Nitz, O. “Generalized Resolvents of Isometric Linear Relations in Pontryagin Spaces, II: Krein-Langer Formula”. Methods of Functional Analysis and Topology 6, no. 3 (September 25, 2000): 72–96. Accessed December 17, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/126.