theorem eq_pow_of_pell {m n k} :
    n ^ k = m ↔ k = 0 ∧ m = 1 ∨0 < k ∧ (n = 0 ∧ m = 0 ∨
      0 < n ∧ ∃ (w a t z : ℕ) (a1 : 1 < a), xn a1 k ≡ yn a1 k * (a - n) + m [MOD t] ∧
      2 * a * n = t + (n * n + 1) ∧ m < t ∧
      n ≤ w ∧ k ≤ w ∧ a * a - ((w + 1) * (w + 1) - 1) * (w * z) * (w * z) = 1) := by
  constructor
  · rintro rfl
    refine' k.eq_zero_or_pos.imp (fun k0 : k = 0 => k0.symm ▸ ⟨rfl, rfl⟩) fun hk => ⟨hk, _⟩
    refine' n.eq_zero_or_pos.imp (fun n0 : n = 0 => n0.symm ▸ ⟨rfl, zero_pow hk⟩) fun hn => ⟨hn, _⟩
    set w := max n k
    have nw : n ≤ w := le_max_left _ _
    have kw : k ≤ w := le_max_right _ _
    have wpos : 0 < w := hn.trans_le nw
    have w1 : 1 < w + 1 := Nat.succ_lt_succ wpos
    set a := xn w1 w
    have a1 : 1 < a := strictMono_x w1 wpos
    have na : n ≤ a := nw.trans (n_lt_xn w1 w).le
    set x := xn a1 k
    set y := yn a1 k
    obtain ⟨z, ze⟩ : w ∣ yn w1 w
    exact modEq_zero_iff_dvd.1 ((yn_modEq_a_sub_one w1 w).trans dvd_rfl.modEq_zero_nat)
    have nt : (↑(n ^ k) : ℤ) < 2 * a * n - n * n - 1 := by
      refine' eq_pow_of_pell_lem hn.ne' hk.ne' _
      calc
        n ^ k ≤ n ^ w := Nat.pow_le_pow_of_le_right hn kw
        _ < (w + 1) ^ w := (Nat.pow_lt_pow_left (Nat.lt_succ_of_le nw) wpos.ne')
        _ ≤ a := xn_ge_a_pow w1 w
    lift (2 * a * n - n * n - 1 : ℤ) to ℕ using (Nat.cast_nonneg _).trans nt.le with t te
    have tm : x ≡ y * (a - n) + n ^ k [MOD t] := by
      apply modEq_of_dvd
      rw [Int.ofNat_add, Int.ofNat_mul, Int.ofNat_sub na, te]
      exact x_sub_y_dvd_pow a1 n k
    have ta : 2 * a * n = t + (n * n + 1) := by
      zify
      rw [te]
      ring_nf
    have zp : a * a - ((w + 1) * (w + 1) - 1) * (w * z) * (w * z) = 1 := ze ▸ pell_eq w1 w
    exact ⟨w, a, t, z, a1, tm, ta, Nat.cast_lt.1 nt, nw, kw, zp⟩
  · rintro (⟨rfl, rfl⟩ | ⟨hk0, ⟨rfl, rfl⟩ | ⟨hn0, w, a, t, z, a1, tm, ta, mt, nw, kw, zp⟩⟩)
    · exact _root_.pow_zero n
    · exact zero_pow hk0
    have hw0 : 0 < w := hn0.trans_le nw
    have hw1 : 1 < w + 1 := Nat.succ_lt_succ hw0
    rcases eq_pell hw1 zp with ⟨j, rfl, yj⟩
    have hj0 : 0 < j := by
      apply Nat.pos_of_ne_zero
      rintro rfl
      exact lt_irrefl 1 a1
    have wj : w ≤ j :=
      Nat.le_of_dvd hj0
        (modEq_zero_iff_dvd.1 <|
          (yn_modEq_a_sub_one hw1 j).symm.trans <| modEq_zero_iff_dvd.2 ⟨z, yj.symm⟩)
    have hnka : n ^ k < xn hw1 j
    calc
      n ^ k ≤ n ^ j := Nat.pow_le_pow_of_le_right hn0 (le_trans kw wj)
      _ < (w + 1) ^ j := (Nat.pow_lt_pow_left (Nat.lt_succ_of_le nw) hj0.ne')
      _ ≤ xn hw1 j := xn_ge_a_pow hw1 j
    have nt : (↑(n ^ k) : ℤ) < 2 * xn hw1 j * n - n * n - 1 :=
      eq_pow_of_pell_lem hn0.ne' hk0.ne' hnka
    have na : n ≤ xn hw1 j := (Nat.le_self_pow hk0.ne' _).trans hnka.le
    have te : (t : ℤ) = 2 * xn hw1 j * n - n * n - 1 := by
      rw [sub_sub, eq_sub_iff_add_eq]
      exact mod_cast ta.symm
    have : xn a1 k ≡ yn a1 k * (xn hw1 j - n) + n ^ k [MOD t] := by
      apply modEq_of_dvd
      rw [te, Nat.cast_add, Nat.cast_mul, Int.ofNat_sub na]
      

Proof state:
case a
m n k : ℕ
hk0 : 0 < k
hn0 : 0 < n
w t z : ℕ
mt : m < t
nw : n ≤ w
kw : k ≤ w
hw0 : 0 < w
hw1 : 1 < w + 1
j : ℕ
yj : w * z = yn hw1 j
a1 : 1 < xn hw1 j
tm : xn a1 k ≡ yn a1 k * (xn hw1 j - n) + m [MOD t]
ta : 2 * xn hw1 j * n = t + (n * n + 1)
zp : xn hw1 j * xn hw1 j - ((w + 1) * (w + 1) - 1) * (w * z) * (w * z) = 1
hj0 : 0 < j
wj : w ≤ j
hnka : n ^ k < xn hw1 j
nt : ↑(n ^ k) < 2 * ↑(xn hw1 j) * ↑n - ↑n * ↑n - 1
na : n ≤ xn hw1 j
te : ↑t = 2 * ↑(xn hw1 j) * ↑n - ↑n * ↑n - 1
⊢ 2 * ↑(xn hw1 j) * ↑n - ↑n * ↑n - 1 ∣ ↑(yn a1 k) * (↑(xn hw1 j) - ↑n) + ↑(n ^ k) - ↑(xn a1 k)

Next tactic:
exact x_sub_y_dvd_pow a1 n k