On the Spectral Determination of the Green's Operator Robert Carroll Indiana University Mathematics Journal 1966 text pdf 10.1512/iumj.1966.15.15067 10.1512/iumj.1966.15.15067 en Indiana Univ. Math. J. 15 (1966) 1003 - 1018 state-of-the-art mathematics http://iumj.org/access/