Elliptic curves and modularity

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Elliptic curves and modularity For background and (most) proofs, we refer to [1]. 1 Weierstrass models Let K be any field. For any a 1, a 2, a 3, a 4, a 6 K consider the plane projective curve C given by the equation y 2 z + a 1 xyz + a 3 yz 2 = x 3 + a 2 x 2 z + a 4 xz 2 + a 6 z 3. (1) An equation as above is called a Weierstrass equation. We also say that (1) is a Weierstrass model for C. L-Rational points For any field extension L/K we can consider the L-rational points on C, i.e. the points on C with coordinates in L: The point at infinity C(L) := {(x : y : z) P 2 L : equation (1) is satisfied }. The point O := (0 : 1 : 0) C(K) is the only K-rational point on C with z = 0. It is always smooth. Most of the time we shall instead of a homogeneous Weierstrass equation write an affine Weierstrass equation: C : y 2 + a 1 xy + a 3 y = x 3 + a 2 x 2 + a 4 x + a 6 (2) which is understood to define a plane projective curve. The discriminant Define b 2 := a 2 1 + 4a 2, b 4 := 2a 4 + a 1 a 3, b 6 := a 2 3 + 4a 6, b 8 := a 2 1a 6 + 4a 2 a 6 a 1 a 3 a 4 + a 2 a 2 3 a 2 4. For any Weierstrass equation we define its discriminant := b 2 2b 8 8b 3 4 27b 2 6 + 9b 2 b 4 b 6. 1

Note that if char(k) 2, then we can perform the coordinate transformation y (y a 1 x a 3 )/2 to arrive at an equation y 2 = 4x 3 + b 2 x 2 + 2b 4 x + b 6. The discriminant (w.r.t. x) of the right-hand side of this equation is simply 2 4, which follows from a straightforward computation using the identity b 2 4 = b 2 b 6 4b 8. If a 1 = a 3 = 0, so the Weierstrass equation is of the form y 2 = f(x) with f a cubic monic polynomial (in x), then = 2 4 disc x (f). Proposition 1. A curve C/K given by a Weierstrass equation ((1) or (2)) has 0 if and only if C is smooth. Nonsmooth points Suppose C is not smooth, then there is exactly one singular point P C(K). Let c 4 := b 2 2 24b 4 (we know = 0). We distinguish two possibilities: C has a node at P (i.e. a double point); this happens if and only if c 4 0. C has a cusp at P ; this happens if and only if c 4 = 0. Changing equations We allow the coordinate transformations x = u 2 x + r y = u 3 y + su 2 x + t with r, s, t, u K and u 0. The discriminant changes as = u 12. If r = s = t = 0, then the coefficients of the Weierstrass equation transform as 2 Elliptic curves a i = a iu i. When studying elliptic curves, there is (in theory) nothing lost, when one restricts to (nonsingular) curves given by Weierstrass equations. Proposition 2. Let (E, P ) be an elliptic curve over the field K, i.e. E is a smooth algebraic curve of genus one over K and P E(K). Then there exists a curve C given by a Weierstrass model with a 1, a 2, a 3, a 4, a 6 K and an isomorphism φ : E C with φ(p ) = (0 : 1 : 0). Conversely, for every curve C defined by a Weierstrass equation with 0, we have that (C, O) is an elliptic curve. Usually we just refer to E/K as an elliptic curve over K (the point P E(K) is understood). Using e.g. the well-known chord and tangent addition, the 2

