SukhorolskyiM. A. “Boundary-Value Problems for the Helmholts Equation in Domains of the Complex Plane”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 3, Mar. 2016, pp. 363-76, http://umj.imath.kiev.ua/index.php/umj/article/view/1844.