Integral Operators and Changes of Density Lutz Weis Indiana University Mathematics Journal 1982 text pdf 10.1512/iumj.1982.31.31010 10.1512/iumj.1982.31.31010 en Indiana Univ. Math. J. 31 (1982) 83 - 96 state-of-the-art mathematics http://iumj.org/access/