intro s _
case hdiff
σ₁ σ₂ t : ℝ
t_ne_zero : t ≠ 0
diff : ∀ (σ : ℝ), DifferentiableAt ℂ ζ (↑σ + ↑t * I)
⊢ ∀ x ∈ [[σ₁, σ₂]], DifferentiableAt ℝ (fun {σ₂} => ζ (↑σ₂ + ↑t * I)) x
case hdiff
σ₁ σ₂ t : ℝ
t_ne_zero : t ≠ 0
diff : ∀ (σ : ℝ), DifferentiableAt ℂ ζ (↑σ + ↑t * I)
s : ℝ
a✝ : s ∈ [[σ₁, σ₂]]
⊢ DifferentiableAt ℝ (fun {σ₂} => ζ (↑σ₂ + ↑t * I)) s