Hembars’ka, S. B., and P. V. Zaderei. “Estimations of the Integral of Modulus for Mixed Derivatives of the Sum of Double Trigonometric Series”. Ukrains’kyi Matematychnyi Zhurnal 68, no. 7 (July 25, 2016): 908-921. Accessed October 1, 2022. http://umj.imath.kiev.ua/index.php/umj/article/view/1890.