Mordell–Weil theorem
In mathematics, the Mordell–Weil theorem states that for an abelian variety formula_1 over a number field formula_2, the group formula_3 of "K"-rational points of formula_1 is a finitely-generated abelian group, called the Mordell–Weil group. The case with formula_1 an elliptic curve formula_6 and formula_2 the field of rational numbers is Mordell's theorem, answering a question apparently posed by Henri Poincaré around 1901; it was proved by Louis Mordell in 1922. It is a foundational theorem of Diophantine geometry and the arithmetic of abelian varieties. History. 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 in establishing the finiteness of the quotient group formula_8 which forms a major step in the proof. Certainly the finiteness of this group is a necessary condition for formula_9 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 to Jacobians of higher genus curves over arbitrary number fields 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 formula_3. 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). Further results. The theorem left unanswered a number of questions: