A minimization Problem and the Regularity of Solutions in the Presence of a Free Boundary Daniel Phillips Indiana University Mathematics Journal 1983 text pdf 10.1512/iumj.1983.32.32001 10.1512/iumj.1983.32.32001 en Indiana Univ. Math. J. 32 (1983) 1 - 17 state-of-the-art mathematics http://iumj.org/access/