MyleikoG. L., and SolodkiiS. G. “Hyperbolic Cross and Complexity of Various Classes of Linear Ill-posed problems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 7, July 2017, pp. 951-63, https://umj.imath.kiev.ua/index.php/umj/article/view/1748.