Bakhtin, A. K., and V. E. V’yun. “Application of a Separating Transformation to Estimates of Inner Radii of Open Sets”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 10, Oct. 2007, pp. 1313–1321, https://umj.imath.kiev.ua/index.php/umj/article/view/3390.