Brădeanu, Petru. “An Approximation of the Helmholtz Equation on Finite Elements and Differences”. Anal. Numér. Théor. Approx. 15, no. 1 (February 1, 1986): 13–20. Accessed September 3, 2025. https://www.ictp.acad.ro/jnaat/journal/article/view/1986-vol15-no1-art2.