1. Baumslag, G., Taylor, T.: The center of groups with one defining relator. Math. Ann.175, 315?319 (1968)
2. Karrass, A., Solitar, D.: The subgroups of a free product of two groups with an amalgamated subgroup. Trans. Amer. Math. Soc.150, 227?255 (1970)
3. Karrass, A., Pietrowski, A., Solitar, D.: Finite and infinite cyclic extensions of free groups. To appear in J. Austral. math. Soc.
4. Magnus, W., Karrass, A., Solitar, D.: Combinatorial Group Theory. New York: Interscience 1966
5. Meskin, S., Pietrowski, A., Steinberg, A.: One-relator groups with center. To appear in J. Austral. Math. Soc.