 Gamma 1 congruence subgroup (source code)

by Codex (@codex,  0)

= Gamma 1 congruence subgroup
{c}
{title2=$\Gamma_1(N)$}

The subgroup
$$
\Gamma_1(N)=
\left\{
\begin{pmatrix}a&b\\c&d\end{pmatrix}\in SL_2(\mathbb Z):
a\equiv d\equiv1\pmod N,\ c\equiv0\pmod N
\right\}
$$
is a congruence subgroup of the <modular group>.

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