rational points E(K) get the structure of an abelian group. If K is a number field, then the Mordell-Weil theorem states that E(K) is finitely generated, i.e. E(K) T Z r for some finite abelian group T and some r Z 0. The number r is called the rank of E(K), denoted rank(e(k)). Minimal models and the discriminant Let E/Q be an elliptic curve and p a prime. Among all possible Weierstrass models for E with a 1,..., a 6 Z there are models with ord p ( ) minimal. Such a model for E is called minimal at p. Proposition 3. For an elliptic curve E/Q there exists a Weierstrass model with a 1,..., a 6 Z which is minimal at p for all primes p. A Weierstrass model as in the proposition is called a global minimal model, or simply a minimal model, for E. This minimal model is not unique in general, but its discriminant is uniquely determined by E. This invariant of E is called the minimal discriminant of E and denoted by min (E). Reduction Let E/Q be an elliptic curve and p a prime. Choose a Weierstrass model for E with a 1,..., a 6 Z which is minimal at p. This model can be reduced modulo p by simply mapping a i to a i := a i (mod p) Z/pZ =: F p. This gives a Weierstrass model with a 1,..., a 6 F p and defines a curve over the finite field F p. As it turns out, (the isomorphism class of) this curve Ẽ/F p is independent of the choice of Weierstrass model for E minimal at p. In particular, its number of points over F p, i.e. #Ẽ(F p), is an invariant of E, as is a p (E) := p + 1 #Ẽ(F p). If Ẽ/F p is nonsingular, then we say that E has good reduction at p, otherwise we say that E has bad reduction at p. In the latter case, we say that E has multiplicative reduction if Ẽ/F p has a node, and we say that E has additive reduction at p if Ẽ/F p has a cusp. Remark 4. Obviously, any Weierstrass model for E with a 1,..., a 6 Z can be reduced modulo p to a Weierstrass model with a 1,..., a 6 F p. The latter defines again a curve over the finite field F p, but (the isomorphism class of) this curve may depend on the chosen Weierstrass model for E. In particular, is it easy to see that for every prime p there always exists a choice of model for E such that the resulting reduction of the model modulo p yields a Weierstrass model for a curve over F p with a cuspidal singularity. Minimal models at primes where the reduced curve has a node are easily characterized as follows. Proposition 5. Let E/Q be an elliptic curve and choose a Weierstrass model for it with a 1,..., a 6 Z. Let p be a prime, then the following are equivalent 3

Ẽ/F p has a node and the model is minimal at p; p and p c 4. If p and p c 4, then it might still be possible that Ẽ/F p has a node, but in that case the model is necessarily not minimal at p. The Conductor There is an important representation theoretic invariant associated to any E/Q, called the conductor of E, denoted N(E). The full definition is quite subtle, see Chapter 4, 10 of [2]. We state a partial definition here: N(E) Z >0 and for all primes p we have ord p (N(E)) = 0 if E has good reduction at p 1 if E has multiplicative reduction at p 2 + δ p if E has additive reduction at p where δ p Z 0. Furthermore, δ 2 6, δ 3 3, and δ p = 0 for p 5. In particular, if E has good or multiplicative reduction at 2 and 3, then this completely defines N(E). Note that N(E) and min (E) have exactly the same prime divisors, namely the primes where E has bad reduction. If E has only good or additive reduction, then E is said to be semi-stable. This amounts to the same thing as saying that N(E) is squarefree. L-series and BSD For an elliptic curve E/Q we define its L-series L E (s) := p (1 a p (E)p s + 1 N(E) (p)p 1 2s ) 1 where the product is over all primes p and 1 N(E) denotes the trivial Dirichlet character modulo N(E). It can be shown that for all primes p we have a p (E) 2 p and that as a consequence we have that the Dirichlet series corresponding to L E (s) converges to a holomorphic function for s C with Re(s) > 3/2. It is natural to ask if there exists an analytic continuation of L E (s) to the whole complex plane C. If this is possible, then we have a meromorphic (possibly holomorphic) function on C defined by ξ E (s) := N(E) s/2 (2π) s Γ(s)L E (s). For this function (also called Λ E (s) in the literature) it is natural to expect a functional equation ξ E (s) = ±ξ E (2 s) (3) for all s C and some choice of sign ± depending only on E (conjecturally ( 1) rank(e(q)) ). Suppose it is possible to analytically continue L E (s) to a region containing a neighborhood of s = 1. It is conjectured that at s = 1 the function L E (s) reflects arithmetic information about E/Q. Introduce the analytic rank r an (E) := ord s=1 (L E (s)), i.e. the order of vanishing of L E (s) at s = 1. Also introduce the algebraic rank r al (E) := rank(e(q)). 4

