\Bibitem{Kob73} \by G.~N.~Kobzev \paper btt-reducibility.~I \jour Algebra Logika \yr 1973 \vol 12 \issue 2 \pages 190--204 \mathnet{http://mi.mathnet.ru/al1376} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=0401447}