Petrina, D. Ya. 2003. “Model BCS Hamiltonian and Approximating Hamiltonian in the Case of Infinite Volume. IV. Two Branches of Their Common Spectra and States”. Ukrains’kyi Matematychnyi Zhurnal 55 (2), 174-96. https://umj.imath.kiev.ua/index.php/umj/article/view/3899.