Published online by Cambridge University Press: 19 June 2017
We discuss the connection between decidability of a theory of a large algebraic extensions of   ${\Bbb Q}$  and the recursiveness of the field as a subset of a fixed algebraic closure. In particular, we prove that if an algebraic extension K of
 ${\Bbb Q}$  and the recursiveness of the field as a subset of a fixed algebraic closure. In particular, we prove that if an algebraic extension K of   ${\Bbb Q}$  has a decidable existential theory, then within any fixed algebraic closure
 ${\Bbb Q}$  has a decidable existential theory, then within any fixed algebraic closure   $\widetilde{\Bbb Q}$  of
 $\widetilde{\Bbb Q}$  of   ${\Bbb Q}$ , the field K must be conjugate over
 ${\Bbb Q}$ , the field K must be conjugate over   ${\Bbb Q}$  to a field which is recursive as a subset of the algebraic closure. We also show that for each positive integer e there are infinitely many e-tuples
 ${\Bbb Q}$  to a field which is recursive as a subset of the algebraic closure. We also show that for each positive integer e there are infinitely many e-tuples   $\sigma\in {\text{Gal}}\left( {\Bbb Q} \right)^e $  such that the field
 $\sigma\in {\text{Gal}}\left( {\Bbb Q} \right)^e $  such that the field   $\widetilde{\Bbb Q}\left( \sigma\right)$  is primitive recursive in
 $\widetilde{\Bbb Q}\left( \sigma\right)$  is primitive recursive in   $\widetilde{\Bbb Q}$  and its elementary theory is primitive recursively decidable. Moreover,
 $\widetilde{\Bbb Q}$  and its elementary theory is primitive recursively decidable. Moreover,   $\widetilde{\Bbb Q}\left( \sigma\right)$  is PAC and
 $\widetilde{\Bbb Q}\left( \sigma\right)$  is PAC and   ${\text{Gal}}\left( {\widetilde{\Bbb Q}\left( \sigma\right)} \right)$  is isomorphic to the free profinite group on e generators.
 ${\text{Gal}}\left( {\widetilde{\Bbb Q}\left( \sigma\right)} \right)$  is isomorphic to the free profinite group on e generators.