Quasiregular Mappings with Finite Multiplicity and Royden Algebras Nathan Soderborg Indiana University Mathematics Journal 1991 text pdf 10.1512/iumj.1991.40.40050 10.1512/iumj.1991.40.40050 en Indiana Univ. Math. J. 40 (1991) 1143 - 1167 state-of-the-art mathematics http://iumj.org/access/