On the Geometry of Two-Dimensional Elastic Stress Fields P. NemenyiA. Saienz Indiana University Mathematics Journal 1952 text pdf 10.1512/iumj.1952.1.51003 10.1512/iumj.1952.1.51003 en Indiana Univ. Math. J. 1 (1952) 73 - 86 state-of-the-art mathematics http://iumj.org/access/