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It's called the Congruence Topologycongruence topology, and is (obviously) always at least as coarse as the profinite topology. If your group has the Congruence Subgroup Property (the modular group doesn't, but $SL_n(\mathbb{Z})$ does for $n>2$) then it's the same as the profinite topology. A google search found, for instance, Section 7.3 of Algebraic theory of the Bianchi groups by Benjamin Fine. |
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It's called the Congruence Topology. If your group has the Congruence Subgroup Property (the modular group doesn't, but $SL_n(\mathbb{Z})$ does for $n>2$) then it's the same as the profinite topology. |
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