Köhnen, W. (1980). Einige Saturationssätze fur n-parametrige Halbgruppen von Operatoren: Some saturation theorems for n-parameter semigroups of operators. Anal. Numér. Théor. Approx., 9(1), 65–73. Retrieved from https://ictp.acad.ro/jnaat/journal/article/view/1980-vol9-no1-art9