MathSciNet bibliographic data MR878891 11E57 (11F06 20H05) Mason, A. W. Free quotients of congruence subgroups of ${\rm SL}_2$${\rm SL}_2$ over a Dedekind ring of arithmetic type contained in a function field. Math. Proc. Cambridge Philos. Soc. 101 (1987), no. 3, 421–429. Article

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