@article{MZ_1968__105_170974,
     author = {Karl Gustaffson},
     title = {Positive {(Noncommuting)} {Operator} {Products} and {Semigroups.}},
     journal = {Mathematische Zeitschrift},
     pages = {160--172},
     year = {1968},
     volume = {105},
     url = {http://geodesic.mathdoc.fr/item/MZ_1968__105_170974/}
}