An Eigenvalue Problem Related to Stability Questions in Perturbed Autonomous Systems W. Loud Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11009 10.1512/iumj.1962.11.11009 en Indiana Univ. Math. J. 11 (1962) 139 - 151 state-of-the-art mathematics http://iumj.org/access/