Rapid perturbational calculations for the Helmholtz equation in two dimensions
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Courant Institute of Mathematical Sciences, New York University, 251 Mercer Street, New York, NY 10012-1110
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Centro de Investigación en Matemáticas, CIMAT A. C., Jalisco s/n, Mineral de Valenciana, C.P. 36240, Guanajuato, Guanajuato