/-- A slightly stronger version of `exists_partition` on which we perform induction on `n`:
for all `ε > 0`, we can partition the remainders of any family of polynomials `A`
into equivalence classes, where the equivalence(!) relation is "closer than `ε`". -/
theorem exists_partition_polynomial_aux (n : ℕ) {ε : ℝ} (hε : 0 < ε) {b : Fq[X]} (hb : b ≠ 0)
    (A : Fin n → Fq[X]) : ∃ t : Fin n → Fin (Fintype.card Fq ^ ⌈-log ε / log (Fintype.card Fq)⌉₊),
      ∀ i₀ i₁ : Fin n, t i₀ = t i₁ ↔
        (cardPowDegree (A i₁ % b - A i₀ % b) : ℝ) < cardPowDegree b • ε := by
  have hbε : 0 < cardPowDegree b • ε := by
    rw [Algebra.smul_def, eq_intCast]
    exact mul_pos (Int.cast_pos.mpr (AbsoluteValue.pos _ hb)) hε
  -- We go by induction on the size `A`.
  induction' n with n ih
  · refine' ⟨finZeroElim, finZeroElim⟩
  -- Show `anti_archimedean` also holds for real distances.
  have anti_archim' : ∀ {i j k} {ε : ℝ},
    (cardPowDegree (A i % b - A j % b) : ℝ) < ε →
      (cardPowDegree (A j % b - A k % b) : ℝ) < ε →
        (cardPowDegree (A i % b - A k % b) : ℝ) < ε := by
    intro i j k ε
    simp_rw [← Int.lt_ceil]
    exact cardPowDegree_anti_archimedean
  obtain ⟨t', ht'⟩ := ih (Fin.tail A)
  -- We got rid of `A 0`, so determine the index `j` of the partition we'll re-add it to.
  rsuffices ⟨j, hj⟩ :
    ∃ j, ∀ i, t' i = j ↔ (cardPowDegree (A 0 % b - A i.succ % b) : ℝ) < cardPowDegree b • ε
  · refine' ⟨Fin.cons j t', fun i₀ i₁ => _⟩
    refine' Fin.cases _ (fun i₀ => _) i₀ <;> refine' Fin.cases _ (fun i₁ => _) i₁
    · simpa using hbε
    · rw [Fin.cons_succ, Fin.cons_zero, eq_comm, AbsoluteValue.map_sub]
      exact hj i₁
    · rw [Fin.cons_succ, Fin.cons_zero]
      exact hj i₀
    · rw [Fin.cons_succ, Fin.cons_succ]
      exact ht' i₀ i₁
  -- `exists_approx_polynomial` guarantees that we can insert `A 0` into some partition `j`,
  -- but not that `j` is uniquely defined (which is needed to keep the induction going).
  obtain ⟨j, hj⟩ : ∃ j, ∀ i : Fin n,
      t' i = j → (cardPowDegree (A 0 % b - A i.succ % b) : ℝ) < cardPowDegree b • ε := by
    by_contra! hg
    obtain ⟨j₀, j₁, j_ne, approx⟩ := exists_approx_polynomial hb hε
      (Fin.cons (A 0) fun j => A (Fin.succ (Classical.choose (hg j))))
    revert j_ne approx
    refine' Fin.cases _ (fun j₀ => _) j₀ <;>
      refine' Fin.cases (fun j_ne approx => _) (fun j₁ j_ne approx => _) j₁
    · exact absurd rfl j_ne
    · rw [Fin.cons_succ, Fin.cons_zero, ← not_le, AbsoluteValue.map_sub] at approx
      have := (Classical.choose_spec (hg j₁)).2
      contradiction
    · rw [Fin.cons_succ, Fin.cons_zero, ← not_le] at approx
      have := (Classical.choose_spec (hg j₀)).2
      contradiction
    · rw [Fin.cons_succ, Fin.cons_succ] at approx
      rw [Ne.def, Fin.succ_inj] at j_ne
      have : j₀ = j₁ := (Classical.choose_spec (hg j₀)).1.symm.trans
        (((ht' (Classical.choose (hg j₀)) (Classical.choose (hg j₁))).mpr approx).trans
          (Classical.choose_spec (hg j₁)).1)
      contradiction
  -- However, if one of those partitions `j` is inhabited by some `i`, then this `j` works.
  by_cases exists_nonempty_j : ∃ j, (∃ i, t' i = j) ∧
      ∀ i, t' i = j → (cardPowDegree (A 0 % b - A i.succ % b) : ℝ) < cardPowDegree b • ε
  · obtain ⟨j, ⟨i, hi⟩, hj⟩ := exists_nonempty_j
    refine' ⟨j, fun i' => ⟨hj i', fun hi' => _root_.trans ((ht' _ _).mpr _) hi⟩⟩
    apply anti_archim' _ hi'
    

Proof state:
Fq : Type u_1
inst✝¹ : Fintype Fq
inst✝ : Field Fq
ε : ℝ
hε : 0 < ε
b : Fq[X]
hb : b ≠ 0
hbε : 0 < cardPowDegree b • ε
n : ℕ
ih :
  ∀ (A : Fin n → Fq[X]),
    ∃ t, ∀ (i₀ i₁ : Fin n), t i₀ = t i₁ ↔ ↑(cardPowDegree (A i₁ % b - A i₀ % b)) < cardPowDegree b • ε
A : Fin (Nat.succ n) → Fq[X]
anti_archim' :
  ∀ {i j k : Fin (Nat.succ n)} {ε : ℝ},
    ↑(cardPowDegree (A i % b - A j % b)) < ε →
      ↑(cardPowDegree (A j % b - A k % b)) < ε → ↑(cardPowDegree (A i % b - A k % b)) < ε
t' : Fin n → Fin (Fintype.card Fq ^ ⌈-log ε / log ↑(Fintype.card Fq)⌉₊)
ht' : ∀ (i₀ i₁ : Fin n), t' i₀ = t' i₁ ↔ ↑(cardPowDegree (Fin.tail A i₁ % b - Fin.tail A i₀ % b)) < cardPowDegree b • ε
j✝ : Fin (Fintype.card Fq ^ ⌈-log ε / log ↑(Fintype.card Fq)⌉₊)
hj✝ : ∀ (i : Fin n), t' i = j✝ → ↑(cardPowDegree (A 0 % b - A (Fin.succ i) % b)) < cardPowDegree b • ε
j : Fin (Fintype.card Fq ^ ⌈-log ε / log ↑(Fintype.card Fq)⌉₊)
hj : ∀ (i : Fin n), t' i = j → ↑(cardPowDegree (A 0 % b - A (Fin.succ i) % b)) < cardPowDegree b • ε
i : Fin n
hi : t' i = j
i' : Fin n
hi' : ↑(cardPowDegree (A 0 % b - A (Fin.succ i') % b)) < cardPowDegree b • ε
⊢ ↑(cardPowDegree (A (Fin.succ i) % b - A 0 % b)) < cardPowDegree b • ε

Next tactic:
rw [AbsoluteValue.map_sub]