On the Hyperboloidal Means and the Wave Equation Yanping Chen Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21034 10.1512/iumj.1972.21.21034 en Indiana Univ. Math. J. 21 (1972) 437 - 447 state-of-the-art mathematics http://iumj.org/access/