Open Mappings on 2-Dimensional Manifolds G. Whyburn Indiana University Mathematics Journal 1961 text pdf 10.1512/iumj.1961.10.10013 10.1512/iumj.1961.10.10013 en Indiana Univ. Math. J. 10 (1961) 181 - 197 state-of-the-art mathematics http://iumj.org/access/