Some Moebius Semigroups on the 2-Sphere C. Ballantine Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11027 10.1512/iumj.1962.11.11027 en Indiana Univ. Math. J. 11 (1962) 455 - 467 state-of-the-art mathematics http://iumj.org/access/