d="Abstract" class="abstractSubHead">Abstract:

Hilbert’s Tenth Problem over the rationals is one of the biggest open problems in the area of undecidability in number theory. In this paper we construct new, computably presentable subrings $R \subseteq \mathbb {Q}$ having the property that Hilbert’s Tenth Problem for $R$, denoted $\operatorname {HTP}(R)$, is Turing equivalent to $\operatorname {HTP}(\mathbb {Q})$.

We are able to put several additional constraints on the rings $R$ that we construct. Given any computable nonnegative real number $r\leq 1$ we construct such rings $R=\mathbb {Z}[\mathcal {S}^{-1}]$ with $\mathcal {S}$ a set of primes of lower density $r$. We also construct examples of rings $R$ for which deciding membership in $R$ is Turing equivalent to deciding $\operatorname {HTP}(R)$ and also equivalent to deciding $\operatorname {HTP}(\mathbb {Q})$. Alternatively, we can make $\operatorname {HTP}(R)$ have arbitrary computably enumerable degree above $\operatorname {HTP}(\mathbb {Q})$. Finally, we show that the same can be done for subrings of number fields and their prime ideals.