@article{DE_1992_28_5_a9,
     author = {S. Baizaev and \`E. Muhamadiev},
     title = {On the index of first-order elliptic operators in the plane},
     journal = {Differencialʹnye uravneni\^a},
     pages = {818--827},
     publisher = {mathdoc},
     volume = {28},
     number = {5},
     year = {1992},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1992_28_5_a9/}
}