1. G. Baumslag and D. Solitar, “Some two-generator one-relator non-Hopfian groups,” Bull. Am. Math. Soc., 68, No. 3, 199-201 (1962).
2. J.-P. Serre, Trees, Springer, Berlin (1980).
3. V. A. Churkin, “Theory of groups that act on trees,” Algebra Logika, 22, No. 2, 159-165 (1983).
4. M. Clay and M. Forester, “On the isomorphism problem for generalized Baumslag–Solitar groups,” preprint, arXiv:math.GR0710.2108vl , 10 Oct. 2007.
5. G. Levitt, “On the automorphism group of generalized Baumslag–Solitar groups,” preprint, arXiv:math.GR/0511083 , 3 Nov. 2005.