@article{CMH_1970__45_139455,
     author = {Peter Hess and Tosio Kato},
     title = {Perturbation of {Closed} {Operators} and {Their} {Adjoints.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {524--529},
     publisher = {mathdoc},
     volume = {45},
     year = {1970},
     zbl = {0263.47015},
     url = {http://geodesic.mathdoc.fr/item/CMH_1970__45_139455/}
}