@article{10_4064_am_22_1_75_89,
     author = {Andrzej Pokrzywa},
     title = {Perturbation of the spectrum of an essentially selfadjoint operator},
     journal = {Applicationes Mathematicae},
     pages = {75--89},
     publisher = {mathdoc},
     volume = {22},
     number = {1},
     year = {1993},
     doi = {10.4064/am-22-1-75-89},
     zbl = {0795.47005},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/am-22-1-75-89/}
}