  The purpose of this work is to improve the estimates for the $\Delta$-GenEO
method from the paper "Overlapping Schwarz methods with GenEO coarse spaces for
indefinite and nonself-adjoint problems" by N. Bootland, V. Dolean, I. G
Graham, C. Ma, R. Scheichl (https://doi.org/10.1093/imanum/drac036) when
applied to the indefinite Helmholtz equation. We derive k-dependent estimates
of quantities of interest ensuring the robustness of the method.
