An Inequality for Positive Linear Integral Operators Eberhard Hopf Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12046 10.1512/iumj.1963.12.12046 en Indiana Univ. Math. J. 12 (1963) 683 - 692 state-of-the-art mathematics http://iumj.org/access/