/-- **Phragmen-Lindelöf principle** in the right half-plane. Let `f : ℂ → E` be a function such that

* `f` is differentiable in the open right half-plane and is continuous on its closure;
* `‖f z‖` is bounded from above by `A * exp(B * (abs z) ^ c)` on the open right half-plane
  for some `c < 2`;
* `‖f z‖` is bounded from above by a constant on the imaginary axis;
* `f x`, `x : ℝ`, tends to zero superexponentially fast as `x → ∞`:
  for any natural `n`, `exp (n * x) * ‖f x‖` tends to zero as `x → ∞`.

Then `f` is equal to zero on the closed right half-plane. -/
theorem eq_zero_on_right_half_plane_of_superexponential_decay (hd : DiffContOnCl ℂ f {z | 0 < z.re})
    (hexp : ∃ c < (2 : ℝ), ∃ B,
      f =O[cobounded ℂ ⊓ 𝓟 {z | 0 < z.re}] fun z => expR (B * abs z ^ c))
    (hre : SuperpolynomialDecay atTop expR fun x => ‖f x‖) (him : ∃ C, ∀ x : ℝ, ‖f (x * I)‖ ≤ C) :
    EqOn f 0 {z : ℂ | 0 ≤ z.re} := by
  rcases him with ⟨C, hC⟩
  -- Due to continuity, it suffices to prove the equality on the open right half-plane.
  suffices ∀ z : ℂ, 0 < z.re → f z = 0 by
    simpa only [closure_setOf_lt_re] using
      EqOn.of_subset_closure this hd.continuousOn continuousOn_const subset_closure Subset.rfl
  -- Consider $g_n(z)=e^{nz}f(z)$.
  set g : ℕ → ℂ → E := fun (n : ℕ) (z : ℂ) => exp z ^ n • f z
  have hg : ∀ n z, ‖g n z‖ = expR z.re ^ n * ‖f z‖ := fun n z ↦ by
    simp only [norm_smul, norm_eq_abs, Complex.abs_pow, abs_exp]
  intro z hz
  -- Since `e^{nz} → ∞` as `n → ∞`, it suffices to show that each `g_n` is bounded from above by `C`
  suffices H : ∀ n : ℕ, ‖g n z‖ ≤ C
  · contrapose! H
    simp only [hg]
    exact (((tendsto_pow_atTop_atTop_of_one_lt (Real.one_lt_exp_iff.2 hz)).atTop_mul
      (norm_pos_iff.2 H) tendsto_const_nhds).eventually (eventually_gt_atTop C)).exists
  intro n
  -- This estimate follows from the Phragmen-Lindelöf principle in the right half-plane.
  refine' right_half_plane_of_tendsto_zero_on_real ((differentiable_exp.pow n).diffContOnCl.smul hd)
    _ _ (fun y => _) hz.le
  · rcases hexp with ⟨c, hc, B, hO⟩
    refine' ⟨max c 1, max_lt hc one_lt_two, n + max B 0, .of_norm_left _⟩
    simp only [hg]
    refine' ((isBigO_refl (fun z : ℂ => expR z.re ^ n) _).mul hO.norm_left).trans (.of_bound 1 _)
    filter_upwards [(eventually_cobounded_le_norm 1).filter_mono inf_le_left] with z hz
    simp only [← Real.exp_nat_mul, ← Real.exp_add, Real.norm_eq_abs, Real.abs_exp, add_mul, one_mul]
    gcongr
    · calc
        z.re ≤ abs z := re_le_abs _
        _ = abs z ^ (1 : ℝ) := (Real.rpow_one _).symm
        _ ≤ abs z ^ max c 1 := Real.rpow_le_rpow_of_exponent_le hz (le_max_right _ _)
    exacts [le_max_left _ _, hz, le_max_left _ _]
  · rw [tendsto_zero_iff_norm_tendsto_zero]; simp only [hg]
    exact hre n
  · 

Proof state:
case H.refine'_3
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℂ E
a b C✝ : ℝ
f g✝ : ℂ → E
z✝ : ℂ
hd : DiffContOnCl ℂ f {z | 0 < z.re}
hexp : ∃ c < 2, ∃ B, f =O[cobounded ℂ ⊓ 𝓟 {z | 0 < z.re}] fun z => expR (B * Complex.abs z ^ c)
hre : SuperpolynomialDecay atTop expR fun x => ‖f ↑x‖
C : ℝ
hC : ∀ (x : ℝ), ‖f (↑x * I)‖ ≤ C
g : ℕ → ℂ → E := fun n z => cexp z ^ n • f z
hg : ∀ (n : ℕ) (z : ℂ), ‖g n z‖ = expR z.re ^ n * ‖f z‖
z : ℂ
hz : 0 < z.re
n : ℕ
y : ℝ
⊢ ‖g n (↑y * I)‖ ≤ C

Next tactic:
rw [hg, ofReal_mul_re, I_re, mul_zero, Real.exp_zero, one_pow, one_mul]