 is ordinary. Then if ρ is ordinary and det ρ = εk−1χ for some integer k ≥ 2 and some χ of finite order, ρ comes from a modular form. (ii) Assume that ρ0 is flat and that p is odd. Then if ρ restricted to a de￾composition group at p is equivalent to a representation on a p-divisible group, again ρ comes from a modular form

MODULAR ELLIPTIC CURVES AND FERMATS LAST THEOREM 447 In case (i) it is not hard to see that if the form exists it has to be of weight 2; in (i) of course it would have weight k. One can of course enlarge this conjecture in several ways, by weakening the conditions in(i) and(ii),by considering other number fields of Q and by considering groups other than GL? We prove two results concerning this conjecture. The first includes hypothesis that Po is modular. Here and for the rest of this paper we assume that p is an odd prime. THEOREM 0. 2. Suppose that po is irreducible and satisfies either(I) (II) above. Suppose also that Po is modular and that (i)po is absolutely irreducible when restricted to Q(V(-1)= (ii)If q =-1 mod p is ramified in Po then either polp is reduc the algebraic closure where Da is a decomposition group at q or polr absolutely irreducible where Ia is an inertia group at g. Then any representation p as in the conjecture does indeed come from a mod- alar form The only condition which really seems essential to our method is the re- quirement that po be modular The most interesting case at the moment is when p=3 and po can be de- fined over F3. Then since PGL2 (F3)a S4 every such representation is modular by the theorem of Langlands and Tunnell mentioned above. In particular, ev ery representation into GL2 (Z3) whose reduction satisfies the given conditions is modular. We deduce: THEOREM 0.3. Suppose that e is an elliptic curve defined over Q and that po is the Galois action on the 3-division points. Suppose that e has the following properties (i)e has good or multiplicative reduction at 3 (i)po is absolutely irreducible when restricted to Q(V-3) (iii) For any q=-1 mod 3 either polD is reducible over the algebraic closure or poll is absolutely irreducible Then e should be modular We should point out that while the properties of the zeta function follo directly from Theorem 0. 2 the stronger version that E is covered by XO(N)

MODULAR ELLIPTIC CURVES AND FERMAT’S LAST THEOREM 447 In case (ii) it is not hard to see that if the form exists it has to be of weight 2; in (i) of course it would have weight k. One can of course enlarge this conjecture in several ways, by weakening the conditions in (i) and (ii), by considering other number fields of Q and by considering groups other than GL2. We prove two results concerning this conjecture. The first includes the hypothesis that ρ0 is modular. Here and for the rest of this paper we will assume that p is an odd prime. Theorem 0.2. Suppose that ρ0 is irreducible and satisfies either (I) or (II) above. Suppose also that ρ0 is modular and that (i) ρ0 is absolutely irreducible when restricted to Q (−1) p−1 2 p  . (ii) If q ≡ −1 mod p is ramified in ρ0 then either ρ0|Dq is reducible over the algebraic closure where Dq is a decomposition group at q or ρ0|Iq is absolutely irreducible where Iq is an inertia group at q. Then any representation ρ as in the conjecture does indeed come from a mod￾ular form. The only condition which really seems essential to our method is the re￾quirement that ρ0 be modular. The most interesting case at the moment is when p = 3 and ρ0 can be de- fined over F3. Then since PGL2(F3) S4 every such representation is modular by the theorem of Langlands and Tunnell mentioned above. In particular, ev￾ery representation into GL2(Z3) whose reduction satisfies the given conditions is modular. We deduce: Theorem 0.3. Suppose that E is an elliptic curve defined over Q and that ρ0 is the Galois action on the 3-division points. Suppose that E has the following properties: (i) E has good or multiplicative reduction at 3. (ii) ρ0 is absolutely irreducible when restricted to Q√−3  . (iii) For any q ≡ −1 mod 3 either ρ0|Dq is reducible over the algebraic closure or ρ0|Iq is absolutely irreducible. Then E should be modular. We should point out that while the properties of the zeta function follow directly from Theorem 0.2 the stronger version that E is covered by X0(N)