Decomposition of Semi-Groups of Partial Isometries L. Wallen Indiana University Mathematics Journal 1971 text pdf 10.1512/iumj.1971.20.20019 10.1512/iumj.1971.20.20019 en Indiana Univ. Math. J. 20 (1971) 207 - 212 state-of-the-art mathematics http://iumj.org/access/