\Bibitem{Sha99} \by S.~A.~Shapovalov \paper The depth invariant for group actions on countable phase spaces \jour Mat. Zametki \yr 1999 \vol 65 \issue 6 \pages 893--907 \mathnet{http://mi.mathnet.ru/mzm1125} \crossref{https://doi.org/10.4213/mzm1125} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=1728289} \zmath{https://zbmath.org/?q=an:1063.37501} \transl \jour Math. Notes \yr 1999 \vol 65 \issue 6 \pages 749--759 \crossref{https://doi.org/10.1007/BF02675590} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=000083786600031}