1. Freie Produkte und ihre Untergruppen;Baer;Comp. Math.,1938
2. On groups acting on locally finite graphs;Bergman;Ann. Math.,1968
3. Homological Algebra;Cartan,1956
4. The fundamental group of one-dimensional spaces;Curtis,1959
5. Singular homology of one-dimensional spaces;Curtis;Ann. Math.,1959