Коробская, А. В. “Semigroup of Integration Operator and Its Properties”. Bulletin of V.N. Karazin Kharkiv National University, Series «Mathematical Modeling. Information Technology. Automated Control Systems», Vol. 24, no. 1105, Apr. 2014, pp. 85-98, https://periodicals.karazin.ua/mia/article/view/14252.