\Bibitem{Smi52}
\by Yu.~M.~Smirnov
\paper Mappings of systems of open sets
\jour Sb. Math.
\yr 1952
\vol 73
\issue 1
\mathnet{http://mi.mathnet.ru//eng/sm5523}
\mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=50263}
\zmath{https://zbmath.org/?q=an:0047.16102}