[1] Wiegold J.:
Semigroup coverings of groups. Mat.-fyz. časop. 11 (1961), 3-13.
Zbl 0104.01902
[3] Neumann B. H.:
An essay on free products of groups with amalgamations. Phil. Trans. Roy. Soc. London Ser. A 246 (1954), 503-554.
MR 0062741 |
Zbl 0057.01702