Module: Mathlib.Analysis.SpecificLimits.FloorPow

Goal state before:
pp: u : ℕ → ℝ
l : ℝ
hmono : Monotone u
hlim :
  ∀ (a : ℝ),
    1 < a →
      ∃ c,
        (∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ a * ↑(c n)) ∧
          Tendsto c atTop atTop ∧ Tendsto (fun n ↦ u (c n) / ↑(c n)) atTop (𝓝 l)
⊢ Tendsto (fun n ↦ u n / ↑n) atTop (𝓝 l)

Tactic:
by
  /- To check the result up to some `ε > 0`, we use a sequence `c` for which the ratio
    `c (N+1) / c N` is bounded by `1 + ε`. Sandwiching a given `n` between two consecutive values of
    `c`, say `c N` and `c (N+1)`, one can then bound `u n / n` from above by `u (c N) / c (N - 1)`
    and from below by `u (c (N - 1)) / c N` (using that `u` is monotone), which are both comparable
    to the limit `l` up to `1 + ε`.
    We give a version of this proof by clearing out denominators first, to avoid discussing the sign
    of different quantities. -/
  have lnonneg : 0 ≤ l := by
    rcases hlim 2 one_lt_two with ⟨c, _, ctop, clim⟩
    have : Tendsto (fun n => u 0 / c n) atTop (𝓝 0) :=
      tendsto_const_nhds.div_atTop (tendsto_natCast_atTop_iff.2 ctop)
    apply le_of_tendsto_of_tendsto' this clim fun n => ?_
    gcongr
    exact hmono zero_le
  have A : ∀ ε : ℝ, 0 < ε → ∀ᶠ n in atTop, u n - n * l ≤ ε * (1 + ε + l) * n := by
    intro ε εpos
    rcases hlim (1 + ε) ((lt_add_iff_pos_right _).2 εpos) with ⟨c, cgrowth, ctop, clim⟩
    have L : ∀ᶠ n in atTop, u (c n) - c n * l ≤ ε * c n := by
      rw [← tendsto_sub_nhds_zero_iff, ← Asymptotics.isLittleO_one_iff ℝ,
        Asymptotics.isLittleO_iff] at clim
      filter_upwards [clim εpos, ctop (Ioi_mem_atTop 0)] with n hn cnpos'
      have cnpos : 0 < c n := cnpos'
      calc
        u (c n) - c n * l = (u (c n) / c n - l) * c n := by field
        _ ≤ ε * c n := by
          gcongr
          refine (le_abs_self _).trans ?_
          simpa using hn
    obtain ⟨a, ha⟩ :
      ∃ a : ℕ, ∀ b : ℕ, a ≤ b → (c (b + 1) : ℝ) ≤ (1 + ε) * c b ∧ u (c b) - c b * l ≤ ε * c b :=
      eventually_atTop.1 (cgrowth.and L)
    let M := ((Finset.range (a + 1)).image fun i => c i).max' (by simp)
    filter_upwards [Ici_mem_atTop M] with n hn
    have exN : ∃ N, n < c N := by
      rcases (tendsto_atTop.1 ctop (n + 1)).exists with ⟨N, hN⟩
      exact ⟨N, by lia⟩
    let N := Nat.find exN
    have ncN : n < c N := Nat.find_spec exN
    have aN : a + 1 ≤ N := by
      by_contra! h
      have cNM : c N ≤ M := by
        apply le_max'
        apply mem_image_of_mem
        exact mem_range.2 h
      exact lt_irrefl _ ((cNM.trans hn).trans_lt ncN)
    have Npos : 0 < N := lt_of_lt_of_le Nat.succ_pos' aN
    have cNn : c (N - 1) ≤ n := by
      have : N - 1 < N := Nat.pred_lt Npos.ne'
      simpa only [not_lt] using Nat.find_min exN this
    have IcN : (c N : ℝ) ≤ (1 + ε) * c (N - 1) := by
      have A : a ≤ N - 1 := (Nat.le_sub_one_iff_lt Npos).mpr aN
      have B : N - 1 + 1 = N := Nat.succ_pred_eq_of_pos Npos
      have := (ha _ A).1
      rwa [B] at this
    calc
      u n - n * l ≤ u (c N) - c (N - 1) * l := by gcongr; exact hmono ncN.le
      _ = u (c N) - c N * l + (c N - c (N - 1)) * l := by ring
      _ ≤ ε * c N + ε * c (N - 1) * l := by
        gcongr
        · exact (ha N (a.le_succ.trans aN)).2
        · linarith only [IcN]
      _ ≤ ε * ((1 + ε) * c (N - 1)) + ε * c (N - 1) * l := by gcongr
      _ = ε * (1 + ε + l) * c (N - 1) := by ring
      _ ≤ ε * (1 + ε + l) * n := by gcongr
  have B : ∀ ε : ℝ, 0 < ε → ∀ᶠ n : ℕ in atTop, (n : ℝ) * l - u n ≤ ε * (1 + l) * n := by
    intro ε εpos
