Height of a rational number Created 2026-09-24 Updated 2026-09-24
For coprime integers with ,This follows directly from the real and p-adic absolute values, or from the height-Mahler measure formula for the primitive polynomial .
Past exam of the mathematics course of the University of Cambridge 2025 iii Paper 166 2 f Solution Created 2026-09-24 Updated 2026-09-25
For coprime integers with ,This follows either directly from the real and p-adic absolute values, or from the height-Mahler measure formula applied to the primitive minimal polynomial .
Past exam of the mathematics course of the University of Cambridge 2025 iii Paper 166 2 h Solution Created 2026-09-24 Updated 2026-09-25
For an integer , let be a root ofIts roots areThe polynomial is irreducible over : its discriminant is , and the product of the coprime consecutive integers and cannot be a square unless both are squares, which is impossible for consecutive positive squares beyond .
The height-Mahler measure formula givesBoth algebraic conjugates of exceed , soConsequentlyFix, for example, . For all sufficiently large , the ratio is larger than by a fixed margin, while . Hence, for every , some sufficiently large satisfies
Past exam of the mathematics course of the University of Cambridge 2026 iii Paper 166 2 a Solution Created 2026-09-24 Updated 2026-09-25
Ifits Mahler measure isIf is the primitive minimal polynomial of an algebraic number and , the height-Mahler measure formula is
Past exam of the mathematics course of the University of Cambridge 2026 iii Paper 166 3 c Solution Created 2026-09-24 Updated 2026-09-25
Let be the monic cubic minimal polynomial of the algebraic integer , let be its conjugates, and let be the two conjugates of a nonrational . Because the degrees three and two are coprime, the fields are linearly disjoint, andis a nonzero integer.
AssumeFor large this makes bounded. Since is a quadratic algebraic integer, the height-Mahler measure formula givesand therefore . The two remaining factors with are bounded, while the three factors with are . Consequently
We now use the standard effective norm form consequence of Part (b). Forwhere is an integral basis of , the effective norm-form height estimate supplies effective constants , depending only on and , such thatfor every nonzero . Its proof factors , balances a generator using the Dirichlet unit theorem, and applies the Baker lower bound for a homogeneous linear form in logarithms to the linear relations defining ; the coprime degrees and exclude a unit-family degeneracy.
Apply this estimate to . Since , the height inequalities imply . HenceChoose the effective value . The last inequality bounds effectively. The rational integers are already covered by the stronger degree-three Liouville approximation theorem, and the Northcott theorem leaves only finitely many remaining quadratic integers of bounded height. Taking the minimum ofover this effective finite set gives an effective and provesfor every .