Bakhtin, A. K. “Sharp Estimates for Inner Radii of Systems of Nonoverlapping Domains and Open Sets”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 12, Dec. 2007, pp. 1601–1618, https://umj.imath.kiev.ua/index.php/umj/article/view/3416.