Cătinaş, E., and I. Păvăloiu. “On Approximating the Eigenvalues and Eigenvectors of Linear Continuous Operators”. Rev. Anal. Numér. Théor. Approx., vol. 26, no. 1, Aug. 1997, pp. 19-27, https://ictp.acad.ro/jnaat/journal/article/view/1997-vol26-nos1-2-art3.