Neittaanmäki, P., and R. Picard. “On the Convergence of the Finite Element Approximation of Eigenfrequencies and Eigenvectors to Maxwell’s Boundary Value Problem”. Annales Fennici Mathematici, vol. 6, no. 2, Aug. 1981, pp. 255-71, doi:10.5186/aasfm.1981.0613.