The Bianchi group $\mathrm{SL}_2(\mathbb{Z}[\omega])$, its congruence subgroup of level $3+\omega$, and the box over a rhombus

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The Bianchi group $\mathrm{SL}_2(\mathbb{Z}[\omega])$, its congruence subgroup of level $3+\omega$, and the box over a rhombus · Prove2Me