Existence of Regular Solutions with Convex Levels for Semilinear Elliptic Equations with Nonmonotone L^1 Nonlinearities Part I: An Approximating Free Boundary Problem Peter LaurenceE. Stredulinsky Indiana University Mathematics Journal 1990 text pdf 10.1512/iumj.1990.39.39051 10.1512/iumj.1990.39.39051 en Indiana Univ. Math. J. 39 (1990) 1081 - 1114 state-of-the-art mathematics http://iumj.org/access/