V. A. Derkach, S. Hassi, and H. de Snoo, “Operator models associated with singular perturbations”, MFAT, vol. 7, no. 3, pp. 1–21, Sep. 2001, Accessed: Feb. 04, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/154