Bakhtin, A. K., and A. L. Targonskii. 2011. “Generalized (n, D)-Ray Systems of Points and Inequalities for Nonoverlapping Domains and Open Sets”. Ukrains’kyi Matematychnyi Zhurnal 63 (7): 867-79. https://umj.imath.kiev.ua/index.php/umj/article/view/2771.