Exploiting minmax characterizations for nonlinear and nonoverdamped eigenvalue problems, we prove the existence of a countable set of eigenvalues converging to and inclusion theorems for a rational spectral problem governing mechanical vibrations of a tube bundle immersed in an incompressible viscous fluid. The paper demonstrates that the variational characterization of eigenvalues is a powerful tool for studying nonoverdamped eigenproblems, and that the appropriate enumeration of the eigenvalues is of predominant importance, whereas the natural ordering of the eigenvalues may yield false conclusions.
Heinrich Voss. "Locating real eigenvalues of a spectral problem in fluid-solid type structures." J. Appl. Math. 2005 (1) 37 - 48, 16 February 2005. https://doi.org/10.1155/JAM.2005.37