Liouville Theorems for Even Order Elliptic Systems G. HileChiping Zhou Indiana University Mathematics Journal 1994 text pdf 10.1512/iumj.1994.43.43016 10.1512/iumj.1994.43.43016 en Indiana Univ. Math. J. 43 (1994) 383 - 410 state-of-the-art mathematics http://iumj.org/access/