-- Mathlib/Analysis/SpecificLimits/FloorPow.lean
theorem tendsto_div_of_monotone_of_exists_subseq_tendsto_div (u : ℕ → ℝ) (l : ℝ)
    (hmono : Monotone u)
    (hlim : ∀ a : ℝ, 1 < a → ∃ c : ℕ → ℕ, (∀ᶠ n in atTop, (c (n + 1) : ℝ) ≤ 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)

Proof state:
case intro.intro.intro
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)
c : ℕ → ℕ
left✝ : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ 2 * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
this : Tendsto (fun n => u 0 / ↑(c n)) atTop (𝓝 0)
⊢ 0 ≤ l

Tactic:
apply le_of_tendsto_of_tendsto' this clim fun n => ?_

Result:
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)
c : ℕ → ℕ
left✝ : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ 2 * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
this : Tendsto (fun n => u 0 / ↑(c n)) atTop (𝓝 0)
n : ℕ
⊢ u 0 / ↑(c n) ≤ u (c n) / ↑(c n)

Proof state:
case h
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
exN : ∃ N, n < c N
⊢ u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n

Tactic:
let N := Nat.find exN

Result:
case h
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
exN : ∃ N, n < c N
N : ℕ := Nat.find exN
⊢ u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n

Proof state:
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
⊢ ∀ᶠ (n : ℕ) in atTop, ↑(c n) * l - u (c n) ≤ ε * ↑(c n)

Tactic:
rw [← tendsto_sub_nhds_zero_iff, ← Asymptotics.isLittleO_one_iff ℝ,
  Asymptotics.isLittleO_iff] at clim

Result:
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : ∀ ⦃c_1 : ℝ⦄, 0 < c_1 → ∀ᶠ (x : ℕ) in atTop, ‖u (c x) / ↑(c x) - l‖ ≤ c_1 * ‖1‖
⊢ ∀ᶠ (n : ℕ) in atTop, ↑(c n) * l - u (c n) ≤ ε * ↑(c n)

Proof state:
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
N : ℕ
hN : n + 1 ≤ c N
⊢ n < c N

Tactic:
omega

Result:
no goals

Proof state:
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
exN : ∃ N, n < c N
N : ℕ := Nat.find exN
ncN : n < c N
aN : a + 1 ≤ N
Npos : 0 < N
cNn : c (N - 1) ≤ n
IcN : ↑(c N) ≤ (1 + ε) * ↑(c (N - 1))
⊢ u (c N) - ↑(c (N - 1)) * l = u (c N) - ↑(c N) * l + (↑(c N) - ↑(c (N - 1))) * l

Tactic:
ring

Result:
no goals

Proof state:
case h
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
exN : ∃ N, n < c N
N : ℕ := Nat.find exN
ncN : n < c N
aN : a + 1 ≤ N
Npos : 0 < N
cNn : c (N - 1) ≤ n
⊢ u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n

Tactic:
have IcN : (c N : ℝ) ≤ (1 + ε) * c (N - 1) := by
  have A : 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 B : N - 1 + 1 = N := Nat.succ_pred_eq_of_pos Npos
  have := (ha _ A).1
  rwa [B] at this

Result:
case h
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
ε : ℝ
εpos : 0 < ε
c : ℕ → ℕ
cgrowth : ∀ᶠ (n : ℕ) in atTop, ↑(c (n + 1)) ≤ (1 + ε) * ↑(c n)
ctop : Tendsto c atTop atTop
clim : Tendsto (fun n => u (c n) / ↑(c n)) atTop (𝓝 l)
L : ∀ᶠ (n : ℕ) in atTop, u (c n) - ↑(c n) * l ≤ ε * ↑(c n)
a : ℕ
ha : ∀ (b : ℕ), a ≤ b → ↑(c (b + 1)) ≤ (1 + ε) * ↑(c b) ∧ u (c b) - ↑(c b) * l ≤ ε * ↑(c b)
M : ℕ := (image (fun i => c i) (range (a + 1))).max' ⋯
n : ℕ
hn : n ∈ Set.Ici M
exN : ∃ N, n < c N
N : ℕ := Nat.find exN
ncN : n < c N
aN : a + 1 ≤ N
Npos : 0 < N
cNn : c (N - 1) ≤ n
IcN : ↑(c N) ≤ (1 + ε) * ↑(c (N - 1))
⊢ u n - ↑n * l ≤ ε * (1 + ε + l) * ↑n