Safonov, V. M., and Y. Y. Trohimchuk. “On One Criterion of Constancy of a Complex Function”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 8, Aug. 1999, pp. 1096–1104, https://umj.imath.kiev.ua/index.php/umj/article/view/4700.