Plastic Disturbances Whose Speed of Propagation is Less than the Velocity of a Shear Wave T. Thomas Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57050 10.1512/iumj.1958.7.57050 en Indiana Univ. Math. J. 7 (1958) 893 - 900 state-of-the-art mathematics http://iumj.org/access/