NguyenD. T., and PhamV. D. “On the Degree of Holomorphic Mappings from an Annulus into an Annulus”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 8, Aug. 2004, pp. 1135–1138, https://umj.imath.kiev.ua/index.php/umj/article/view/3828.