Multiple Positive Fixed Points of Nonlinear Operators on Ordered Banach Spaces Richard LeggettLynn Williams Indiana University Mathematics Journal 1979 text pdf 10.1512/iumj.1979.28.28046 10.1512/iumj.1979.28.28046 en Indiana Univ. Math. J. 28 (1979) 673 - 688 state-of-the-art mathematics http://iumj.org/access/