@article{MZ_1980__171_172923,
     author = {Harald Luschgy},
     title = {Invariant {Extensions} of {Positive} {Operators.}},
     journal = {Mathematische Zeitschrift},
     pages = {75--82},
     publisher = {mathdoc},
     volume = {171},
     year = {1980},
     zbl = {0437.47028},
     url = {http://geodesic.mathdoc.fr/item/MZ_1980__171_172923/}
}