-- Mathlib/Combinatorics/Additive/AP/Three/Behrend.lean
theorem roth_lower_bound_explicit (hN : 4096 ≤ N) :
    (N : ℝ) * exp (-4 * √(log N)) < rothNumberNat N

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
⊢ ↑N * rexp (-4 * √(log ↑N)) < ↑(rothNumberNat N)

Tactic:
have hd : 0 < dValue N := dValue_pos (hN.trans' <| by norm_num1)

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
⊢ ↑N * rexp (-4 * √(log ↑N)) < ↑(rothNumberNat N)

Proof state:
case refine_1
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this✝ : (2 * dValue N - 1) ^ n ≤ N
this : rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))
⊢ 1 < N

Tactic:
exact hN.trans_lt' (by norm_num1)

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
⊢ 8 ≤ 4096

Tactic:
norm_num1

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
⊢ ↑N * rexp (-4 * √(log ↑N)) < ↑(rothNumberNat N)

Tactic:
have hn : 0 < (n : ℝ) := cast_pos.2 (nValue_pos <| hN.trans' <| by norm_num1)

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
⊢ ↑N * rexp (-4 * √(log ↑N)) < ↑(rothNumberNat N)

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ ↑N * rexp (-4 * √(log ↑N)) < ↑(rothNumberNat N)

Tactic:
calc
  _ ≤ (N ^ (nValue N : ℝ)⁻¹ / rexp 1 : ℝ) ^ (n - 2) / n := ?_
  _ < _ := by gcongr; exacts [(tsub_pos_of_lt hn₂).ne', bound hN]
  _ ≤ rothNumberNat ((2 * dValue N - 1) ^ n) := bound_aux hd.ne' hn₂.le
  _ ≤ rothNumberNat N := mod_cast rothNumberNat.mono this

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ ↑N * rexp (-4 * √(log ↑N)) ≤ (↑N ^ (↑(nValue N))⁻¹ / rexp 1) ^ (n - 2) / ↑n

Proof state:
case refine_2
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this✝ : (2 * dValue N - 1) ^ n ≤ N
this : rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))
⊢ 1 < N

Tactic:
exact hN.trans_lt' (by norm_num1)

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))

Tactic:
rw [← exp_add, ← add_mul]

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ rexp (-4 * √(log ↑N)) = rexp ((-2 + -2) * √(log ↑N))

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ rexp (-4 * √(log ↑N)) = rexp ((-2 + -2) * √(log ↑N))

Tactic:
norm_num

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
⊢ 2 ≤ 4096

Tactic:
norm_num1

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this✝ : (2 * dValue N - 1) ^ n ≤ N
this : rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))
⊢ 1 < 4096

Tactic:
norm_num1

Result:
no goals

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ ↑N * rexp (-4 * √(log ↑N)) ≤ (↑N ^ (↑(nValue N))⁻¹ / rexp 1) ^ (n - 2) / ↑n

Tactic:
rw [← rpow_natCast, div_rpow (rpow_nonneg hN₀.le _) (exp_pos _).le, ← rpow_mul hN₀.le,
  inv_mul_eq_div, cast_sub hn₂.le, cast_two, same_sub_div hn.ne', exp_one_rpow,
  div_div, rpow_sub hN₀, rpow_one, div_div, div_eq_mul_inv]

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ ↑N * rexp (-4 * √(log ↑N)) ≤ ↑N * (↑N ^ (2 / ↑n) * (rexp (↑n - 2) * ↑n))⁻¹

Proof state:
case refine_2
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this✝ : (2 * dValue N - 1) ^ n ≤ N
this : rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))
⊢ 1 < ↑N

Tactic:
rw [one_lt_cast]

Result:
case refine_2
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this✝ : (2 * dValue N - 1) ^ n ≤ N
this : rexp (-4 * √(log ↑N)) = rexp (-2 * √(log ↑N)) * rexp (-2 * √(log ↑N))
⊢ 1 < N

Proof state:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ rexp (-4 * √(log ↑N)) ≤ (↑N ^ (2 / ↑n) * (rexp (↑n - 2) * ↑n))⁻¹

Tactic:
rw [mul_inv, mul_inv, ← exp_neg, ← rpow_neg (cast_nonneg _), neg_sub, ← div_eq_mul_inv]

Result:
N : ℕ
hN : 4096 ≤ N
n : ℕ := nValue N
hn : 0 < ↑n
hd : 0 < dValue N
hN₀ : 0 < ↑N
hn₂ : 2 < n
this : (2 * dValue N - 1) ^ n ≤ N
⊢ rexp (-4 * √(log ↑N)) ≤ ↑N ^ (-(2 / ↑n)) * (rexp (2 - ↑n) / ↑n)