Improvements to the theoretical estimates of the Schwarz preconditioner with Δ-GenEO coarse space for the indefinite Helmholtz problem

The purpose of this work is to improve the estimates for the Δ-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.