/--
This is a proof that the number produced using `psp_from_prime` is actually pseudoprime to base `b`.
The primary purpose of this lemma is to help prove `exists_infinite_pseudoprimes`.

We use <https://primes.utm.edu/notes/proofs/a_pseudoprimes.html> as a rough outline of the proof.
-/
private theorem psp_from_prime_psp {b : ℕ} (b_ge_two : 2 ≤ b) {p : ℕ} (p_prime : p.Prime)
    (p_gt_two : 2 < p) (not_dvd : ¬p ∣ b * (b ^ 2 - 1)) : FermatPsp (psp_from_prime b p) b := by
  unfold psp_from_prime
  set A := (b ^ p - 1) / (b - 1)
  set B := (b ^ p + 1) / (b + 1)
  -- Inequalities
  have hi_A : 1 < A := a_id_helper (Nat.succ_le_iff.mp b_ge_two) (Nat.Prime.one_lt p_prime)
  have hi_B : 1 < B := b_id_helper (Nat.succ_le_iff.mp b_ge_two) p_gt_two
  have hi_AB : 1 < A * B := one_lt_mul'' hi_A hi_B
  have hi_b : 0 < b := by linarith
  have hi_p : 1 ≤ p := Nat.one_le_of_lt p_gt_two
  have hi_bsquared : 0 < b ^ 2 - 1 := by
    -- Porting note: was `by nlinarith [Nat.one_le_pow 2 b hi_b]`
    have h0 := mul_le_mul b_ge_two b_ge_two zero_le_two hi_b.le
    have h1 : 1 < 2 * 2 := by linarith
    have := tsub_pos_of_lt (h1.trans_le h0)
    rwa [pow_two]
  have hi_bpowtwop : 1 ≤ b ^ (2 * p) := Nat.one_le_pow (2 * p) b hi_b
  have hi_bpowpsubone : 1 ≤ b ^ (p - 1) := Nat.one_le_pow (p - 1) b hi_b
  -- Other useful facts
  have p_odd : Odd p := p_prime.odd_of_ne_two p_gt_two.ne.symm
  have AB_not_prime : ¬Nat.Prime (A * B) := Nat.not_prime_mul hi_A hi_B
  have AB_id : A * B = (b ^ (2 * p) - 1) / (b ^ 2 - 1) := AB_id_helper _ _ b_ge_two p_odd
  have hd : b ^ 2 - 1 ∣ b ^ (2 * p) - 1 := by
    simpa only [one_pow, pow_mul] using nat_sub_dvd_pow_sub_pow _ 1 p
  -- We know that `A * B` is not prime, and that `1 < A * B`. Since two conditions of being
  -- pseudoprime are satisfied, we only need to show that `A * B` is probable prime to base `b`
  refine' ⟨_, AB_not_prime, hi_AB⟩
  -- Used to prove that `2 * p * (b ^ 2 - 1) ∣ (b ^ 2 - 1) * (A * B - 1)`.
  have ha₁ : (b ^ 2 - 1) * (A * B - 1) = b * (b ^ (p - 1) - 1) * (b ^ p + b) := by
    apply_fun fun x => x * (b ^ 2 - 1) at AB_id
    rw [Nat.div_mul_cancel hd] at AB_id
    apply_fun fun x => x - (b ^ 2 - 1) at AB_id
    nth_rw 2 [← one_mul (b ^ 2 - 1)] at AB_id
    rw [← Nat.mul_sub_right_distrib, mul_comm] at AB_id
    rw [AB_id]
    exact bp_helper hi_b hi_p
  -- If `b` is even, then `b^p` is also even, so `2 ∣ b^p + b`
  -- If `b` is odd, then `b^p` is also odd, so `2 ∣ b^p + b`
  have ha₂ : 2 ∣ b ^ p + b := by
    -- Porting note: golfed
    rw [← even_iff_two_dvd, Nat.even_add, Nat.even_pow' p_prime.ne_zero]
  -- Since `b` isn't divisible by `p`, `b` is coprime with `p`. we can use Fermat's Little Theorem
  -- to prove this.
  have ha₃ : p ∣ b ^ (p - 1) - 1 := by
    have : ¬p ∣ b := mt (fun h : p ∣ b => dvd_mul_of_dvd_left h _) not_dvd
    have : p.Coprime b := Or.resolve_right (Nat.coprime_or_dvd_of_prime p_prime b) this
    have : IsCoprime (b : ℤ) ↑p := this.symm.isCoprime
    have : ↑b ^ (p - 1) ≡ 1 [ZMOD ↑p] := Int.ModEq.pow_card_sub_one_eq_one p_prime this
    have : ↑p ∣ ↑b ^ (p - 1) - ↑1 := mod_cast Int.ModEq.dvd (Int.ModEq.symm this)
    exact mod_cast this
  -- Because `p - 1` is even, there is a `c` such that `2 * c = p - 1`. `nat_sub_dvd_pow_sub_pow`
  -- implies that `b ^ c - 1 ∣ (b ^ c) ^ 2 - 1`, and `(b ^ c) ^ 2 = b ^ (p - 1)`.
  have ha₄ : b ^ 2 - 1 ∣ b ^ (p - 1) - 1 := by
    cases' p_odd with k hk
    have : 2 ∣ p - 1 := ⟨k, by simp [hk]⟩
    cases' this with c hc
    have : b ^ 2 - 1 ∣ (b ^ 2) ^ c - 1 := by
      simpa only [one_pow] using nat_sub_dvd_pow_sub_pow _ 1 c
    

Proof state:
case intro.intro
b : ℕ
b_ge_two : 2 ≤ b
p : ℕ
p_prime : Prime p
p_gt_two : 2 < p
not_dvd : ¬p ∣ b * (b ^ 2 - 1)
A : ℕ := (b ^ p - 1) / (b - 1)
B : ℕ := (b ^ p + 1) / (b + 1)
hi_A : 1 < A
hi_B : 1 < B
hi_AB : 1 < A * B
hi_b : 0 < b
hi_p : 1 ≤ p
hi_bsquared : 0 < b ^ 2 - 1
hi_bpowtwop : 1 ≤ b ^ (2 * p)
hi_bpowpsubone : 1 ≤ b ^ (p - 1)
AB_not_prime : ¬Prime (A * B)
AB_id : A * B = (b ^ (2 * p) - 1) / (b ^ 2 - 1)
hd : b ^ 2 - 1 ∣ b ^ (2 * p) - 1
ha₁ : (b ^ 2 - 1) * (A * B - 1) = b * (b ^ (p - 1) - 1) * (b ^ p + b)
ha₂ : 2 ∣ b ^ p + b
ha₃ : p ∣ b ^ (p - 1) - 1
k : ℕ
hk : p = 2 * k + 1
c : ℕ
hc : p - 1 = 2 * c
this : b ^ 2 - 1 ∣ (b ^ 2) ^ c - 1
⊢ b ^ 2 - 1 ∣ b ^ (p - 1) - 1

Next tactic:
have : b ^ 2 - 1 ∣ b ^ (2 * c) - 1 := by rwa [← pow_mul] at this