Glavan, V. A. “The Structure of Linear Extensions With the Favard Type Conditions II. Linear Extensions With the Additivity Property of Recurrent Motions”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 4 (April 25, 1993): 466–471. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5833.