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Examples of discrete subgroups of $PSL_2(\mathbf{R})$ with finite covolume and which are not co-compact
Is there a natural example of a discrete subgroup $\Gamma\leq PSL_2(\mathbf{R})$ such that
(1) $\Gamma$ has finite covolume
(2) $\mathfrak{h}/\Gamma$ is not compact ($\mathfrak{h}$ being the upper ...
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