Rudenko, A. V. “Approximation of Densities of Absolutely Continuous Components of Measures in a Hilbert Space Using the Ornstein-Uhlenbeck Semigroup”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 12, Dec. 2004, pp. 1654-6, https://umj.imath.kiev.ua/index.php/umj/article/view/3872.