1. V. D. Koshmanenko, Singular Linear Forms in the Theory of Perturbed of Self-Adjoint Operators [in Russian], Naukova Dumka, Kiev (1993).
2. Yu. M. Berezanskii, Expansion of Self-Adjoint Operators in Eigenfunctions [in Russian], Naukova Dumka, Kiev (1965)
3. V. D. Koshmanenko, “Perturbations of self-adjoint operators by singular bilinear forms,” Ukr. Mat. Zh., 41, No. 16, 3–18 (1989).
4. V. D. Koshmanenko and S. Ota, “On characteristic properties of singular operators,” Ukr. Mat. Zh., 48, No. 11, 1184–1493 (1996).
5. J. F. Brasche, V. Koshmanenko, and H. Neidhardt, “Some new aspects of Krein's extension theory,” Ukr. Mat. Zh., 46, No. 1, 37–54 (1994).