### Quick description

There is no general algorithm that will determine whether a given finitely presented group is finite or not. However, there are a number of strategies to try.

### Prerequisites

Basic definitions of combinatorial group theory.

### General discussion

Suppose that is a finitely presented group. It is easy to find quotients of ; adding relations to the presentation and determining the consequences amounts to passing to a quotient of by the normal closure of the relations. Then if you can add relations to and end up with a group that you know to be infinite, it follows that the original group was infinite.

### Example 1

Let be the finitely presented group with presentation . Trying to determine explicitly whether there are infinitely many reduced words in seems hopelessly complicated. Instead, let's notice that if we add the relation , then this group collapses to , which we recognize as the infinite Coxeter group .

## Comments

## methods to show that a group is infinite

Tue, 09/06/2009 - 17:32 — Danny Calegari (not verified)I have a couple of posts on my blog (http://lamington.wordpress.com) with lots of examples of ways to show that a group is finite/infinite, probably too many/too specialized for the tricki. You are welcome to mine these posts for anything you think is relevant.

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