Perturbation of the Eigenvalues of Quadratic Matrix Polynomials. - Heinz Langer; Branko Najman; Kresimir Veselic (SIAM J. Matrix Anal. Appl., 1992)

Triple Variational Principles for Eigenvalues of Self-Adjoint Operators and Operator Functions. - David Eschwé; Heinz Langer (SIAM J. Math. Anal., 2002)

On the Spectrum of the Magnetohydrodynamic Mean-Field α2-Dynamo Operator. - Uwe Günther; Heinz Langer; Christiane Tretter (SIAM J. Math. Anal., 2010)