Module: Mathlib.Analysis.Analytic.Composition

Goal state before:
pp: 𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : NormedAddCommGroup F
inst✝² : NormedSpace 𝕜 F
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace 𝕜 G
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
hq : 0 < q.radius
hp : 0 < p.radius
⊢ ∃ r > 0, Summable fun i ↦ ‖q.compAlongComposition p i.snd‖₊ * r ^ i.fst

Tactic:
by
  /- This follows from the fact that the growth rate of `‖qₙ‖` and `‖pₙ‖` is at most geometric,
    giving a geometric bound on each `‖q.compAlongComposition p op‖`, together with the
    fact that there are `2^(n-1)` compositions of `n`, giving at most a geometric loss. -/
  rcases ENNReal.lt_iff_exists_nnreal_btwn.1 (lt_min zero_lt_one hq) with ⟨rq, rq_pos, hrq⟩
  rcases ENNReal.lt_iff_exists_nnreal_btwn.1 (lt_min zero_lt_one hp) with ⟨rp, rp_pos, hrp⟩
  simp only [lt_min_iff, ENNReal.coe_lt_one_iff, ENNReal.coe_pos] at hrp hrq rp_pos rq_pos
  obtain ⟨Cq, _hCq0, hCq⟩ : ∃ Cq > 0, ∀ n, ‖q n‖₊ * rq ^ n ≤ Cq :=
    q.nnnorm_mul_pow_le_of_lt_radius hrq.2
  obtain ⟨Cp, hCp1, hCp⟩ : ∃ Cp ≥ 1, ∀ n, ‖p n‖₊ * rp ^ n ≤ Cp := by
    rcases p.nnnorm_mul_pow_le_of_lt_radius hrp.2 with ⟨Cp, -, hCp⟩
    exact ⟨max Cp 1, le_max_right _ _, fun n => (hCp n).trans (le_max_left _ _)⟩
  let r0 : ℝ≥0 := (4 * Cp)⁻¹
  have r0_pos : 0 < r0 := inv_pos.2 (mul_pos zero_lt_four (zero_lt_one.trans_le hCp1))
  set r : ℝ≥0 := rp * rq * r0
  have r_pos : 0 < r := mul_pos (mul_pos rp_pos rq_pos) r0_pos
  have I :
    ∀ i : Σ n : ℕ, Composition n, ‖q.compAlongComposition p i.2‖₊ * r ^ i.1 ≤ Cq / 4 ^ i.1 := by
    rintro ⟨n, c⟩
    have A := calc
      ‖q c.length‖₊ * rq ^ n ≤ ‖q c.length‖₊ * rq ^ c.length :=
        mul_le_mul' le_rfl (pow_le_pow_of_le_one rq.2 hrq.1.le c.length_le)
      _ ≤ Cq := hCq _
    have B := calc
      (∏ i, ‖p (c.blocksFun i)‖₊) * rp ^ n = ∏ i, ‖p (c.blocksFun i)‖₊ * rp ^ c.blocksFun i := by
        simp only [Finset.prod_mul_distrib, Finset.prod_pow_eq_pow_sum, c.sum_blocksFun]
      _ ≤ ∏ _i : Fin c.length, Cp := Finset.prod_le_prod' fun i _ => hCp _
      _ = Cp ^ c.length := by simp
      _ ≤ Cp ^ n := pow_right_mono₀ hCp1 c.length_le
    calc
      ‖q.compAlongComposition p c‖₊ * r ^ n ≤
          (‖q c.length‖₊ * ∏ i, ‖p (c.blocksFun i)‖₊) * r ^ n := by
        grw [q.compAlongComposition_nnnorm p c]
      _ = ‖q c.length‖₊ * rq ^ n * ((∏ i, ‖p (c.blocksFun i)‖₊) * rp ^ n) * r0 ^ n := by
        ring
      _ ≤ Cq * Cp ^ n * r0 ^ n := mul_le_mul' (mul_le_mul' A B) le_rfl
      _ = Cq / 4 ^ n := by
        simp only [r0]
        simp [field, mul_pow]
  refine ⟨r, r_pos, NNReal.summable_of_le I ?_⟩
  simp_rw [div_eq_mul_inv]
  refine Summable.mul_left _ ?_
  have : ∀ n : ℕ, HasSum (fun c : Composition n => (4 ^ n : ℝ≥0)⁻¹) (2 ^ (n - 1) / 4 ^ n) := by
    intro n
    convert! hasSum_fintype fun c : Composition n => (4 ^ n : ℝ≥0)⁻¹
    simp [Finset.card_univ, composition_card, div_eq_mul_inv]
  refine NNReal.summable_sigma.2 ⟨fun n => (this n).summable, (NNReal.summable_nat_add_iff 1).1 ?_⟩
  convert! (NNReal.summable_geometric (NNReal.div_lt_one_of_lt one_lt_two)).mul_left (1 / 4) using 1
  ext1 n
  rw [(this _).tsum_eq, add_tsub_cancel_right]
  simp [field, pow_succ, mul_pow, show (4 : ℝ≥0) = 2 * 2 by norm_num]

Goal state after: