Scattering Resonances of a Helmholtz Resonator Peter HislopAndre Martinez Indiana University Mathematics Journal 1991 text pdf 10.1512/iumj.1991.40.40034 10.1512/iumj.1991.40.40034 en Indiana Univ. Math. J. 40 (1991) 767 - 788 state-of-the-art mathematics http://iumj.org/access/