@article{MM2_1981__34_154769,
     author = {W.M. Greenlee},
     title = {Stability {Theorems} for {Singular} {Perturbation} of {Eigenvalues.}},
     journal = {Manuscripta mathematica},
     pages = {157--174},
     year = {1981},
     volume = {34},
     zbl = {0489.47004},
     url = {http://geodesic.mathdoc.fr/item/MM2_1981__34_154769/}
}