On Some Transformation Semigroups Invariant under Euclidean or Non-Euclidean Isometries Charles Loewner Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58028 10.1512/iumj.1959.8.58028 en Indiana Univ. Math. J. 8 (1959) 393 - 409 state-of-the-art mathematics http://iumj.org/access/