Banakh, T. O., I. V. Protasov, and K. D. Protasova. “Descriptive Complexity of the Sizes of Subsets of groups”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 9, Sept. 2017, pp. 1280-3, https://umj.imath.kiev.ua/index.php/umj/article/view/1780.