The Ubiquity of Elliptic Curves Joseph Silverman Brown

The Ubiquity of Elliptic Curves Joseph Silverman (Brown University) MAA Invited Address Baltimore – January 18, 2003

Elliptic Curves Geometry, Algebra, Analysis and Beyond…

What is an Elliptic Curve? • An elliptic curve is a curve that’s also naturally a group. • The group law on an elliptic curve can be described: • Geometrically using intersection theory • Algebraically using polynomial equations • Analytically using complex analytic functions • Elliptic curves appear in many diverse areas of mathematics, ranging from number theory to complex analysis, and from cryptography to mathematical physics. -4 -

The Equation of an Elliptic Curve An Elliptic Curve is a curve given by an equation E : y 2 = f(x) for a cubic or quartic polynomial f(x) We also require that the polynomial f(x) has no double roots. This ensures that the curve is nonsingular. After a change of variables, the equation takes the simpler form E : y 2 = x 3 + A x + B Finally, for reasons to be explained shortly, we toss in an extra point O “at infinity, ” so E is really the set E = { (x, y) : y 2 = x 3 + A x + B } { O } -5 -

A Typical Elliptic Curve E E : Y 2 = X 3 – 5 X + 8 Surprising Fact: We can use geometry to make the points of an elliptic curve into a group. -6 -

The Group Law on an Elliptic Curve

Adding Points P + Q on E R Q P P+Q -8 -

Doubling a Point P on E Tangent Line to E at P R P 2*P -9 -

Vertical Lines and an Extra Point at Infinity O Add an extra point O “at infinity. ” The point O lies on every vertical line. P Q = –P Vertical lines have no third intersection point - 10 -

Properties of “Addition” on E Theorem: The addition law on E has the following properties: a) P + O = O + P = P for all P E. b) P + (–P) = O for all P E. c) (P + Q) + R = P + (Q + R) for all P, Q, R E. d) P + Q = Q + P for all P, Q E. In other words, the addition law + makes the points of E into a commutative group. All of the group properties are trivial to check except for the associative law (c). The associative law can be verified by a lengthy computation using explicit formulas, or by using more advanced algebraic or analytic methods. - 11 -

Algebraic Formulas for Addition on E Suppose that we want to add the points P 1 = (x 1, y 1) and P 2 = (x 2, y 2) on the elliptic curve E : y 2 = x 3 + Ax + B. Quite a mess!!!!! But… Crucial Observation: If A and B are in a field K and if P 1 and P 2 have coordinates in K, then P 1+ P 2 and 2 P 1 have coordinates in K. - 12 -

The Group of Points on E with Coordinates in a Field K The elementary observation on the previous slide leads to an important result: Theorem (Poincaré, 1900): Let K be a field and suppose that an elliptic curve E is given by an equation of the form y 2 = x 3 + A x + B with A, B K. Let E(K) be the set of points of E with coordinates in K, E(K) = { (x, y) E : x, y K } { O }. Then E(K) is a subgroup of E. - 13 -

Elliptic Curves and Complex Analysis Or…How the Elliptic Curve Acquired Its Unfortunate Moniker

The Arc Length of an Ellipse The arc length of a (semi)circle is given by the familiar integral x 2+y 2=a 2 -a a The arc length of a (semi)ellipse is more complicated b -a x 2/a 2 + y 2/b 2 = 1 a - 15 -

The Arc Length of an Ellipse Let k 2 = 1 – b 2/a 2 and change variables x ax. Then the arc length of an ellipse is An Elliptic Curve! with y 2 = (1 – x 2) (1 – k 2 x 2) = quartic in x. An elliptic integral is an integral , where R(x, y) is a rational function of the coordinates (x, y) on an “elliptic curve” E : y 2 = f(x) = cubic or quartic in x. - 16 -

