congruence subgroup
Urs Schreiber
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congruence subgroup
Contents
Definition
Of the modular group
Let n ∈ ℕ n \in \mathbb{N} be a natural number . Write
p n : SL 2 ( ℤ ) → SL 2 ( ℤ / n ℤ )
p_n \;\colon\; SL_2(\mathbb{Z}) \to SL_2(\mathbb{Z}/n\mathbb{Z})
for the projection from the Moebius special linear group
SL
(
2
,
ℤ
SL(2,\mathbb{Z}
induced by the quotient projection ℤ → ℤ / n ℤ \mathbb{Z} \to \mathbb{Z}/n\mathbb{Z} to the integers modulo n .
The mod-n n congruence subgroups of the special linear group SL 2 ( ℤ )...