TACHEV, Gancho. “On Two Modified Phillips Operators: Dedicated to Professor Heiner Gonska on the Occasion of His 70th Anniversary”. Studia Universitatis Babeș-Bolyai Mathematica, vol. 64, no. 3, Sept. 2019, pp. 305–312, https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/2295.