Theorem: tendsto_div_of_monotone_of_exists_subseq_tendsto_div
File: Mathlib/Analysis/SpecificLimits/FloorPow.lean

Proof steps:
state_before: 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)
state_after: 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)
lnonneg : 0 ≤ l
⊢ Tendsto (fun n => u n / ↑n) atTop (𝓝 l)
tactic: 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 _)
state_before: 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)
lnonneg : 0 ≤ l
A : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n
⊢ Tendsto (fun n => u n / ↑n) atTop (𝓝 l)
state_after: 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)
lnonneg : 0 ≤ l
A : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n
B : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, ↑n * l - u n ≤ ε * (1 + l) * ↑n
⊢ Tendsto (fun n => u n / ↑n) atTop (𝓝 l)
tactic: 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
          simp only [cnpos.ne', Ne, Nat.cast_eq_zero, not_false_iff, neg_sub, field_simps]
        _ ≤ 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 linarith only [hN]⟩
    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 := by
      apply @Nat.le_of_add_le_add_right a 1 (N - 1)
      rw [Nat.sub_add_cancel Npos]
      exact 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
state_before: 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)
lnonneg : 0 ≤ l
A : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n
B : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, ↑n * l - u n ≤ ε * (1 + l) * ↑n
⊢ Tendsto (fun n => u n / ↑n) atTop (𝓝 l)
state_after: case refine_1
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)
lnonneg : 0 ≤ l
A : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n
B : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, ↑n * l - u n ≤ ε * (1 + l) * ↑n
d : ℝ
hd : d < l
⊢ ∀ᶠ (b : ℕ) in atTop, d < u b / ↑b

case refine_2
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)
lnonneg : 0 ≤ l
A : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n
B : ∀ (ε : ℝ), 0 < ε → ∀ᶠ (n : ℕ) in atTop, ↑n * l - u n ≤ ε * (1 + l) * ↑n
d : ℝ
hd : d > l
⊢ ∀ᶠ (b : ℕ) in atTop, u b / ↑b < d
tactic: refine tendsto_order.2 ⟨fun d hd => ?_, fun d hd => ?_⟩