Solodkii, S. G. “Complexity of Fredholm Equations of the Second Kind With Kernels from Anisotropic Classes of Differentiable Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 4, Apr. 1996, pp. 525-32, https://umj.imath.kiev.ua/index.php/umj/article/view/5315.