Viscosity Solutions of Second Order Fully Nonlinear Elliptic Equations with State Constraints Markos Katsoulakis Indiana University Mathematics Journal 1994 text pdf 10.1512/iumj.1994.43.43020 10.1512/iumj.1994.43.43020 en Indiana Univ. Math. J. 43 (1994) 493 - 519 state-of-the-art mathematics http://iumj.org/access/