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It is known that the $j$-function takes algebraic values when evaluated at imaginary quadratic integers. This is a result that was proved by Schneider in 1937 apparently. To be precise, Schneider proved that

If $z \in \mathbb{H} = \{ z \in \mathbb{C} \mid \Im{(z)} > 0 \}$ and $j(z)$ is an algebraic integer. Then $z$ is algebraic if and only if $z$ is an imaginary quadratic integer.

I would really appreciate if someone can provide me some nice reference(s) where this result is proved.

Thank you very much for any help.

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