Differential Operators with Perturbed Domains C. Swanson Indiana University Mathematics Journal 1957 text pdf 10.1512/iumj.1957.6.56046 10.1512/iumj.1957.6.56046 en Indiana Univ. Math. J. 6 (1957) 823 - 846 state-of-the-art mathematics http://iumj.org/access/