TITLE: Algorithms in hyperbolic groups
QUESTION [15 upvotes]: I'm stuck in some algorithms in hyperbolic groups, which may be rather simple.
Let $G$ be a hyperbolic group given by a finite presentation. It is known that the hyperbolicity constant $\delta$ can be computable from the given presentation (by papasoglu, but I could not find/read the related reference). 
We also know there exists Dehn presentation for $G$ which allows to solve word problems in $G$. Here is my question. Is it clear how to obtain a Dehn presentation in practice?
In general, every relator in Dehn presentation is of form $wv$, where $w$ is a word of length upper bounded by a constant, for example, $8\delta+1$, and $v$ is of length strictly less than $u$ such that $u, v$ represent the same group element. So there are at most finitely many such relators.
But in practice, how to list these relators? Of course, $u, v$ are of finite length. There are finitely many choices to list. But how to know whether $u, v$ give the same group element? Is it amount to saying that $G$ has a soluble word problem?
In some practical cases, we can do it by other means. For example, in surface groups, we can use the group action on the hyperbolic plane to efficatively decide whether $u$ and $v$ give the same transformation. 
So in general, is there a way to do this in algorithms? or researchers in math are just satisfied with the EXISTENCE of Dehn algorithms?

REPLY [14 votes]: At the risk of being subjective (and possibly even argumentative), I feel like I should offer an answer to the implicit question, answered piquantly by Derek Holt: 

'a large proportion of researchers in Geometric Group Theory appear to be interested only in the existence of algorithms and their complexity, rather than in actually using them to do computations'.

It's really wonderful for group theorists when we have algorithms that can actually be run on examples we care about. Derek has done a lot of great work on this, as have many others.  A different recent example which I find very striking is Calegari--Walker's algorithm for finding surface subgroups of ascending HNN extensions of free groups, which they were able to use to find a surface subgroup in an old example of Sapir.  (No surprise it hadn't been found by hand - the surface was of genus 32!)
But general results about the (non-)existence of algorithms or their complexity play another, completely different, role in group theory. Many researchers (I count myself among this number) are less interested in particular groups than in classes of groups: finitely presented groups, hyperbolic groups etc.  In this setting, algorithmic results are not about computations at all, but rather play an important strategic role.  They help us to orient ourselves in these classes of groups, get a sense of how 'wild' or 'tame' they are, and work out which sorts of questions are reasonable to pursue.
Here's a story which I think illustrates this point nicely.
A famous theorem of Adyan and Rabin (ca. 1956) asserts that the triviality problem for finitely presented groups is not algorithmically solvable.  This result in turn relies on the Novikov--Boone theorem, which exhibits a group with unsolvable word problem.  One reasonable interpretation is that the class of finitely presented groups is too 'wild' to classify. This is an important strategic result in combinatorial group theory, and one might be tempted to deduce that one should give up on the theory of finitely presented groups altogether.
Now fast forward to about 1980, and Gromov's discovery that, although the class of finitely presented groups is 'wild', a 'random' finitely presented group is in fact hyperbolic, and so has many nice algorithmic properties (linear Dehn function, automaticity etc).  I think of this result as 'rescuing' combinatorial group theory from the Adyan--Rabin nightmare, using geometry.
Without any computation, these theorems give us great insight into how we should think about finitely presented groups: in short, an arbitrary fp group is intractable, but a generic fp group is quite nice.
Finally, it may be worth pointing out that these general facts are not just attractive; they have enormous 'strategic' importance, closing off certain directions of research and consequently focussing attention on others.  For instance, it can be deduced from the Adyan--Rabin theorem that it's impossible to classify smooth 4-manifolds.  Since classifying 3-manifolds (in the form of geometrization) was the dominant research direction in 3-manifold topology for decades, just imagine how different 4-manifold topology might be if a classification were believed possible!