Geometric group action

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In mathematics, specifically geometric group theory, a geometric group action is a certain type of action of a discrete group on a metric space.

[edit] Definition

In geometric group theory, a geometry is any proper, geodesic metric space. An action of a finitely-generated group G on a geometry X is geometric if it satisfies the following conditions:

Each element of G acts as an isometry of X.
The action is cocompact, i.e. the quotient space X/G is a compact space.
The action is properly discontinuous, with each point having a finite stabilizer.

[edit] Uniqueness

If a group G acts geometrically upon two geometries X and Y, then X and Y are quasi-isometric. Since any group acts geometrically on its own Cayley graph, any space on which G acts geometrically is quasi-isometric to the Cayley graph of G.