The Joining of Exchange Geometries Henry Crapo Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17052 10.1512/iumj.1968.17.17052 en Indiana Univ. Math. J. 17 (1968) 837 - 852 state-of-the-art mathematics http://iumj.org/access/