\Bibitem{Ber50} \by S.~N.~Bernshtein \paper On the curvature of surfaces \jour Russian Math. Surveys \yr 1950 \vol 5 \issue 4 \mathnet{http://mi.mathnet.ru//eng/rm8456} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=36559} \zmath{https://zbmath.org/?q=an:0036.23001}