Lower Bounds for Eigenvalues Norman Bazley Indiana University Mathematics Journal 1961 text pdf 10.1512/iumj.1961.10.10019 10.1512/iumj.1961.10.10019 en Indiana Univ. Math. J. 10 (1961) 289 - 307 state-of-the-art mathematics http://iumj.org/access/