/-- Given `f : NormedAddGroupHom G H` for some complete `G` and a subgroup `K` of `H`, if every
element `x` of `K` has a preimage under `f` whose norm is at most `C*‖x‖` then the same holds for
elements of the (topological) closure of `K` with constant `C+ε` instead of `C`, for any
positive `ε`.
-/
theorem controlled_closure_of_complete {f : NormedAddGroupHom G H} {K : AddSubgroup H} {C ε : ℝ}
    (hC : 0 < C) (hε : 0 < ε) (hyp : f.SurjectiveOnWith K C) :
    f.SurjectiveOnWith K.topologicalClosure (C + ε) := by
  rintro (h : H) (h_in : h ∈ K.topologicalClosure)
  -- We first get rid of the easy case where `h = 0`.
  by_cases hyp_h : h = 0
  · rw [hyp_h]
    use 0
    simp
  /- The desired preimage will be constructed as the sum of a series. Convergence of
    the series will be guaranteed by completeness of `G`. We first write `h` as the sum
    of a sequence `v` of elements of `K` which starts close to `h` and then quickly goes to zero.
    The sequence `b` below quantifies this. -/
  set b : ℕ → ℝ := fun i => (1 / 2) ^ i * (ε * ‖h‖ / 2) / C
  have b_pos : ∀ i, 0 < b i := by
    intro i
    field_simp [hC]
    exact
      div_pos (mul_pos hε (norm_pos_iff.mpr hyp_h)) (mul_pos (by norm_num : (0 : ℝ) < 2 ^ i * 2) hC)
  obtain
    ⟨v : ℕ → H, lim_v : Tendsto (fun n : ℕ => ∑ k in range (n + 1), v k) atTop (𝓝 h), v_in :
      ∀ n, v n ∈ K, hv₀ : ‖v 0 - h‖ < b 0, hv : ∀ n > 0, ‖v n‖ < b n⟩ :=
    controlled_sum_of_mem_closure h_in b_pos
  /- The controlled surjectivity assumption on `f` allows to build preimages `u n` for all
    elements `v n` of the `v` sequence. -/
  have : ∀ n, ∃ m' : G, f m' = v n ∧ ‖m'‖ ≤ C * ‖v n‖ := fun n : ℕ => hyp (v n) (v_in n)
  choose u hu hnorm_u using this
  /- The desired series `s` is then obtained by summing `u`. We then check our choice of
    `b` ensures `s` is Cauchy. -/
  set s : ℕ → G := fun n => ∑ k in range (n + 1), u k
  have : CauchySeq s := by
    apply NormedAddCommGroup.cauchy_series_of_le_geometric'' (by norm_num) one_half_lt_one
    rintro n (hn : n ≥ 1)
    calc
      ‖u n‖ ≤ C * ‖v n‖ := hnorm_u n
      _ ≤ C * b n := by gcongr; exact (hv _ <| Nat.succ_le_iff.mp hn).le
      _ = (1 / 2) ^ n * (ε * ‖h‖ / 2) := by simp [mul_div_cancel' _ hC.ne.symm]
      _ = ε * ‖h‖ / 2 * (1 / 2) ^ n := mul_comm _ _
  -- We now show that the limit `g` of `s` is the desired preimage.
  obtain ⟨g : G, hg⟩ := cauchySeq_tendsto_of_complete this
  refine' ⟨g, _, _⟩
  · -- We indeed get a preimage. First note:
    have : f ∘ s = fun n => ∑ k in range (n + 1), v k := by
      ext n
      simp [map_sum, hu]
    /- In the above equality, the left-hand-side converges to `f g` by continuity of `f` and
      definition of `g` while the right-hand-side converges to `h` by construction of `v` so
      `g` is indeed a preimage of `h`. -/
    rw [← this] at lim_v
    exact tendsto_nhds_unique ((f.continuous.tendsto g).comp hg) lim_v
  · -- Then we need to estimate the norm of `g`, using our careful choice of `b`.
    suffices : ∀ n, ‖s n‖ ≤ (C + ε) * ‖h‖
    exact le_of_tendsto' (continuous_norm.continuousAt.tendsto.comp hg) this
    intro n
    have hnorm₀ : ‖u 0‖ ≤ C * b 0 + C * ‖h‖ := by
      have :=
        calc
          ‖v 0‖ ≤ ‖h‖ + ‖v 0 - h‖ := norm_le_insert' _ _
          _ ≤ ‖h‖ + b 0 := by gcongr
      

Proof state:
G : Type u_1
inst✝² : NormedAddCommGroup G
inst✝¹ : CompleteSpace G
H : Type u_2
inst✝ : NormedAddCommGroup H
f : NormedAddGroupHom G H
K : AddSubgroup H
C ε : ℝ
hC : 0 < C
hε : 0 < ε
hyp : NormedAddGroupHom.SurjectiveOnWith f K C
h : H
h_in : h ∈ AddSubgroup.topologicalClosure K
hyp_h : ¬h = 0
b : ℕ → ℝ := fun i => (1 / 2) ^ i * (ε * ‖h‖ / 2) / C
b_pos : ∀ (i : ℕ), 0 < b i
v : ℕ → H
lim_v : Tendsto (fun n => ∑ k in range (n + 1), v k) atTop (𝓝 h)
v_in : ∀ (n : ℕ), v n ∈ K
hv₀ : ‖v 0 - h‖ < b 0
hv : ∀ n > 0, ‖v n‖ < b n
u : ℕ → G
hu : ∀ (n : ℕ), f (u n) = v n
hnorm_u : ∀ (n : ℕ), ‖u n‖ ≤ C * ‖v n‖
s : ℕ → G := fun n => ∑ k in range (n + 1), u k
this✝ : CauchySeq s
g : G
hg : Tendsto s atTop (𝓝 g)
n : ℕ
this : ‖v 0‖ ≤ ‖h‖ + b 0
⊢ ‖u 0‖ ≤ C * b 0 + C * ‖h‖

Next tactic:
calc
        ‖u 0‖ ≤ C * ‖v 0‖ := hnorm_u 0
        _ ≤ C * (‖h‖ + b 0) := by gcongr
        _ = C * b 0 + C * ‖h‖ := by rw [add_comm, mul_add]