On the Fredholm Eigenvalues of Plane Domains Glenn Schober Indiana University Mathematics Journal 1967 text pdf 10.1512/iumj.1967.16.16036 10.1512/iumj.1967.16.16036 en Indiana Univ. Math. J. 16 (1967) 535 - 541 state-of-the-art mathematics http://iumj.org/access/