Mogilevskii, V. I. “Boundary Triplets and Krein Type Resolvent Formula for Symmetric Operators With Unequal Defect Numbers”. Methods of Functional Analysis and Topology 12, no. 3 (September 25, 2006): 258–280. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/319.