On the Equation of the Vibrating String William Feller Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58023 10.1512/iumj.1959.8.58023 en Indiana Univ. Math. J. 8 (1959) 339 - 348 state-of-the-art mathematics http://iumj.org/access/