1. Baumslag G. and Solitar D., “Some two-generator one-relator non-Hopfian groups,” Bull. Amer. Math. Soc., 68, No. 3, 199–201 (1962).
2. Dudkin F. A., “Baumslag-Solitar groups and their subgroups,” in: Itogi Nauki. Yug Rossii. Matematicheskiĭ Forum. Vol. 6: Groups and Graphs [in Russian], YuMI VNTs RAN i PSO-A, Vladikavkaz, 2012, pp. 21–28.
3. Serre J.-P., Trees, Springer-Verlag, Berlin, Heidelberg, and New York (1980).
4. Churkin V. A., “Theory of groups that act on trees,” Algebra and Logic, 22, No. 2, 159–165 (1983).
5. Clay M. and Forester M., “Whitehead moves for G-trees,” Bull. London Math. Soc., 41, No. 2, 205–212 (2009).