field_simp [g]
h2 :
  ∑ x ∈ Finset.filter (fun x => Even x) (Finset.range 1319), 1 / (↑x + 1) +
      ∑ x ∈ Finset.filter (fun x => ¬Even x) (Finset.range 1319), 1 / (↑x + 1) =
    ∑ x ∈ Finset.range 1319, 1 / (↑x + 1)
g : ℕ ↪ ℕ := { toFun := fun x => 2 * x + 1, inj' := ⋯ }
h4 : Finset.map g (Finset.range 659) = Finset.filter (fun x => ¬Even x) (Finset.range 1319)
x : ℕ
a✝ : x ∈ Finset.range 659
⊢ 1 / (2 * (↑x + 1)) = 1 / (↑(g x) + 1)
h2 :
  ∑ x ∈ Finset.filter (fun x => Even x) (Finset.range 1319), 1 / (↑x + 1) +
      ∑ x ∈ Finset.filter (fun x => ¬Even x) (Finset.range 1319), 1 / (↑x + 1) =
    ∑ x ∈ Finset.range 1319, 1 / (↑x + 1)
g : ℕ ↪ ℕ := { toFun := fun x => 2 * x + 1, inj' := ⋯ }
h4 : Finset.map g (Finset.range 659) = Finset.filter (fun x => ¬Even x) (Finset.range 1319)
x : ℕ
a✝ : x ∈ Finset.range 659
⊢ 2 * ↑x + 1 + 1 = 2 * (↑x + 1)