Let $\Gamma$ be a finitely generated subgroup of $PSL_2(\mathbb R)$.

I'm looking for effective criteria (idealy necessary and sufficient but just necessary would be a good start) ensuring that $\Gamma$ is

1.- Fuchsian (ie. $\Gamma$ is a discrete subgroup); or

2.- Fuchsian of the first type (ie. the Riemann surface $\mathbb H/\Gamma$ is of finite hyperbolic area); or

3.- Fuchsian and co-compact (ie. the Riemann surface $\mathbb H/\Gamma$ is compact).

I'm not familiar with this field, but I know that there are a lot of works on this question, especially when $\Gamma$ is generated by two elements. In this case, one can certainly find some satisfying criteria or algorithms in Gilman's memoir `Two-generators discrete subgroups of $PSL_2(\mathbb R)$'.

Main question: what about the case when $\Gamma$ is generated by more than two elements?

I think that $\Gamma$ is Fuchsian iff every subgroup generated by 2 elements of $\Gamma$ is discrete (is it true? A confirmation and better, a reference, would be welcome). Anyway, I don't see how this could be used in an effective manner to verify that a given $\Gamma$ is discrete or not...

Main question (made more precise): assume that $\Gamma$ is generated by $n\geq 3$ elements. Is there a criterion involving the $n$ generators of $\Gamma$ all together that must be verified if this group is Fuchsian (or Fuchsian of the first type, or Fuchsian and cocompact)?


Secondary question: in case of two-generators subgroups, are the existing criteria/algorithms about the properties 1, 2 and 3 implemented? If yes, where is it possible to find softwares/codes allowing to make explicit computations?

Thanks in advance.

  • $\begingroup$ W.M. Goldman, Topological components of spaces of representations, Inventiones 1988 and references therein might be relevant. $\endgroup$ – few_reps Mar 28 '14 at 13:04
  • $\begingroup$ And for the computation of Euler classes, J.Barge, E. Ghys, Cocycles d'Euler et de Maslov, Math. Ann. 1992. $\endgroup$ – few_reps Mar 28 '14 at 13:10
  • $\begingroup$ I don't know if it is the same content as his memoir, but there is An algorithm for 2-Generator Fuchsian groups by Gilman and Maskit. See also Polynomial complexity of the Gilman-Maskit discreteness algorithm by Yicheng Jiang. $\endgroup$ – Aurel Mar 28 '14 at 14:22
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    $\begingroup$ Maybe you could bruteforce the problem: enumerate larger and larger balls in the word metric, and hope to either get a counter-example to Jorgensen's inequality or find a finite-sided Dirichlet domain and apply Poincaré's theorem. $\endgroup$ – Aurel Mar 28 '14 at 14:32
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    $\begingroup$ Yes, this is how Jorgensen's algorithm works. $\endgroup$ – Misha Mar 28 '14 at 19:39

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