Presentation complex
# Presentation complex

In geometric group theory, a presentation complex is a 2-dimensional cell complex associated to any presentation of a group G. The complex has a single vertex, and one loop at the vertex for each generator of G. There is one 2-cell for each relation in the presentation, with the boundary of the 2-cell attached along the appropriate word.

## Properties

* The fundamental group of the presentation complex is the group G itself.
* The universal cover of the presentation complex is a Cayley complex for G, whose 1-skeleton is the Cayley graph of G.
* Any presentation complex for G is the 2-skeleton of an Eilenberg–MacLane space $K(G,1)$.

## Examples

Let $G=\mathbb {Z} ^{2}$ be the two-dimensional integer lattice, with presentation

    $G=\langle x,y|xyx^{-1}y^{-1}\rangle .$

Then the presentation complex for G is a torus, obtained by gluing the opposite sides of a square, the 2-cell, which are labelled x and y. All four corners of the square are glued into a single vertex, the 0-cell of the presentation complex, while a pair consisting of a longtitudal and meridian circles on the torus, intersecting at the vertex, constitutes its 1-skeleton.

The associated Cayley complex is a regular tiling of the plane by unit squares. The 1-skeleton of this complex is a Cayley graph for $\mathbb {Z} ^{2}$.

Let $G=\mathbb {Z} _{2}*\mathbb {Z} _{2}$ be the Infinite dihedral group, with presentation $\langle a,b\mid a^{2},b^{2}\rangle$. The presentation complex for $G$ is $\mathbb {RP} ^{2}\vee \mathbb {RP} ^{2}$, the wedge sum of projective planes. For each path, there is one 2-cell glued to each loop, which provides the standard cell structure for each projective plane. The Cayley complex is an infinite string of spheres.