Elliptic Integrals and Elliptic Functions The circular integral is equal to sin-1(w). Its inverse function w = sin(z) is periodic with period 2. The elliptic integral has an inverse w = (z) with two independent complex periods 1 and 2. (z + 1) = (z + 2) = (z) Doubly periodic functions are called for all z C. elliptic functions. - 17 -

Elliptic Functions and Elliptic Curves The -function and its derivative satisfy an algebraic relation This equation looks familiar The double periodicity of (z) means that it is a function on the quotient space C/L, where L is the lattice L = { n 1 1 + n 2 2 : n 1, n 2 Z }. 1 1 + 2 L 2 (z) and ’(z) are functions on a fundamental parallelogram - 18 -

The Complex Points on an Elliptic Curve The -function gives a complex analytic isomorphism E(C) Parallelogram with opposite sides identified = a torus Thus the points of E with coordinates in the complex numbers C form a torus, that is, the surface of a donut. E(C) = - 19 -

Elliptic Curves and Number Theory Rational Points on Elliptic Curves

E(Q) : The Group of Rational Points A fundamental and ancient problem in number theory is that of solving polynomial equations using integers or rational numbers. The description of E(Q) is a landmark in the modern study of Diophantine equations. Theorem (Mordell, 1922): Let E be an elliptic curve given by an equation E : y 2 = x 3 + A x + B with A, B Q. There is a finite set of points P 1, P 2, …, Pr so that every point P in E(Q) can be obtained as a sum P = n 1 P 1 + n 2 P 2 + … + n r. P r with n 1, …, nr Z. In other words, E(Q) is a finitely generated group. - 21 -

E(Q) : The Group of Rational Points The elements of finite order in the group E(Q) are quite well understood. Theorem (Mazur, 1977): The group E(Q) contains at most 16 points of finite order. The minimal number of points needed to generate the group E(Q) is much more mysterious! Conjecture: The number of points needed to generate E(Q) may be arbitrarily large. Current World Record: There is an elliptic curve with Number of generators for E(Q) 23. - 22 -

E(Fp) : The Group of Points Modulo p Number theorists also like to solve polynomial equations modulo p. This is much easier than finding solutions in Q, since there are only finitely many solutions in the finite field Fp! One expects E(Fp) to have approximately p+1 points. A famous theorem of Hasse (later vastly generalized by Weil and Deligne) quantifies this expectation. Theorem (Hasse, 1922): An elliptic curve equation E : y 2 x 3 + A x + B (modulo p) has p+1+ solutions (x, y) mod p, where the error satisfies - 26 -

Elliptic Curves and Cryptography

The (Elliptic Curve) Discrete Log Problem Let A be a group and let P and Q be known elements of A. The Discrete Logarithm Problem (DLP) is to find an m summands integer m satisfying Q = P + … + P = m. P. • There are many cryptographic constructions based on the difficulty of solving the DLP in various finite groups. • The first group used for this purpose (Diffie-Hellman 1976) was the multiplicative group Fp* in a finite field. • Koblitz and Miller (1985) independently suggested using the group E(Fp) of points modulo p on an elliptic curve. • At this time, the best algorithms for solving the elliptic curve discrete logarithm problem (ECDLP) are much less efficient than the algorithms for solving DLP in Fp* or factoring large integers. - 29 -

Elliptic Curve Diffie-Hellman Key Exchange Public Knowledge: A group E(Fp) and a point P of order n. BOB ALICE Choose secret 0 < b < n Choose secret 0 < a < n Compute QBob = b. P Compute QAlice = a. P Send QBob to Alice to Bob Send QAlice Compute b. QAlice Compute a. QBob and Alice have the shared value b. QAlice = ab. P = a. QBob Presumably(? ) recovering ab. P from a. P and b. P requires solving the elliptic curve discrete logarithm problem. - 30 -

Elliptic Curves and Classical Physics

The Elliptic Curve and the Pendulum - 32 -

The Elliptic Curve and the Pendulum In freshman physics, one assumes that q is small and derives the formula This leads to a simple harmonic motion for the pendulum. But this formula is only a rough approximation. The actual differential equation for the pendulum is - 33 -

