Analytic continuations with respect to a parameter of the Green function of exterior boundary value problems for the two-dimensional Helmholtz equation. II

This paper gives a construction of some special potentials for the two-dimensional Helmholtz equation. With these potentials, one can establish the existence of Green's functions of exterior boundary-value problems for each $k$ from the complex upper $k$-plane, and the analyticity of these functions in that region. More than that, one can establish the existence of an analytic continuation to the region
$$ \{0>\operatorname{Im}k>-\beta(1+|\operatorname{Re}k|^{1/3},\,|\operatorname{Re}k|>0\} $$
for some $\beta>0$, with estimates characterizing the behavior of the Green functions for large absolute values of $k$.
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