A Counter-Example to the Singular Sequence Problem Jurgen Voigt Indiana University Mathematics Journal 1990 text pdf 10.1512/iumj.1990.39.39003 10.1512/iumj.1990.39.39003 en Indiana Univ. Math. J. 39 (1990) 27 - 30 state-of-the-art mathematics http://iumj.org/access/