Karwowski W, Koshmanenko VD. On the definition of singular bilinear forms and singular linear operators. Ukr. Mat. Zhurn. [Internet]. 1993 Aug. 25 [cited 2025 Dec. 5];45(8):1084–1089. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/5903