Generation Gaps and Abelianised Defects of Free Products
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Let G be a group of the form G_1* ... *G_n, the free product of n subgroups, and let M be a ZG-module of the form $\bigoplus_{i=1}^n M_i \otimes_{\mathbb{Z}G_i} \mathbb{Z}G$. We shall give formulae in various situations for $d_{ZG}(M)$, the minimum number of elements required to generate M. In particular if C_1,C_2 are non-trivial finite cyclic groups of coprime orders, $G = (C_1 \times Z) * (C_2 \times Z)$ and $F/R \cong G$ is the free presentation obtained from the natural free presentations of the two factors, then the number of generators of the relation module, $d_{\mathbb{Z}G}(R/R')$ is three. It seems plausible that the minimum number of relators of G should be 4, and this would give a finitely presented group with positive relation gap. However we cannot prove this last statement.
18 pages, minor changes. To appear in J. Group Theory
18 pages, minor changes. To appear in J. Group Theory