Existence and Uniqueness of Positive Eigenfunctions for a Class of Quasilinear Elliptic Boundary Value Problems of Sublinear Type Charles Coffman Indiana University Mathematics Journal 1973 text pdf 10.1512/iumj.1973.22.22085 10.1512/iumj.1973.22.22085 en Indiana Univ. Math. J. 22 (1973) 1017 - 1027 state-of-the-art mathematics http://iumj.org/access/