Sur la Presque-Periodicite des Solutions de l'equation des Ondes Non Homogene S. Zaidman Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58025 10.1512/iumj.1959.8.58025 en Indiana Univ. Math. J. 8 (1959) 369 - 382 state-of-the-art mathematics http://iumj.org/access/