exact DifferentiableAt.add_const (c := t * I) <| differentiableAt_ofReal _
case hdiff.hf
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 => ↑σ₂ + ↑t * I) s
no goals