## Rewriting Techniques in Theoretical Physics## Évelyne Contejean, Antoine Coste and Benjamin Monate |

Abstract:This paper presents a general method for studying some quotients of the special linear groupSL_{2}over the integers, which are of fundamental interest in the field of statistical physics. Our method automatically helps in validating some conjectures due to physicists, such as conjectures stating that a set of equations completely describes a finite given quotient ofSL_{2}. In a first step, we show that in the cases we are interested in, the usual presentation of finitely generated groups with some constant generators and a binary concatenation can be turned into an equivalent one with unary generators. In a second step, when the completion of the transformed set of equations terminates, we show how to compute directly the associated normal forms automaton. According to the presence of loops, we are able to decide the finiteness of the quotient, and to compute its cardinality. When the quotient is infinite, the automaton gives some hints on what kind of equations are needed in order to insure the finiteness of the quotient.

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