Congruence subgroups

This defines congruence subgroups of SL(2, ℤ) such as Γ(N), Γ₀(N) and Γ₁(N) for N a natural number.

It also contains basic results about congruence subgroups.

@[simp]

theorem SL_reduction_mod_hom_val (N : ℕ) (γ : Matrix.SpecialLinearGroup (Fin 2) ℤ) (i : Fin 2) (j : Fin 2) : ↑((Matrix.SpecialLinearGroup.map (Int.castRingHom (ZMod N))) γ) i j = ↑(↑γ i j)

def Gamma (N : ℕ) : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)

The full level N congruence subgroup of SL(2, ℤ) of matrices that reduce to the identity modulo N.

Equations

• Gamma N = MonoidHom.ker (Matrix.SpecialLinearGroup.map (Int.castRingHom (ZMod N)))

theorem Gamma_mem' (N : ℕ) (γ : Matrix.SpecialLinearGroup (Fin 2) ℤ) : γ ∈ Gamma N ↔ (Matrix.SpecialLinearGroup.map (Int.castRingHom (ZMod N))) γ = 1

@[simp]

theorem Gamma_mem (N : ℕ) (γ : Matrix.SpecialLinearGroup (Fin 2) ℤ) : γ ∈ Gamma N ↔ ↑(↑γ 0 0) = 1 ∧ ↑(↑γ 0 1) = 0 ∧ ↑(↑γ 1 0) = 0 ∧ ↑(↑γ 1 1) = 1

theorem Gamma_normal (N : ℕ) : Subgroup.Normal (Gamma N)

theorem Gamma_one_top : Gamma 1 = ⊤

theorem Gamma_zero_bot : Gamma 0 = ⊥

def Gamma0 (N : ℕ) : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)

The congruence subgroup of SL(2, ℤ) of matrices whose lower left-hand entry reduces to zero modulo N.

Equations

• Gamma0 N = { toSubmonoid := { toSubsemigroup := { carrier := {g : Matrix.SpecialLinearGroup (Fin 2) ℤ | ↑(↑g 1 0) = 0}, mul_mem' := ⋯ }, one_mem' := ⋯ }, inv_mem' := ⋯ }

@[simp]

theorem Gamma0_mem (N : ℕ) (A : Matrix.SpecialLinearGroup (Fin 2) ℤ) : A ∈ Gamma0 N ↔ ↑(↑A 1 0) = 0

theorem Gamma0_det (N : ℕ) (A : ↥(Gamma0 N)) : ↑(Matrix.det ↑↑A) = 1

def Gamma0Map (N : ℕ) : ↥(Gamma0 N) →* ZMod N

The group homomorphism from Gamma0 to ZMod N given by mapping a matrix to its lower right-hand entry.

Equations

• Gamma0Map N = { toOneHom := { toFun := fun (g : ↥(Gamma0 N)) => ↑(↑↑g 1 1), map_one' := ⋯ }, map_mul' := ⋯ }

def Gamma1' (N : ℕ) : Subgroup ↥(Gamma0 N)

The congruence subgroup Gamma1 (as a subgroup of Gamma0) of matrices whose bottom row is congruent to (0,1) modulo N.

Equations

• Gamma1' N = MonoidHom.ker (Gamma0Map N)

@[simp]

theorem Gamma1_mem' (N : ℕ) (γ : ↥(Gamma0 N)) : γ ∈ Gamma1' N ↔ (Gamma0Map N) γ = 1

theorem Gamma1_to_Gamma0_mem (N : ℕ) (A : ↥(Gamma0 N)) : A ∈ Gamma1' N ↔ ↑(↑↑A 0 0) = 1 ∧ ↑(↑↑A 1 1) = 1 ∧ ↑(↑↑A 1 0) = 0

def Gamma1 (N : ℕ) : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)

The congruence subgroup Gamma1 of SL(2, ℤ) consisting of matrices whose bottom row is congruent to (0,1) modulo N.

Equations

• Gamma1 N = Subgroup.map (MonoidHom.comp (Subgroup.subtype (Gamma0 N)) (Subgroup.subtype (Gamma1' N))) ⊤

@[simp]

theorem Gamma1_mem (N : ℕ) (A : Matrix.SpecialLinearGroup (Fin 2) ℤ) : A ∈ Gamma1 N ↔ ↑(↑A 0 0) = 1 ∧ ↑(↑A 1 1) = 1 ∧ ↑(↑A 1 0) = 0

theorem Gamma1_in_Gamma0 (N : ℕ) : Gamma1 N ≤ Gamma0 N

def IsCongruenceSubgroup (Γ : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)) : Prop

A congruence subgroup is a subgroup of SL(2, ℤ) which contains some Gamma N for some (N : ℕ+).

Equations

• IsCongruenceSubgroup Γ = ∃ (N : ℕ+), Gamma ↑N ≤ Γ

theorem isCongruenceSubgroup_trans (H : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)) (K : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)) (h : H ≤ K) (h2 : IsCongruenceSubgroup H) : IsCongruenceSubgroup K

theorem Gamma_is_cong_sub (N : ℕ+) : IsCongruenceSubgroup (Gamma ↑N)

theorem Gamma1_is_congruence (N : ℕ+) : IsCongruenceSubgroup (Gamma1 ↑N)

theorem Gamma0_is_congruence (N : ℕ+) : IsCongruenceSubgroup (Gamma0 ↑N)

theorem Gamma_cong_eq_self (N : ℕ) (g : ConjAct (Matrix.SpecialLinearGroup (Fin 2) ℤ)) : g • Gamma N = Gamma N

theorem conj_cong_is_cong (g : ConjAct (Matrix.SpecialLinearGroup (Fin 2) ℤ)) (Γ : Subgroup (Matrix.SpecialLinearGroup (Fin 2) ℤ)) (h : IsCongruenceSubgroup Γ) : IsCongruenceSubgroup (g • Γ)