\Bibitem{GurKhr94} \by A.~P.~Gurevich, A.~P.~Khromov \paper First and second order differentiation operators with weight functions of variable sign \jour Mat. Zametki \yr 1994 \vol 56 \issue 1 \pages 3--15 \mathnet{http://mi.mathnet.ru/mzm2218} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=1309814} \zmath{https://zbmath.org/?q=an:0835.34093} \transl \jour Math. Notes \yr 1994 \vol 56 \issue 1 \pages 653--661 \crossref{https://doi.org/10.1007/BF02110552} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=A1994RH68200001}