-- Mathlib/NumberTheory/ArithmeticFunction.lean
theorem sum_eq_iff_sum_smul_moebius_eq_on [AddCommGroup R] {f g : ℕ → R}
    (s : Set ℕ) (hs : ∀ m n, m ∣ n → n ∈ s → m ∈ s) :
    (∀ n > 0, n ∈ s → (∑ i ∈ n.divisors, f i) = g n) ↔
      ∀ n > 0, n ∈ s → (∑ x ∈ n.divisorsAntidiagonal, μ x.fst • g x.snd) = f n

Proof state:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
⊢ ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n

Tactic:
let F := fun (n : ℕ) => ∑ x ∈ n.divisorsAntidiagonal, μ x.fst • g x.snd

Result:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
⊢ ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n

Proof state:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∑ d ∈ n.divisors, F d = g n

Tactic:
apply sum_eq_iff_sum_smul_moebius_eq.mpr _ n hn

Result:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∀ n > 0, ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = F n

Proof state:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
this : ∑ d ∈ n.divisors, μ (n / d) • G d = f n
⊢ ∀ x ∈ n.divisors, μ (n / x) • g x = μ (n / x) • G x

Tactic:
intro d hd

Result:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
this : ∑ d ∈ n.divisors, μ (n / d) • G d = f n
d : ℕ
hd : d ∈ n.divisors
⊢ μ (n / d) • g d = μ (n / d) • G d

Proof state:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∀ n > 0, ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = F n

Tactic:
intro _ _

Result:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
n : ℕ
hn : n > 0
hnP : n ∈ s
n✝ : ℕ
a✝ : n✝ > 0
⊢ ∑ x ∈ n✝.divisorsAntidiagonal, μ x.1 • g x.2 = F n✝

Proof state:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
this : ∑ d ∈ n.divisors, μ (n / d) • G d = f n
⊢ ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n

Tactic:
rw [Nat.sum_divisorsAntidiagonal' (f := fun x y => μ x • g y), ← this, sum_congr rfl]

Result:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
this : ∑ d ∈ n.divisors, μ (n / d) • G d = f n
⊢ ∀ x ∈ n.divisors, μ (n / x) • g x = μ (n / x) • G x

Proof state:
case mp
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
⊢ ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n

Tactic:
intro n hn hnP

Result:
case mp
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n

Proof state:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
⊢ (∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n) → ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n

Tactic:
intro h

Result:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
⊢ ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n

Proof state:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
⊢ ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n

Tactic:
intro n hn hnP

Result:
case mpr
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n
F : ℕ → R := fun n => ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∑ i ∈ n.divisors, f i = g n

Proof state:
case mp
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∑ x ∈ n.divisorsAntidiagonal, μ x.1 • g x.2 = f n

Tactic:
suffices ∑ d ∈ n.divisors, μ (n/d) • G d = f n from by
  rw [Nat.sum_divisorsAntidiagonal' (f := fun x y => μ x • g y), ← this, sum_congr rfl]
  intro d hd
  rw [← h d (Nat.pos_of_mem_divisors hd) <| hs d n (Nat.dvd_of_mem_divisors hd) hnP]

Result:
case mp
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
⊢ ∑ d ∈ n.divisors, μ (n / d) • G d = f n

Proof state:
R : Type u_1
inst✝ : AddCommGroup R
f g : ℕ → R
s : Set ℕ
hs : ∀ (m n : ℕ), m ∣ n → n ∈ s → m ∈ s
h : ∀ n > 0, n ∈ s → ∑ i ∈ n.divisors, f i = g n
G : ℕ → R := fun n => ∑ i ∈ n.divisors, f i
n : ℕ
hn : n > 0
hnP : n ∈ s
n✝ : ℕ
a✝ : n✝ > 0
⊢ ∑ i ∈ n✝.divisors, f i = G n✝

Tactic:
rfl

Result:
no goals