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4. A. M. Il’in and A. A. Ershov, “Asymptotics of two-dimensional integrals depending singularly on a small parameter,” Proc. Steklov Inst. Math. 268 (Suppl. 1), S131–S142 (2010).
5. A. M. Il’in and A. R. Danilin, Asymptotic Methods in Analysis (Fizmatlit, Moscow, 2009) [in Russian].