Northcott theorem Created 2026-09-24 Updated 2026-09-24
There are only finitely many algebraic numbers of bounded degree and bounded Absolute multiplicative Weil height. The finite set can be enumerated effectively from the bounded coefficients of their primitive minimal polynomials.
Past exam of the mathematics course of the University of Cambridge 2025 iii Paper 166 2 a Solution Created 2026-09-24 Updated 2026-09-24
Let be a number field. Its places consist of its real embeddings, conjugate pairs of complex embeddings, and the finite places associated with nonzero prime ideals of its ring of integers of a number field. At a real or complex place use the usual absolute value. If lies over the prime number with ramification index , normalize its absolute value byThese normalizations extend the standard absolute values on .
Write for the local degree of a place. Thus is at a real place, at a complex place, and at a finite place. The Absolute multiplicative Weil height isThe product formula shows that this is unchanged when is replaced by a larger number field containing .
Past exam of the mathematics course of the University of Cambridge 2026 iii Paper 166 2 b Solution Created 2026-09-24 Updated 2026-09-24
Choose a number field containing . At every place of , putThe triangle inequality giveswhere at every non-Archimedean place because the coefficients are integers, while at an Archimedean place one may takeRaise these inequalities to the local weights and multiply over all places. The definition of the Absolute multiplicative Weil height and the product formula then giveThis is the height bound for a polynomial evaluation.