Geodesic
Problem of focusing and the asymptotics of the spectral function of the Laplace–Beltrami operator. I
Zapiski Nauchnykh Seminarov POMI, Mathematical problems in the theory of wave propagation. Part 9, Tome 78 (1978), pp. 3-19 Cet article a éte moissonné depuis la source Math-Net.Ru

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The connection between the asymptotics of the spectral function and the formal shortwave expansion of the solution of the problem of the asymptotics of the Green function near a geodesically concave boundary of a two-dimensional surface is considered in the paper. 
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     author = {V. M. Babich and B. M. Levitan},
     title = {Problem of focusing and the asymptotics of the spectral function of the {Laplace{\textendash}Beltrami} {operator.~I}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {3--19},
     year = {1978},
     volume = {78},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1978_78_a0/}
}
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V. M. Babich; B. M. Levitan. Problem of focusing and the asymptotics of the spectral function of the Laplace–Beltrami operator. I. Zapiski Nauchnykh Seminarov POMI, Mathematical problems in the theory of wave propagation. Part 9, Tome 78 (1978), pp. 3-19. http://geodesic.mathdoc.fr/item/ZNSL_1978_78_a0/