Haussmann, Werner, and Hans-Bernd Knoop. “Konvergenzordnung Einer Folge Positiver Linearer Operatoren: Convergence Order of a Sequence of Positive Linear Operators”. Anal. Numér. Théor. Approx., vol. 4, no. 2, Aug. 1975, pp. 123-30, https://ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no2-art2.