Bakushinsky, A., and A. Leonov. “New a Posteriori Error Estimates for Approximate Solutions to Iregular Operator Equations”. Numerical Methods and Programming (Vychislitel’nye Metody I Programmirovanie), vol. 15, no. 40, June 2014, pp. 359-6, https://en.num-meth.ru/index.php/journal/article/view/774.