Fundamental groups and group presentations with bounded relator lengths

We study the geometry of compact geodesic spaces with trivial first Betti number admitting large finite groups of isometries. We show that if a finite group $G$ acts by isometries on a compact geodesic space $X$ whose first Betti number vanishes, then diam$(X)\mathrm{/}$diam$(X\mathrm{/}G)\le 4\sqrt{\mathrm{\mid }G\mathrm{\mid }}$. For a group $G$ and a finite symmetric generating set $S$, $P_k(\Gamma (G, S))$denotesthe2-dimensionalCW-complexwhose1-skeletonistheCayleygraph$\Gamma$of$G$withrespectto$S$andwhose2-cellsare$m$-gonsfor$0 \leq m \leq k$,definedbythesimplegraphloopsoflength$m$in$\Gamma$, up to cyclic permutations. Let $G$ be a finite abelian group with $\mathrm{\mid }G\mathrm{\mid }\ge 3$ and $S$ a symmetric set of generators for which $P_k(\mathrm{\Gamma }(G,S))$ has trivial first Betti number. We show that the first nontrivial eigenvalue $-{\lambda }_1$ of the Laplacian on the Cayley graph satisfies $\lambda_1 \geq 2 - 2 \cos ( 2 \pi / k ) $. We also give an explicit upper bound on the diameter of the Cayley graph of $G$ with respect to $S$ of the form $O (k^2 \vert S \vert \log \vert G \vert )$. Related explicit bounds for the Cheeger constant and Kazhdan constant of the pair $(G,S)$ are also obtained.