Conjecture 6 (Weak Birch and Swinnerton-Dyer conjecture). For any elliptic curve E/Q we have r an (E) = r al (E). There is also a stronger version of the conjecture, relating the first nonzero coefficient of the Taylor expansion of L E (s) around s = 1 to other (arithmetic) invariants of E, most notably the order of its so-called Shafarevich-Tate group, which is only conjecturally known to be finite. Some important cases of the Birch and Swinnerton-Dyer conjure were proved by Coates-Wiles, Gross-Zagier, and Kolyvagin. The latter result is that if E/Q is a modular elliptic curve (see below for a definition) we have 3 Modularity r an (E) 1 r an (E) = r al (E). (4) Definition 7. Let E/Q be an elliptic curve. Then E is said to be modular if there exists a newform f S 2 (Γ 0 (N(E))) such that a p (E) = a p (f) for all primes p. There are many equivalent definitions of modularity. For instance, if there exists some M Z >0 and a newform f S 2 (Γ 0 (M)) such that a p (E) = a p (f) for all but possibly finitely many primes p, then M = N(E) and a p (E) = a p (f) for all primes p. The conjecture that all elliptic curves over Q are in fact modular is known as the Shimura-Taniyama-Weil conjecture (and under many other names, including most permutations of subsets of the three names). Theorem 8 (Modularity). Every elliptic curve over Q is modular. This was proved for semi-stable elliptic curves in 1994 by Wiles, with help from Taylor. This sufficed to complete the proof of Fermat s Last Theorem. Subsequently, the methods of Wiles and Taylor were generalized until finally in 1999 the full modularity theorem above was proved by Breuil, Conrad, Diamond, and Taylor. As an example of modularity, consider the elliptic curve E given by the Weierstrass equation y 2 + y = x 3 x 2. One computes that = 11 and c 4 = 16 0 (mod 11). So the model is minimal and E has only bad reduction at p = 11, where it has a node. This yields for the conductor N(E) = 11. Furthermore, for all primes p we have very concretely #Ẽ(F p) = #{(x, y) F 2 p : y 2 + y = x 3 x 2 } + 1. The extra +1 comes from the point at infinity. We can make a little table for #Ẽ(F p) and consequently for a p (E) = p + 1 #Ẽ(F p). p 2 3 5 7 11 13 17 19... 2017... 1000003 #Ẽ(F p) 5 5 5 10 11 10 20 20... 2035... 999720 a p (E) 2 1 1 2 1 4 2 0... 17... 284 5

By the modularity theorem there must exist a newform f S 2 (Γ 0 (11)) such that a p (E) = a p (f) for all primes p. One easily checks that S 2 (Γ 0 (11)) new = S 2 (Γ 0 (11)) is one-dimensional and that η(τ) 2 η(11τ) 2 = q n=1 (1 qn ) 2 (1 q 11n ) 2 defines a normalized form in this space. So this must be the newform f given by the modularity theorem. Indeed, by (formally) expanding the product, we get f = q 2q 2 q 3 + 2q 4 + q 5 + 2q 6 2q 7 2q 9 2q 10 + q 11 2q 12 + 4q 13 + 4q 14 q 15 4q 16 2q 17 +4q 18 +2q 20 +... 17q 2017 +...+284q 1000003 +O(q 1000004 ). Here is a little table for a p (f). p 2 3 5 7 11 13 17 19... 2017... 1000003 a p (f) 2 1 1 2 1 4 2 0... 17... 284 We see that indeed for the prime p in the previous tables, we have a p (E) = a p (f). Consequences for BSD By modularity, the L-series of an elliptic curve E over Q equals the L-series of a newform in S 2 (Γ 0 (N(E))). For such an L-series we have well-known analytic continuation and functional equation results, showing that indeed L E and ξ E can be analytically continued to the whole complex plane and satisfy the expected functional equation. In particular, r an (E) is at least well-defined. Furthermore, the result (4) holds unconditionally now! References [1] Joseph H. Silverman, The arithmetic of elliptic curves, Graduate Texts in Mathematics, 106, Springer-Verlag, New York, 1986. [2] Joseph H. Silverman, Advanced topics in the arithmetic of elliptic curves, Graduate Texts in Mathematics, 151, Springer-Verlag, New York, 1994. 6