Theorem: ArithmeticFunction.IsMultiplicative.mul
File: Mathlib/NumberTheory/ArithmeticFunction.lean

Proof steps:
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
⊢ (f * g).IsMultiplicative
state_after: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
⊢ ∀ {m n : ℕ}, m.Coprime n → (f * g) (m * n) = (f * g) m * (f * g) n
tactic: refine ⟨by simp [hf.1, hg.1], ?_⟩
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
⊢ ∀ {m n : ℕ}, m.Coprime n → (f * g) (m * n) = (f * g) m * (f * g) n
state_after: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
⊢ ∀ {m n : ℕ},
    m.Coprime n →
      ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
        (∑ x ∈ m.divisorsAntidiagonal, f x.1 * g x.2) * ∑ x ∈ n.divisorsAntidiagonal, f x.1 * g x.2
tactic: simp only [mul_apply]
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
⊢ ∀ {m n : ℕ},
    m.Coprime n →
      ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
        (∑ x ∈ m.divisorsAntidiagonal, f x.1 * g x.2) * ∑ x ∈ n.divisorsAntidiagonal, f x.1 * g x.2
state_after: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
    (∑ x ∈ m.divisorsAntidiagonal, f x.1 * g x.2) * ∑ x ∈ n.divisorsAntidiagonal, f x.1 * g x.2
tactic: intro m n cop
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
    (∑ x ∈ m.divisorsAntidiagonal, f x.1 * g x.2) * ∑ x ∈ n.divisorsAntidiagonal, f x.1 * g x.2
state_after: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
    ∑ x ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal, f x.1.1 * g x.1.2 * (f x.2.1 * g x.2.2)
tactic: rw [sum_mul_sum, ← sum_product']
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2 =
    ∑ x ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal, f x.1.1 * g x.1.2 * (f x.2.1 * g x.2.2)
state_after: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal, f x.1.1 * g x.1.2 * (f x.2.1 * g x.2.2) =
    ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2
tactic: symm
state_before: R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∑ x ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal, f x.1.1 * g x.1.2 * (f x.2.1 * g x.2.2) =
    ∑ x ∈ (m * n).divisorsAntidiagonal, f x.1 * g x.2
state_after: case hi
R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∀ a ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal,
    (match a with
      | ((i, j), k, l) => (i * k, j * l)) ∈
      (m * n).divisorsAntidiagonal

case i_inj
R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ Set.InjOn
    (fun x =>
      match x with
      | ((i, j), k, l) => (i * k, j * l))
    ↑(m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal)

case i_surj
R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ Set.SurjOn
    (fun x =>
      match x with
      | ((i, j), k, l) => (i * k, j * l))
    ↑(m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal) ↑(m * n).divisorsAntidiagonal

case h
R : Type u_1
inst✝ : CommSemiring R
f g : ArithmeticFunction R
hf : f.IsMultiplicative
hg : g.IsMultiplicative
m n : ℕ
cop : m.Coprime n
⊢ ∀ a ∈ m.divisorsAntidiagonal ×ˢ n.divisorsAntidiagonal,
    f a.1.1 * g a.1.2 * (f a.2.1 * g a.2.2) =
      f
          (match a with
            | ((i, j), k, l) => (i * k, j * l)).1 *
        g
          (match a with
            | ((i, j), k, l) => (i * k, j * l)).2
tactic: apply sum_nbij fun ((i, j), k, l) ↦ (i * k, j * l)