Petrina, D. Ya. “Spectrum and States of the BCS Hamiltonian in a Finite Domain. II. Spectra of Excitations”.
Ukrains’kyi Matematychnyi Zhurnal 53, no. 8 (August 25, 2001): 1080–1100. Accessed April 11, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/4328.