\Bibitem{Tik60} \by V.~M.~Tikhomirov \paper Letter to the editors \jour Russian Math. Surveys \yr 1960 \vol 15 \issue 6 \mathnet{http://mi.mathnet.ru/eng/rm6800} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=117489} \zmath{https://zbmath.org/?q=an:0097.09103}