W. Karwowski and V. D. Koshmanenko, “On the definition of singular bilinear forms and singular linear operators”, Ukr. Mat. Zhurn., vol. 45, no. 8, pp. 1084–1089, Aug. 1993, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5903