Published online by Cambridge University Press: 11 October 2016
We study the set ${\mathcal{S}}$ of labelled seeds of a cluster algebra of rank n inside a field
${\mathcal{F}}$ as a homogeneous space for the group M n of (globally defined) mutations and relabellings. Regular equivalence relations on
${\mathcal{S}}$ are associated to subgroups W of AutM n (
${\mathcal{S}}$ ), and we thus obtain groupoids W\
${\mathcal{S}}$ . We show that for two natural choices of equivalence relation, the corresponding groups W c and W + act on
${\mathcal{F}}$ , and the groupoids W c\
${\mathcal{S}}$ and W +\
${\mathcal{S}}$ act on the model field
${\mathcal{K}}$ =ℚ(x 1,. . .,x n ). The groupoid W +\
${\mathcal{S}}$ is equivalent to Fock–Goncharov's cluster modular groupoid. Moreover, W c is isomorphic to the group of cluster automorphisms, and W + to the subgroup of direct cluster automorphisms, in the sense of Assem–Schiffler–Shramchenko.
We also prove that, for mutation classes whose seeds have mutation finite quivers, the stabiliser of a labelled seed under M n determines the quiver of the seed up to ‘similarity’, meaning up to taking opposites of some of the connected components. Consequently, the subgroup W c is the entire automorphism group of ${\mathcal{S}}$ in these cases.