Uniqueness under Nonlinear Boundary Conditions for Elliptic Problems Simon Levin Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17031 10.1512/iumj.1968.17.17031 en Indiana Univ. Math. J. 17 (1968) 507 - 521 state-of-the-art mathematics http://iumj.org/access/