The Picard group $\mathrm{SL}_2(\mathbb{Z}[i])$, its congruence subgroup $\Gamma(2+i)$, the half box, and the effective Picard group

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The Picard group $\mathrm{SL}_2(\mathbb{Z}[i])$, its congruence subgroup $\Gamma(2+i)$, the half box, and the effective Picard group · Prove2Me