    rcases hlim (1 + ε) ((lt_add_iff_pos_right _).2 εpos) with ⟨c, cgrowth, ctop, clim⟩
    have L : ∀ᶠ n : ℕ in atTop, (c n : ℝ) * l - u (c n) ≤ ε * c n := by
      rw [← tendsto_sub_nhds_zero_iff, ← Asymptotics.isLittleO_one_iff ℝ,
        Asymptotics.isLittleO_iff] at clim
      filter_upwards [clim εpos, ctop (Ioi_mem_atTop 0)] with n hn cnpos'
      have cnpos : 0 < c n := cnpos'
      calc
        (c n : ℝ) * l - u (c n) = -(u (c n) / c n - l) * c n := by field
        _ ≤ ε * c n := by
          gcongr
          refine le_trans (neg_le_abs _) ?_
          simpa using hn
    obtain ⟨a, ha⟩ :
      ∃ a : ℕ,
        ∀ b : ℕ, a ≤ b → (c (b + 1) : ℝ) ≤ (1 + ε) * c b ∧ (c b : ℝ) * l - u (c b) ≤ ε * c b :=
      eventually_atTop.1 (cgrowth.and L)
    let M := ((Finset.range (a + 1)).image fun i => c i).max' (by simp)
    filter_upwards [Ici_mem_atTop M] with n hn
    have exN : ∃ N, n < c N := by
      rcases (tendsto_atTop.1 ctop (n + 1)).exists with ⟨N, hN⟩
      exact ⟨N, by lia⟩
    let N := Nat.find exN
    have ncN : n < c N := Nat.find_spec exN
    have aN : a + 1 ≤ N := by
      by_contra! h
      have cNM : c N ≤ M := by
        apply le_max'
        apply mem_image_of_mem
        exact mem_range.2 h
      exact lt_irrefl _ ((cNM.trans hn).trans_lt ncN)
    have Npos : 0 < N := lt_of_lt_of_le Nat.succ_pos' aN
    have aN' : a ≤ N - 1 := (Nat.le_sub_one_iff_lt Npos).mpr aN
    have cNn : c (N - 1) ≤ n := by
      have : N - 1 < N := Nat.pred_lt Npos.ne'
      simpa only [not_lt] using Nat.find_min exN this
    calc
      (n : ℝ) * l - u n ≤ c N * l - u (c (N - 1)) := by
        gcongr
        exact hmono cNn
      _ ≤ (1 + ε) * c (N - 1) * l - u (c (N - 1)) := by
        gcongr
        have B : N - 1 + 1 = N := Nat.succ_pred_eq_of_pos Npos
        simpa [B] using (ha _ aN').1
      _ = c (N - 1) * l - u (c (N - 1)) + ε * c (N - 1) * l := by ring
      _ ≤ ε * c (N - 1) + ε * c (N - 1) * l := add_le_add (ha _ aN').2 le_rfl
      _ = ε * (1 + l) * c (N - 1) := by ring
      _ ≤ ε * (1 + l) * n := by gcongr
  refine tendsto_order.2 ⟨fun d hd => ?_, fun d hd => ?_⟩
  · obtain ⟨ε, hε, εpos⟩ : ∃ ε : ℝ, d + ε * (1 + l) < l ∧ 0 < ε := by
      have L : Tendsto (fun ε => d + ε * (1 + l)) (𝓝[>] 0) (𝓝 (d + 0 * (1 + l))) := by
        apply Tendsto.mono_left _ nhdsWithin_le_nhds
        exact tendsto_const_nhds.add (tendsto_id.mul tendsto_const_nhds)
      simp only [zero_mul, add_zero] at L
      exact (((tendsto_order.1 L).2 l hd).and self_mem_nhdsWithin).exists
    filter_upwards [B ε εpos, Ioi_mem_atTop 0] with n hn npos
    simp_rw [div_eq_inv_mul]
    calc
      d < (n : ℝ)⁻¹ * n * (l - ε * (1 + l)) := by
        rw [inv_mul_cancel₀, one_mul]
        · linarith only [hε]
        · exact Nat.cast_ne_zero.2 (ne_of_gt npos)
      _ = (n : ℝ)⁻¹ * (n * l - ε * (1 + l) * n) := by ring
      _ ≤ (n : ℝ)⁻¹ * u n := by gcongr; linarith only [hn]
  · obtain ⟨ε, hε, εpos⟩ : ∃ ε : ℝ, l + ε * (1 + ε + l) < d ∧ 0 < ε := by
      have L : Tendsto (fun ε => l + ε * (1 + ε + l)) (𝓝[>] 0) (𝓝 (l + 0 * (1 + 0 + l))) := by
        apply Tendsto.mono_left _ nhdsWithin_le_nhds
        exact
          tendsto_const_nhds.add
            (tendsto_id.mul ((tendsto_const_nhds.add tendsto_id).add tendsto_const_nhds))
      simp only [zero_mul, add_zero] at L
      exact (((tendsto_order.1 L).2 d hd).and self_mem_nhdsWithin).exists
    filter_upwards [A ε εpos, Ioi_mem_atTop 0] with n hn (npos : 0 < n)
    calc
      u n / n ≤ (n * l + ε * (1 + ε + l) * n) / n := by gcongr; linarith only [hn]
      _ = (l + ε * (1 + ε + l)) := by field
      _ < d := hε

Goal state after: