\Bibitem{Gli52} \by E.~V.~Glivenko \paper On planar variation \jour Sb. Math. \yr 1952 \vol 72 \issue 3 \mathnet{http://mi.mathnet.ru//eng/sm5447} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=48555} \zmath{https://zbmath.org/?q=an:0046.28701}