The Intermediate Problems and the Maximum-Minimum Theory of Eigenvalues Alexander Weinstein Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12016 10.1512/iumj.1963.12.12016 en Indiana Univ. Math. J. 12 (1963) 235 - 245 state-of-the-art mathematics http://iumj.org/access/