\Bibitem{Sid80} \by A.~F.~Sidorenko \paper Letter to the editors \jour Mat. Zametki \yr 1980 \vol 27 \issue 4 \pages 667 \mathnet{http://mi.mathnet.ru/mzm6515} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=572840}