\Bibitem{Fro74} \by Yu.~N.~Frolov \paper On a~uniqueness problem \jour Mat. Zametki \yr 1974 \vol 16 \issue 6 \pages 871--878 \mathnet{http://mi.mathnet.ru/mzm7528} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=372169} \zmath{https://zbmath.org/?q=an:0306.30009} \transl \jour Math. Notes \yr 1974 \vol 16 \issue 6 \pages 1116--1121 \crossref{https://doi.org/10.1007/BF01098435}