Bel’skiiS. A. “On Piecewise-Constant Approximation of Continuous Functions of N Variables in Integral Metrics”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 3, Mar. 2002, pp. 293-0, https://umj.imath.kiev.ua/index.php/umj/article/view/4066.