On the Completeness of the Boussinesq-Papkovich Stress Functions R. EubanksE. Sternberg Indiana University Mathematics Journal 1956 text pdf 10.1512/iumj.1956.5.55027 10.1512/iumj.1956.5.55027 en Indiana Univ. Math. J. 5 (1956) 735 - 746 state-of-the-art mathematics http://iumj.org/access/