rw [← Finset.sum_sdiff (s₁ := Finset.Ico 1 N) (s₂ := Finset.Ico 0 N) this]
N : ℕ
Npos : 0 < N
this : Finset.Ico 1 N ⊆ Finset.Ico 0 N
⊢ ∑ x ∈ Finset.Ico 0 N, (↑x)⁻¹ = ∑ x ∈ Finset.Ico 1 N, (↑x)⁻¹
N : ℕ
Npos : 0 < N
this : Finset.Ico 1 N ⊆ Finset.Ico 0 N
⊢ ∑ x ∈ Finset.Ico 0 N \ Finset.Ico 1 N, (↑x)⁻¹ + ∑ x ∈ Finset.Ico 1 N, (↑x)⁻¹ = ∑ x ∈ Finset.Ico 1 N, (↑x)⁻¹