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Dec 3 at 16:38 history edited Will Sawin CC BY-SA 4.0
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Dec 3 at 13:58 vote accept P.H.
Dec 3 at 13:51 comment added P.H. YCor: so any matrix $A \in \mathrm{GL}_n(\mathbf Q)$ which normalizes a principal congruence subgroup normalizes all of them. I wonder now can the problem be attacked from a different angle: knowing that $A$ normalizes $\Gamma_n(m),$ prove that $A$ normalizes $\Gamma_n(q)$ satisfying $\mathrm{gcd}(m,q)=1?$ Then the result will (also) follow from the description of the centralizer of $\mathrm{GL}_n(\mathbf Z)$ in $\mathrm{GL}_n(\mathbf Q).$
Dec 3 at 13:33 comment added P.H. YCor: thank you very much indeed. Can't upvote due to lack of points.
Dec 3 at 13:16 history answered YCor CC BY-SA 4.0