@article{IM_1973__19_142192,
     author = {W. Hansen},
     title = {Perturbation of {Harmonic} {Spaces} and {Construction} of {Semigroups.}},
     journal = {Inventiones mathematicae},
     pages = {149--164},
     publisher = {mathdoc},
     volume = {19},
     year = {1973},
     zbl = {0258.31005},
     url = {http://geodesic.mathdoc.fr/item/IM_1973__19_142192/}
}