DavydovN. A. “Two Theorems in the Theory of Summation of Numerical Series by Lower Triangular Positive Monotone Matrices”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 4, Apr. 1999, pp. 553-5, https://umj.imath.kiev.ua/index.php/umj/article/view/4639.