Dobrynskii, V. A. “On the Structure of the Set of Nonwandering Points of a Pair of Coupled Quadratic Maps”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 12, Dec. 1999, pp. 1704–1709, https://umj.imath.kiev.ua/index.php/umj/article/view/4773.