# Groups, Geometry, and Dynamics

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**Volume 10, Issue 2, 2016, pp. 753–770**

**DOI: 10.4171/GGD/364**

Published online: 2016-06-09

Growth tight actions of product groups

Christopher H. Cashen^{[1]}and Jing Tao

^{[2]}(1) University of Vienna, Wien, Austria

(2) University of Oklahoma, Norman, USA

A group action on a metric space is called growth tight if the exponential growth rate of the group with respect to the induced pseudo-metric is strictly greater than that of its quotients. A prototypical example is the action of a free group on its Cayley graph with respect to a free generating set. More generally, with Arzhantseva we have shown that group actions with strongly contracting elements are growth tight.

Examples of non-growth tight actions are product groups acting on the $L^1$ products of Cayley graphs of the factors.

In this paper we consider actions of product groups on product spaces, where each factor group acts with a strongly contracting element on its respective factor space. We show that this action is growth tight with respect to the $L^p$ metric on the product space, for all $1 < p ≤ \infty$. In particular, the $L^\infty$ metric on a product of Cayley graphs corresponds to a word metric on the product group. This gives the rst examples of groups that are growth tight with respect to an action on one of their Cayley graphs and non-growth tight with respect to an action on another, answering a question of Grigorchuk and de la Harpe.

*Keywords: *Growth tight, exponential growth rate, product groups

Cashen Christopher, Tao Jing: Growth tight actions of product groups. *Groups Geom. Dyn.* 10 (2016), 753-770. doi: 10.4171/GGD/364