Belopolskaya, Ya. I., et al. “Two-Dimensional Helmholtz Resonator With Two Close Point-Like Windows: Regularization for the Neumann Case”. Methods of Functional Analysis and Topology, vol. 28, no. 2, June 2022, pp. 95-104, https://doi.org/https://doi.org/10.31392/MFAT-npu26_2.2022.01.