\Bibitem{Oma73} \by A.~I.~Omarov \paper Subsystems of reduced powers \jour Algebra Logika \yr 1973 \vol 12 \issue 1 \pages 74--82 \mathnet{http://mi.mathnet.ru/al1369} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=0323558}