\Bibitem{Pro94} \by I.~V.~Protasov \paper Letter to the editor \jour Mat. Zametki \yr 1994 \vol 56 \issue 1 \pages 153 \mathnet{http://mi.mathnet.ru/mzm3906} \zmath{https://zbmath.org/?q=an:0833.22005} \transl \jour Math. Notes \yr 1994 \vol 56 \issue 1 \pages 759 \crossref{https://doi.org/10.1007/BF02110572}