D. Y. Petrina, “Spectrum and States of the BCS Hamiltonian in a Finite Domain. II. Spectra of Excitations”, Ukr. Mat. Zhurn., vol. 53, no. 8, pp. 1080–1100, Aug. 2001, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4328