Petrina, D. Ya. “BCS Model Hamiltonian of the Theory of Superconductivity As a Quadratic Form”. Ukrains’kyi Matematychnyi Zhurnal 56, no. 3 (March 25, 2004): 309–338. Accessed April 27, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3756.