A Functional Equation for the Rayleigh Quotient for Eigenvalues, and Some Applications J. DiazF. Metcalf Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17038 10.1512/iumj.1968.17.17038 en Indiana Univ. Math. J. 17 (1968) 623 - 630 state-of-the-art mathematics http://iumj.org/access/