\Bibitem{Lev69}
\by E.~M.~Levich
\paper On a problem of P.~Hall
\jour Dokl. Akad. Nauk SSSR
\yr 1969
\vol 188
\issue 6
\pages 1241--1243
\mathnet{http://mi.mathnet.ru/dan34970}
\mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=0251152}
\zmath{https://zbmath.org/?q=an:0215.10102}