This is an expanded note prepared for a 40-minute elementary introduction to the Birch and Swinnerton-Dyer conjecture presented at the farewell party for Chen-Yu Chi, who was leaving Harvard after his 8 years as a graduate student and a junior fellow here. As majority of the audience are in fields orthogonal to number-theoretic studies, such a talk can easily succeed in convincing the audience that there is a dry, ridiculous, but famous conjecture coming out of nothing; crazy number theorists have wasted their life to contribute to the list of partial results, which cannot even be claimed to be a long list.
Hopefully I have failed in that way.
A Diophantine equation is a polynomial equation with integer unknowns, the study of which dates back to the ancient Greeks. The word "Diophantine" comes from the Greek mathematician Diophantus of Alexandria of the 3rd century, the author of a series of books titled Arithmetica. In 1637, Pierre de Fermat scribbled the famous Fermat's last theorem on the margin of his own copy of Arithmetica. The efforts toward its proof has been widely regarded as one of the most glorious stories of mathematics of all time.
Diophantus studied many problems which are essentially quadratic equations from the modern point of view. For example, which integer triples are the lengths of a right triangle? In other words, what are the rational solutions to the equation ? Notice that the latter equation defines a smooth curve of genus 0 and we are concerned with the set of rational points .
So the problem of finding all rational points of a genus 0 curve reduces to finding one rational point. Because each genus 0 curve can be embedded into as a plane conic, over a number field, this is achieved by a special case of the Hasse principle for quadratic forms. For example, when , has a point over if and only if it has a point over and over every local field . The latter can be determined handily using Hilbert symbols.
The following theorem of Faltings is rather remarkable.
In particular, we know that for any , the Fermat equation has at most finitely many solutions, which is far beyond the scope of any elementary methods. Faltings' 18-page proof uses a wide range of techniques from algebraic number theory and algebraic geometry: at least to me, a whole semester seminar on going through this masterpiece is a truly amusing experience.
Finding these finitely many points is another interesting unsolved problem which we will not go into here. Let us concentrate on the case of genus 1 curves. Let be a smooth curve of genus 1 with a rational point (such a curve is called an elliptic curve), then admits a group law with the identity element . In particular, is an abelian group. The old Mordell-Weil theorem asserts that this group cannot be enormous.
In other words, as an abstract group, can be decomposed as a direct some of finitely many copies of and a finite abelian group. Here the integer is called the rank of . Notice that can be both finite (when ) and infinite (when ). This somehow reflects the tension between the infiniteness on curves of genus 0 and finiteness on curves of higher genus, making the case of elliptic curves most interesting and intricate.
The arithmetics of elliptic curves is also a significant node of the giant web of arithmetic problems. Some examples are in order.
The following statements and conjectures apply for any elliptic curves over a global field . For simplicity, we consider the case .
Mazur's torsion theorem classifies all possible torsion part of . His proof is a paradigm of using scheme-theoretic methods to draw concrete arithmetic consequences.
On the contrary, we understand far less about the rank of . For example, it is not known which integer values can take; it is even not known whether can take arbitrarily large values. People intend to believe the following conjectures about the rank .
Tate-Shafarevich (1967) proved the unboundedness of the rank over functions fields. The unboundedness in the number field case is still unknown: the current record over is kept by Elkies (2006), he found an elliptic curve with at least 28 independent -points via the theory of Mordell-Weil lattices of elliptic K3 surfaces and searching techniques.
It is not even known whether the average rank has an upper bound until the recent seminal work of Bhargava-Shankar (2010). They used methods from the geometry of numbers to count certain integral orbits of the space of binary quartic forms to show that the average rank, if it exists, is less than .
Miraculously, the rank of an elliptic curve, which we do not have understand well, is related to the analytic properties of its -function . To motivate this, recall that associated to each algebraic variety over a finite field , its zeta function is a rational function of (the Weil conjecture). In our case, taking the reduction mod of an elliptic curve over gives an elliptic curve over (here we ignore what happens for the bad primes). Its zeta function is where . Analogous to the Euler factors of the Riemann zeta function, we define the local -factor of to be When evaluating its value at , we retrieve the arithmetic information at , Notice that each point in reduces to a point in . So when has high rank, then tends to be small. Birch and Swinnerton-Dyer did numerical experiments and suggested the heuristic
The -function of is defined to be the product of all local -factors, Formally evaluating the value at gives So intuitively the rank of will correspond to the value of at 1: the larger is, the "smaller" is. However, the value of at does not make sense since the product of only converges when . Nevertheless, if can be continued to an analytic function on the whole of , it may be reasonable to believe that the behavior of at contains the arithmetic information of the rank of . The famous Birch and Swinnerton-Dyer conjecture asserts that
For this reason, we call the number the analytic rank of . Even explicit constants are conjecturally described: Here is the Tate-Shafarevich group, measuring the failure of the Hasse principle for -torsors; is the regulator with respect to the Neron-Tate height pairing on ; is the period, a product of local periods obtained by integrating the Neron differential on .
This remarkable conjecture has been chosen as one of the seven Millennium Prize problems by the Clay Institute, with a million-dollar prize for its solution. It is even more remarkable when we notice that it is so "meaningless": the left-hand-side has no meaning because we did not even know whether was defined round at the time when conjecture was formulated (except for elliptic curves with complex multiplication due to the work of Deuring and Weil in the 1950s), and the right-hand-side has no meaning either because we do not know in general that is finite! Nevertheless, this is indeed a good situation for mathematicians: we have more flexibility to figure out what the truth is.
Here are some consequences of the BSD conjecture.
As we mentioned, the formulation of the BSD conjecture relies on the following two conjectures.
Thanks to the modularity theorem of Wiles and others, we now know can be always continued analytically to the whole of , but the finiteness of is still largely open: in each case that the finiteness of is known, the BSD conjecture of is proved along with it too.
There is much evidence in favor of the BSD conjecture, we now list a few of them. The function field case is of better shape than the number field case due to
The best general result to date for the BSD conjecture over number fields is due to the groundbreaking Gross-Zagier formula relating the central derivative and the heights of Heegner points on defined over an imaginary quadratic field. The Gross-Zagier formula has the following implication.
Building on the Gross-Zagier formula and his theory of Euler systems, Kolyvagin proved that
There is a weaker version of BSD concerning the parity of ranks.
Notice that the parity of the analytic rank is directly related to the root number, the sign of the functional equation of . The parity conjecture is more tractable because the root number can be defined independent on any conjectures using local Galois actions on the Tate module.
Nekovar (2006) extended the parity conjecture to totally real fields. Dokchitser-Dokchitser (2009) proved the -parity conjecture, which allows us to weaker the condition of finiteness of to the finiteness of the -primary part for a prime .
Lectures on the conjecture of Birch and Swinnerton-Dyer, Arithmetic of L-functions, AMS PCMI publications, 2011, 169-210.
Ranks "cheat Sheet", 2011, http://math.uci.edu/~asilverb/connectionstalk.pdf.
The Arithmetic of Elliptic Curves (Graduate Texts in Mathematics), Springer, 2010.