J. F. Brasche, V. D. Koshmanenko, and H. Neidhardt, “New aspects of Krein’s extension theory”, Ukr. Mat. Zhurn., vol. 46, no. 1-2, pp. 37–54, Feb. 1994, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5768