Zabolotskii, N. V., and M. R. Mostova. “Logarithmic Derivative and the Angular Density of Zeros for a Zero-Order Entire Function”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 4, Apr. 2014, pp. 473–481, https://umj.imath.kiev.ua/index.php/umj/article/view/2149.