Nicomedes, Williams L., Klaus-Jürgen Bathe, Fernando J. S. Moreira, and Renato C. Mesquita. “The Method of Finite Spheres in Acoustic Wave Propagation through Nonhomogeneous Media: Inf-Sup Stability Conditions”. Vietnam Journal of Mechanics 42, no. 3 (September 27, 2020): 209–237. Accessed November 8, 2025. https://vjmech.vast.vn/vjmech/article/view/15336.