On the Unique Continuation Problem of Elliptic Partial Differential Equations Irwin Bernstein Indiana University Mathematics Journal 1961 text pdf 10.1512/iumj.1961.10.10039 10.1512/iumj.1961.10.10039 en Indiana Univ. Math. J. 10 (1961) 579 - 606 state-of-the-art mathematics http://iumj.org/access/