@article{1103043343, author = {Kramer, Henry P.}, title = {Perturbation of differential operators.}, journal = {Pacific J. Math.}, volume = {7}, number = {4}, year = {1957}, pages = { 1405-1435}, language = {en}, url = {http://dml.mathdoc.fr/item/1103043343} }
Kramer, Henry P. Perturbation of differential operators.. Pacific J. Math., Tome 7 (1957) no. 4, pp. 1405-1435. http://gdmltest.u-ga.fr/item/1103043343/