A Quadratic Integral Equation, II Roger Nussbaum Indiana University Mathematics Journal 1981 text pdf 10.1512/iumj.1981.30.30065 10.1512/iumj.1981.30.30065 en Indiana Univ. Math. J. 30 (1981) 871 - 906 state-of-the-art mathematics http://iumj.org/access/