@article{RM_1960_15_3_a15,
     author = {A. I. Vol'pert},
     title = {On the index and normal solvability of boundary value problems for elliptic systems of differential equations in the plane},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     year = {1960},
     volume = {15},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1960_15_3_a15/}
}