On Gaussian Sums over GL(2, Z/p^n) Roy Fuller Indiana University Mathematics Journal 1974 text pdf 10.1512/iumj.1974.23.23082 10.1512/iumj.1974.23.23082 en Indiana Univ. Math. J. 23 (1974) 1013 - 1020 state-of-the-art mathematics http://iumj.org/access/