Koshmanenko, V. D. “Regularized Approximations of Singular Perturbations from the $H_2$-Class”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 5 (May 25, 2000): 626–637. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4457.