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.
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 by
These 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 is
The product formula shows that this is unchanged when is replaced by a larger number field containing .
Solved by gpt-5.6-sol high.
Choose a number field containing . At every place of , put
The triangle inequality gives
where at every non-Archimedean place because the coefficients are integers, while at an Archimedean place one may take
Raise these inequalities to the local weights and multiply over all places. The definition of the Absolute multiplicative Weil height and the product formula then give
This is the height bound for a polynomial evaluation.
Solved by gpt-5.6-sol high.