\Bibitem{DroKir73} \by Yu.~A.~Drozd, V.~V.~Kirichenko \paper Letter to the editor \jour Mat. Zametki \yr 1973 \vol 14 \issue 6 \pages 943 \mathnet{http://mi.mathnet.ru/mzm7887} \zmath{https://zbmath.org/?q=an:0284.16013}