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\bibitem{Caratelli_etal2011} {\textquotedblleft}\textit{The Robin problem for the Helmholtz equation in a starlike planar domain}{\textquotedblright}. Caratelli D, Gielis J, Natalini P, Ricci PE, Tavkhelidze I, Georgian mathematical journal \textbf{18}, 465 (2011).   http://doi.org/10.1515/GMJ.2011.0031

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