@article{MZ_1982__180_173183,
     author = {Christer Borell},
     title = {Positivity {Improving} {Operators} and {Hypercontractivity.}},
     journal = {Mathematische Zeitschrift},
     pages = {225--234},
     year = {1982},
     volume = {180},
     zbl = {0472.47015},
     url = {http://geodesic.mathdoc.fr/item/MZ_1982__180_173183/}
}