Approximationseigenschaften der Gammaoperatoren. M. Müller; A. Lupas — 1967 Mathematische Zeitschrift
Approximation properties of a sequence of linear positive operators. A. Lupas; I. Lazarevic — 1975 Publications de l'Institut Mathématique [Elektronische Ressource]
Approximation Properties of the Mn-Operators. (Short Communication). A. Lupas; M.W. Müller — 1970 Aequationes mathematicae