@article{ZNSL_1976_62_a1,
     author = {V. M. Babich and N. S. Grigor'ev},
     title = {Nonspectral singularities of {Green's} function for the {Helmholtz} equation in the exterior of an arbitrary convex polygon},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {21--26},
     year = {1976},
     volume = {62},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1976_62_a1/}
}