Module: Mathlib.Analysis.Normed.Operator.Banach

Goal state before:
pp: 𝕜 : Type u_1
𝕜' : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜
inst✝⁹ : NontriviallyNormedField 𝕜'
σ : 𝕜 →+* 𝕜'
E : Type u_3
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : NormedSpace 𝕜 E
F : Type u_4
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace 𝕜' F
f : E →SL[σ] F
σ' : 𝕜' →+* 𝕜
inst✝⁴ : RingHomInvPair σ σ'
inst✝³ : RingHomIsometric σ
inst✝² : RingHomIsometric σ'
inst✝¹ : CompleteSpace F
inst✝ : CompleteSpace E
surj : Surjective ⇑f
⊢ ∃ C > 0, ∀ (y : F), ∃ x, f x = y ∧ ‖x‖ ≤ C * ‖y‖

Tactic:
by
  obtain ⟨C, C0, hC⟩ := exists_approx_preimage_norm_le f surj
  /- Second step of the proof: starting from `y`, we want an exact preimage of `y`. Let `g y` be
    the approximate preimage of `y` given by the first step, and `h y = y - f(g y)` the part that
    has no preimage yet. We will iterate this process, taking the approximate preimage of `h y`,
    leaving only `h^2 y` without preimage yet, and so on. Let `u n` be the approximate preimage
    of `h^n y`. Then `u` is a converging series, and by design the sum of the series is a
    preimage of `y`. This uses completeness of `E`. -/
  choose g hg using hC
  let h y := y - f (g y)
  have hle : ∀ y, ‖h y‖ ≤ 1 / 2 * ‖y‖ := by
    intro y
    rw [← dist_eq_norm, dist_comm]
    exact (hg y).1
  refine ⟨2 * C + 1, by linarith, fun y => ?_⟩
  have hnle : ∀ n : ℕ, ‖h^[n] y‖ ≤ (1 / 2) ^ n * ‖y‖ := by
    intro n
    induction n with
    | zero => simp only [one_div, one_mul, iterate_zero_apply, pow_zero, le_rfl]
    | succ n IH =>
      rw [iterate_succ']
      apply le_trans (hle _) _
      rw [pow_succ', mul_assoc]
      gcongr
  let u n := g (h^[n] y)
  have ule : ∀ n, ‖u n‖ ≤ (1 / 2) ^ n * (C * ‖y‖) := fun n ↦ by
    apply le_trans (hg _).2
    calc
      C * ‖h^[n] y‖ ≤ C * ((1 / 2) ^ n * ‖y‖) := by gcongr; exact hnle n
      _ = (1 / 2) ^ n * (C * ‖y‖) := by ring
  have sNu : Summable fun n => ‖u n‖ := by
    refine .of_nonneg_of_le (fun n => norm_nonneg _) ule ?_
    exact Summable.mul_right _ (summable_geometric_of_lt_one (by simp) (by norm_num))
  have su : Summable u := sNu.of_norm
  let x := tsum u
  have x_ineq : ‖x‖ ≤ (2 * C + 1) * ‖y‖ :=
    calc
      ‖x‖ ≤ ∑' n, ‖u n‖ := norm_tsum_le_tsum_norm sNu
      _ ≤ ∑' n, (1 / 2) ^ n * (C * ‖y‖) :=
        sNu.tsum_le_tsum ule <| Summable.mul_right _ summable_geometric_two
      _ = (∑' n, (1 / 2) ^ n) * (C * ‖y‖) := tsum_mul_right
      _ = 2 * C * ‖y‖ := by rw [tsum_geometric_two, mul_assoc]
      _ ≤ 2 * C * ‖y‖ + ‖y‖ := le_add_of_nonneg_right (norm_nonneg y)
      _ = (2 * C + 1) * ‖y‖ := by ring
  have fsumeq : ∀ n : ℕ, f (∑ i ∈ Finset.range n, u i) = y - h^[n] y := by
    intro n
    induction n with
    | zero => simp [f.map_zero]
    | succ n IH => rw [sum_range_succ, f.map_add, IH, iterate_succ_apply', sub_add]
  have : Tendsto (fun n => ∑ i ∈ Finset.range n, u i) atTop (𝓝 x) := su.hasSum.tendsto_sum_nat
  have L₁ : Tendsto (fun n => f (∑ i ∈ Finset.range n, u i)) atTop (𝓝 (f x)) :=
    (f.continuous.tendsto _).comp this
  simp only [fsumeq] at L₁
  have L₂ : Tendsto (fun n => y - h^[n] y) atTop (𝓝 (y - 0)) := by
    refine tendsto_const_nhds.sub ?_
    rw [tendsto_iff_norm_sub_tendsto_zero]
    simp only [sub_zero]
    refine squeeze_zero (fun _ => norm_nonneg _) hnle ?_
    rw [← zero_mul ‖y‖]
    refine (_root_.tendsto_pow_atTop_nhds_zero_of_lt_one ?_ ?_).mul tendsto_const_nhds <;> norm_num
  have feq : f x = y - 0 := tendsto_nhds_unique L₁ L₂
  rw [sub_zero] at feq
  exact ⟨x, feq, x_ineq⟩

Goal state after: