T. I. . Zelenyak, “Concerning the use of variational methods in the theory of parabolic equations ”, Ukr. Mat. Zhurn., vol. 44, no. 9, pp. 1284–1287, Oct. 1992, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/8183