Regularity of Solutions to Boundary Value Problems for First Order Systems D. Tartakoff Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21089 10.1512/iumj.1972.21.21089 en Indiana Univ. Math. J. 21 (1972) 1113 - 1129 state-of-the-art mathematics http://iumj.org/access/