1. Pontryagin, L.S. Hermitian operators in spaces with indefinite metric. Izv. Akad. Nauk SSSR, Ser. Mat. 8, 243-280 (1944).
2. Krein, M.G. On an application of the fixed point principle in the theory of linear transformations of spaces with an indefinite metric. Uspekhi Mat. Nauk. 50,no. 2(36), 180-190 (1950).
3. Krein, M.G. A new application of the fixed-point principle in the theory of operators in a space with indefinite metric. Dokl. Akad. Nauk SSSR. 154, no. 5, 1023-1026 (1964).
4. Krein, M.G. and Langer, G.K. The defect subspaces and generalized resolvents of a Hermitian operator in the space ΠK.Functional Anal. Appl. 5, no. 2, 126-146; no. 3, 217-228 (1971).
5. Langer, G.K. On J-Hermitian operators. Dokl. Akad. Nauk SSSR 134, no. 2, 263-266 (1960).