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 .
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 .
For an integer , let be a root of
Its roots are
The 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 gives
Both algebraic conjugates of exceed , so
Consequently
Fix, for example, . For all sufficiently large , the ratio is larger than by a fixed margin, while . Hence, for every , some sufficiently large satisfies
If
its Mahler measure is
If is the primitive minimal polynomial of an algebraic number and , the height-Mahler measure formula is
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, and
is a nonzero integer.
Assume
For large this makes bounded. Since is a quadratic algebraic integer, the height-Mahler measure formula gives
and 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). For
where is an integral basis of , the effective norm-form height estimate supplies effective constants , depending only on and , such that
for 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 . Hence
Choose 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 of
over this effective finite set gives an effective and proves
for every .