@article{700077,
     title = {On the approximation of eigenvalues of non-selfadjoint operators},
     booktitle = {Proceedings of Equadiff III},
     series = {GDML\_Books},
     publisher = {Univ. J. E. Purkyn\v e - P\v r\'\i rodov\v edeck\'a fakulta},
     address = {Brno},
     year = {1973},
     pages = {15-19},
     mrnumber = {MR0353694},
     zbl = {0323.65032},
     url = {http://dml.mathdoc.fr/item/700077}
}

Bramble, J. H. On the approximation of eigenvalues of non-selfadjoint operators, dans Proceedings of Equadiff III, GDML_Books,  (1973), pp. 15-19. http://gdmltest.u-ga.fr/item/700077/