set_option maxHeartbeats 300000 in
/-- If two formal multilinear series have positive radius of convergence, then the terms appearing
in the definition of their composition are also summable (when multiplied by a suitable positive
geometric term). -/
theorem comp_summable_nnreal (q : FormalMultilinearSeries 𝕜 F G) (p : FormalMultilinearSeries 𝕜 E F)
    (hq : 0 < q.radius) (hp : 0 < p.radius) :
    ∃ r > (0 : ℝ≥0),
      Summable fun i : Σ n, Composition n => ‖q.compAlongComposition p i.2‖₊ * r ^ i.1 := 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.comp_along_composition 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_le_pow_right hCp1 c.length_le
    calc
      ‖q.compAlongComposition p c‖₊ * r ^ n ≤
          (‖q c.length‖₊ * ∏ i, ‖p (c.blocksFun i)‖₊) * r ^ n :=
        mul_le_mul' (q.compAlongComposition_nnnorm p c) le_rfl
      _ = ‖q c.length‖₊ * rq ^ n * ((∏ i, ‖p (c.blocksFun i)‖₊) * rp ^ n) * r0 ^ n := by
        simp only [mul_pow]; 

Proof state:
𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
H : Type u_5
inst✝⁸ : NontriviallyNormedField 𝕜
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
hq : 0 < radius q
hp : 0 < radius p
rq rp : ℝ≥0
hrp : rp < 1 ∧ ↑rp < radius p
hrq : rq < 1 ∧ ↑rq < radius q
rp_pos : 0 < rp
rq_pos : 0 < rq
Cq : ℝ≥0
_hCq0 : Cq > 0
hCq : ∀ (n : ℕ), ‖q n‖₊ * rq ^ n ≤ Cq
Cp : ℝ≥0
hCp1 : Cp ≥ 1
hCp : ∀ (n : ℕ), ‖p n‖₊ * rp ^ n ≤ Cp
r0 : ℝ≥0 := (4 * Cp)⁻¹
r0_pos : 0 < r0
r : ℝ≥0 := rp * rq * r0
r_pos : 0 < r
n : ℕ
c : Composition n
A : ‖q (Composition.length c)‖₊ * rq ^ n ≤ Cq
B : (∏ i : Fin (Composition.length c), ‖p (Composition.blocksFun c i)‖₊) * rp ^ n ≤ Cp ^ n
⊢ (‖q (Composition.length c)‖₊ * ∏ x : Fin (Composition.length c), ‖p (Composition.blocksFun c x)‖₊) *
      (rp ^ n * rq ^ n * (4 * Cp)⁻¹ ^ n) =
    ‖q (Composition.length c)‖₊ * rq ^ n *
        ((∏ x : Fin (Composition.length c), ‖p (Composition.blocksFun c x)‖₊) * rp ^ n) *
      (4 * Cp)⁻¹ ^ n

Next tactic:
ring