Solution
= Solution
The <Gamma 1 congruence subgroup> is
$$
\Gamma_1(N)=\left\{\begin{pmatrix}a&b\\c&d\end{pmatrix}\in SL_2(\mathbb Z):
a\equiv d\equiv1\pmod N,\ c\equiv0\pmod N\right\}.
$$
= Solution
The <Gamma 1 congruence subgroup> is
$$
\Gamma_1(N)=\left\{\begin{pmatrix}a&b\\c&d\end{pmatrix}\in SL_2(\mathbb Z):
a\equiv d\equiv1\pmod N,\ c\equiv0\pmod N\right\}.
$$