\Bibitem{Gam90} \by A.~N.~Gamova \paper Effective oracles \jour Algebra Logika \yr 1990 \vol 29 \issue 6 \pages 637--647 \mathnet{http://mi.mathnet.ru/al2126} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=1294233}