@article{MZ_1973__131_171897,
     author = {T. Ando and Takashi Sekiguchi and T. Suzuki},
     title = {Approximation by {Positive} {Operators.}},
     journal = {Mathematische Zeitschrift},
     pages = {273--282},
     year = {1973},
     volume = {131},
     zbl = {0243.47026},
     url = {http://geodesic.mathdoc.fr/item/MZ_1973__131_171897/}
}