An Irreducible Graph Consisting of a Single Block Thomas BrownR. Duke Indiana University Mathematics Journal 1966 text pdf 10.1512/iumj.1966.15.15008 10.1512/iumj.1966.15.15008 en Indiana Univ. Math. J. 15 (1966) 129 - 135 state-of-the-art mathematics http://iumj.org/access/