@article{10_4153_CJM_1972_009_8,
     author = {Hughes, Edward},
     title = {Perturbation {Theorems} for {Relative} {Spectral} {Problems}},
     journal = {Canadian journal of mathematics},
     pages = {72--81},
     year = {1972},
     volume = {24},
     number = {1},
     doi = {10.4153/CJM-1972-009-8},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1972-009-8/}
}