@article{DE_1999_35_5_a15,
     author = {A. Yu. Savin and B. Yu. Sternin and B.-W. Schulze},
     title = {On invariant index formulas for spectral boundary value problems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {705--714},
     publisher = {mathdoc},
     volume = {35},
     number = {5},
     year = {1999},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1999_35_5_a15/}
}