How to Solve the Pendulum Equation - 34 -

How to Solve the Pendulum Equation An Elliptic Integral!!! An Elliptic Curve!!! Conclusion: tan(q /2) = Elliptic Function of t - 35 -

Elliptic Curves and Topology

Cobordism and Genus An important object in topology is the (complex oriented) cobordism ring W. For our purposes, it is enough to know that W is a polynomial ring in infinitely many variables: W= C[T 2, T 4, T 6, T 8, …]. (T 2 n is the cobordism class of projective space CPn. ) A (complex) genus is a ring homomorphism F : W C. The genus F is characterized by its logarithm - 37 -

What Makes a Genus Elliptic? A genus is a ring homomorphism, so it satisfies F(U x V) = F(U) F(V). Here U and V are (cobordism classes) of complex manifolds. It is interesting to impose a stronger multiplicative property: Let W V be a fiber bundle with fiber U, i. e. , W is a twisted product of U and V. Then we still require that F(W) = F(U) F(V). Ochanine proved that the logarithm of F is an elliptic integral! A genus whose logarithm is an elliptic integral is called an Elliptic Genus. - 38 -

Elliptic Curves and Modern Physics

Elliptic Curves and String Theory In string theory, the notion of a point-like particle is replaced by a curve-like string. As a string moves through space-time, it traces out a surface. For example, a single string that moves around and returns to its starting position will trace a torus. So the path traced by a string looks like an elliptic curve! In quantum theory, physicists like to compute averages over all possible paths, so when using strings, they need to compute integrals over the space of all elliptic curves. - 40 -

Elliptic Curves and Number Theory Fermat’s Last Theorem

Fermat’s Last Theorem and Fermat Curves Fermat’s Last Theorem says that if n > 2, then the equation an + bn = cn has no solutions in nonzero integers a, b, c. It is enough to prove the case that n = 4 (already done by Fermat himself) and the case that n = p is an odd prime. If we let x = a/c and y = b/c, then solutions to Fermat’s equation give rational points on the Fermat curve xp + yp = 1. But Fermat’s curve is not an elliptic curve. So how can elliptic curves be used to study Fermat’s problem? - 42 -

Elliptic Curves and Fermat’s Last Theorem Gerhard Frey (and others) suggested using an hypothetical solution (a, b, c) of Fermat’s equation to “manufacture” an elliptic curve Ea, b, c : y 2 = x (x – ap) (x + bp). Frey suggested that Ea, b, c would be such a strange curve, it shouldn’t exist at all. More precisely, Frey doubted that Ea, b, c could be modular. Ribet verified Frey’s intuition by proving that Ea, b, c is indeed not modular. Wiles completed the proof of Fermat’s Last Theorem by showing that (most) elliptic curves, in particular elliptic curves like Ea, b, c, are modular. - 43 -

Elliptic Curves and Fermat’s Last Theorem Ea, b, c : y 2 = x (x – ap) (x + bp) To Summarize: Suppose that ap + bp = cp with abc 0. Ribet proved that Ea, b, c is not modular Wiles proved that Ea, b, c is modular. Conclusion: The equation ap + bp = cp has no solutions. But what does it mean for an elliptic curve E to be modular? - 44 -

Elliptic Curves and Modularity There are many equivalent definitions, none of them particularly intuitive. Here’s one: E is modular if it is parameterized by modular forms! A modular form is a function f(t) with the property The variable t represents the elliptic curve Et whose lattice is Lt = {n 1+n 2 t : n 1, n 2 Z}. So just as in string theory, the space of all elliptic curves makes an unexpected appearance. - 45 -

Conclusion - 46 -

The Ubiquity of Elliptic Curves Joseph Silverman (Brown University) MAA Invited Address Baltimore – January 18, 2003