Mordell-Weil theorem
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In mathematics, the Mordell-Weil theorem states that for an abelian variety A over a number field K, the group A(K) of K-rational points of A is a finitely-generated abelian group. The case with A an elliptic curve E and K the rational number field Q is Mordell's theorem, answering a question apparently posed by Poincaré around 1908; it was proved by Louis Mordell in 1922.
The tangent-chord process (one form of addition theorem on a cubic curve) had been known as far back as the seventeenth century. The process of infinite descent of Fermat was well known, but Mordell succeeded with in establishing a result on the quotient group
- E(Q)/2E(Q)
which forms a major step in the proof. Certainly the finiteness of this group is a necessary condition for E(Q) to be finitely-generated; and it shows that the rank is finite. This turns out to be the essential difficulty. It can be proved by direct analysis of the doubling of a point on E.
Some years later André Weil took up the subject, producing the generalisation in his doctoral dissertation published in 1928. More abstract methods were required, to carry out a proof with the same basic structure. The second half of the proof needs some type of height function, in terms of which to bound the 'size' of points of A(K). Some measure of the co-ordinates will do; heights are logarithmic, so that (roughly speaking) it is a question of how many digits are required to write down a set of homogeneous coordinates. For an abelian variety, there is no a priori preferred representation, though, as a projective variety.
Both halves of the proof have been improved significantly, by subsequent technical advances: in Galois cohomology as applied to descent, and in the study of the best height functions (which are quadratic forms). The theorem left unanswered a number of questions:
- Calculation of the rank (still a demanding computational problem, and not always effective, as far as we know).
- Meaning of the rank: see Birch and Swinnerton-Dyer conjecture.
- For a curve C in its Jacobian variety as A, can the intersection of C with A(K) be infinite? (Not unless C = A, according to Mordell's conjecture, proved by Faltings.)
- In the same context, can C contain infinitely many torsion points of A? (No, according to the Manin-Mumford conjecture proved by Raynaud, other than in the elliptic curve case.)
See also: arithmetic of abelian varieties.