@article{10_1017_S0017089516000446,
     author = {ZHANG, LIANG and TANG, XIANHUA},
     title = {PERTURBATIONS {FROM} {INDEFINITE} {SYMMETRIC} {ELLIPTIC} {BOUNDARY} {VALUE} {PROBLEMS}},
     journal = {Glasgow mathematical journal},
     pages = {635--648},
     year = {2017},
     volume = {59},
     number = {3},
     doi = {10.1017/S0017089516000446},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089516000446/}
}