Spacapan, Igor, and Miroslav Premrov. “Modal Analysis of Wave Motion in Inhomogeneous Waveguides Which Are Modelled by FEM”. Computer Assisted Methods in Engineering and Science, vol. 11, no. 2-3, Jan. 2023, pp. 137-44, https://cames3.ippt.pan.pl/index.php/cames/article/view/1030.