Pereverzev, S. V. “On Complexity of the Problem of Finding Solution for the Fredholm Second Type Equations With Smooth Nuclei. I ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 1, Dec. 1987, pp. 84-91, https://umj.imath.kiev.ua/index.php/umj/article/view/9060.