-
Characterizing integers among rational numbers with a universal-existential formula
- American Journal of Mathematics
- Johns Hopkins University Press
- Volume 131, Number 3, June 2009
- pp. 675-682
- 10.1353/ajm.0.0057
- Article
- Additional Information
- Purchase/rental options available:
We prove that ${\Bbb Z}$ in definable in ${\Bbb Q}$ by a formula with two universal quantifiers followed by seven existential quantifiers. It follows that there is no algorithm for deciding, given an algebraic family of ${\Bbb Q}$-morphisms, whether there exists one that is surjective on rational points. We also give a formula, again with universal quantifiers followed by existential quantifiers, that in any number field defines the ring of integers.