-- Mathlib/Analysis/SpecialFunctions/Log/Base.lean
/-- Induction principle for intervals of real numbers: if a proposition `P` is true
on `[x₀, r * x₀)` and if `P` for `[x₀, r^n * x₀)` implies `P` for `[r^n * x₀, r^(n+1) * x₀)`,
then `P` is true for all `x ≥ x₀`. -/
lemma Real.induction_Ico_mul {P : ℝ → Prop} (x₀ r : ℝ) (hr : 1 < r) (hx₀ : 0 < x₀)
    (base : ∀ x ∈ Set.Ico x₀ (r * x₀), P x)
    (step : ∀ n : ℕ, n ≥ 1 → (∀ z ∈ Set.Ico x₀ (r ^ n * x₀), P z) →
      (∀ z ∈ Set.Ico (r ^ n * x₀) (r ^ (n+1) * x₀), P z)) :
    ∀ x ≥ x₀, P x

Proof state:
case succ
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
n : ℕ
ih : ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
⊢ ∀ x ∈ Ico x₀ (r ^ (n + 1 + 1) * x₀), P x

Tactic:
exact fun x hx => (Ico_subset_Ico_union_Ico hx).elim (ih x) (step (n + 1) (by simp) ih _)

Result:
no goals

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ P x

Tactic:
refine this ⌊logb r (x / x₀)⌋₊ x ?_

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ x ∈ Ico x₀ (r ^ (⌊logb r (x / x₀)⌋₊ + 1) * x₀)

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ x₀ ≤ x ∧ logb r (x / x₀) < ↑⌊logb r (x / x₀)⌋₊ + 1

Tactic:
exact ⟨hx, Nat.lt_floor_add_one _⟩

Result:
no goals

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
⊢ ∀ x ≥ x₀, P x

Tactic:
suffices ∀ n : ℕ, ∀ x ∈ Set.Ico x₀ (r ^ (n + 1) * x₀), P x by
  intro x hx
  have hx' : 0 < x / x₀ := div_pos (hx₀.trans_le hx) hx₀
  refine this ⌊logb r (x / x₀)⌋₊ x ?_
  rw [mem_Ico, ← div_lt_iff₀ hx₀, ← rpow_natCast, ← logb_lt_iff_lt_rpow hr hx', Nat.cast_add,
    Nat.cast_one]
  exact ⟨hx, Nat.lt_floor_add_one _⟩

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
⊢ ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
⊢ ∀ x ≥ x₀, P x

Tactic:
intro x hx

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
⊢ P x

Proof state:
case zero
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
⊢ ∀ x ∈ Ico x₀ (r ^ (0 + 1) * x₀), P x

Tactic:
simpa using base

Result:
no goals

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ x ∈ Ico x₀ (r ^ (⌊logb r (x / x₀)⌋₊ + 1) * x₀)

Tactic:
rw [mem_Ico, ← div_lt_iff₀ hx₀, ← rpow_natCast, ← logb_lt_iff_lt_rpow hr hx', Nat.cast_add,
  Nat.cast_one]

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ x₀ ≤ x ∧ logb r (x / x₀) < ↑⌊logb r (x / x₀)⌋₊ + 1

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
⊢ P x

Tactic:
have hx' : 0 < x / x₀ := div_pos (hx₀.trans_le hx) hx₀

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
this : ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ≥ x₀
hx' : 0 < x / x₀
⊢ P x

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
n : ℕ
ih : ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x
x : ℝ
hx : x ∈ Ico x₀ (r ^ (n + 1 + 1) * x₀)
⊢ n + 1 ≥ 1

Tactic:
simp

Result:
no goals

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
n : ℕ
⊢ ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x

Tactic:
induction n with
| zero => simpa using base
| succ n ih =>
  exact fun x hx => (Ico_subset_Ico_union_Ico hx).elim (ih x) (step (n + 1) (by simp) ih _)

Result:
no goals

Proof state:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
⊢ ∀ (n : ℕ), ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x

Tactic:
intro n

Result:
P : ℝ → Prop
x₀ r : ℝ
hr : 1 < r
hx₀ : 0 < x₀
base : ∀ x ∈ Ico x₀ (r * x₀), P x
step : ∀ n ≥ 1, (∀ z ∈ Ico x₀ (r ^ n * x₀), P z) → ∀ z ∈ Ico (r ^ n * x₀) (r ^ (n + 1) * x₀), P z
n : ℕ
⊢ ∀ x ∈ Ico x₀ (r ^ (n + 1) * x₀), P x