@article{RCM_1973__7_181602,
     author = {D. K. Rao},
     title = {Symmetric perturbation of a self-adjoint operator},
     journal = {Revista colombiana de matematicas},
     pages = {81--84},
     publisher = {mathdoc},
     volume = {7},
     year = {1973},
     zbl = {0295.47020},
     url = {http://geodesic.mathdoc.fr/item/RCM_1973__7_181602/}
}