@article{10_4153_CJM_1978_053_7,
     author = {Lee, S. J.},
     title = {Perturbation of {Direct} {Sum} {Differential} {Operators}},
     journal = {Canadian journal of mathematics},
     pages = {600--630},
     year = {1978},
     volume = {30},
     number = {3},
     doi = {10.4153/CJM-1978-053-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1978-053-7/}
}