On Some Transformation Semigroups Charles Loewner Indiana University Mathematics Journal 1956 text pdf 10.1512/iumj.1956.5.55029 10.1512/iumj.1956.5.55029 en Indiana Univ. Math. J. 5 (1956) 791 - 804 state-of-the-art mathematics http://iumj.org/access/