\Bibitem{Mys49} \by A.~D.~Myshkis \paper On a criterion for subharmonicity \jour Sb. Math. \yr 1949 \vol 67 \issue 2 \mathnet{http://mi.mathnet.ru//eng/sm6005} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=32870} \zmath{https://zbmath.org/?q=an:0039.10903}