Motions in Two Dimensional Indefinite Finsler Spaces John Beem Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21044 10.1512/iumj.1972.21.21044 en Indiana Univ. Math. J. 21 (1972) 551 - 555 state-of-the-art mathematics http://iumj.org/access/