D. Y. Petrina, “BCS Model Hamiltonian of the Theory of Superconductivity as a Quadratic Form”, Ukr. Mat. Zhurn., vol. 56, no. 3, pp. 309–338, Mar. 2004, Accessed: Apr. 23, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3756