VarbanetsP. D., and KirbatS. A. “On the Mean Value of the Function $\overline{S}_k(n)$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 4, Apr. 2011, pp. 448-5, https://umj.imath.kiev.ua/index.php/umj/article/view/2730.