Defining $\mathbb{Z}$ in $\mathbb{Q}$

We show that ${\mathbb Z}$ is definable in ${\mathbb Q}$ by a universal
first-order formula in the language of rings. We also present an
$\forall\exists$-formula for ${\mathbb Z}$ in ${\mathbb Q}$ with just one
universal quantifier. We exhibit new diophantine subsets of ${\mathbb Q}$ like
the complement of the image of the norm map under a quadratic extension, and we
give an elementary proof of the fact that the set of non-squares is
diophantine. Finally, we show that there is no existential formula for
${\mathbb Z}$ in ${\mathbb Q}$, provided one assumes a strong variant of the
Bombieri-Lang Conjecture for varieties over ${\mathbb Q}$ with many ${\mathbb
Q}$-rational points.