On Functions with Almost Periodic or Almost Automorphic First Differences I. Berg Indiana University Mathematics Journal 1970 text pdf 10.1512/iumj.1970.19.19022 10.1512/iumj.1970.19.19022 en Indiana Univ. Math. J. 19 (1970) 239 - 245 state-of-the-art mathematics http://iumj.org/access/