Brusentsev, A. G. “A Haracterization of Closure of the Set of Compactly Supported Functions in Dirichlet Generalized Integral Metric and Its Applications”. Methods of Functional Analysis and Topology, vol. 15, no. 3, Sept. 2009, pp. 237-50, https://zen.imath.kiev.ua/index.php/mfat/article/view/421.