E. DANICKI, “Spectral theory of natural unidirectionality of SPUDT for BG waves”, Arch. Acoust., vol. 16, no. 1, pp. 73–78, Mar. 1991, Accessed: Apr. 17, 2026. [Online]. Available: https://acoustics.ippt.pan.pl/index.php/aa/article/view/1211