1.
Lee IW, Jung HJ, Kim MC. Efficient free vibration analysis of large structures with close or multiple natural frequencies. Part I: Undamped structures. Comp. Assist. Methods Eng. Sci. 1999;6(3-4):403-414. Accessed February 23, 2026. https://cames.ippt.pan.pl/index.php/cames/article/view/1299