On the arithmetic dimension of triangle groups

TL;DR

This paper investigates the arithmetic dimension of hyperbolic triangle groups, establishing finiteness results for fixed dimensions and providing an algorithm to enumerate all such triples with bounded arithmetic dimension.

Contribution

It generalizes previous work by Takeuchi, proving finiteness for triples with a given arithmetic dimension and offering an efficient enumeration algorithm.

Findings

Finiteness of triples with fixed arithmetic dimension proven.

An efficient algorithm for enumerating triples with bounded arithmetic dimension developed.

Explicit classification of triples with arithmetic dimension 1 by Takeuchi confirmed.

Abstract

Let $\Delta=\Delta(a,b,c)$ be a hyperbolic triangle group, a Fuchsian group obtained from reflections in the sides of a triangle with angles $\pi/a,\pi/b,\pi/c$ drawn on the hyperbolic plane. We define the arithmetic dimension of $\Delta$ to be the number of split real places of the quaternion algebra generated by $\Delta$ over its (totally real) invariant trace field. Takeuchi has determined explicitly all triples $(a,b,c)$ with arithmetic dimension $1$, corresponding to the arithmetic triangle groups. We show more generally that the number of triples with fixed arithmetic dimension is finite, and we present an efficient algorithm to completely enumerate the list of triples of bounded arithmetic dimension.