theorem dickson_one_one_zmod_p (p : ℕ) [Fact p.Prime] : dickson 1 (1 : ZMod p) p = X ^ p := by
  -- Recall that `dickson_one_one_eval_add_inv` characterises `dickson 1 1 p`
  -- as a polynomial that maps `x + x⁻¹` to `x ^ p + (x⁻¹) ^ p`.
  -- Since `X ^ p` also satisfies this property in characteristic `p`,
  -- we can use a variant on `Polynomial.funext` to conclude that these polynomials are equal.
  -- For this argument, we need an arbitrary infinite field of characteristic `p`.
  obtain ⟨K, _, _, H⟩ : ∃ (K : Type) (_ : Field K), ∃ _ : CharP K p, Infinite K := by
    let K := FractionRing (Polynomial (ZMod p))
    let f : ZMod p →+* K := (algebraMap _ (FractionRing _)).comp C
    have : CharP K p := by
      rw [← f.charP_iff_charP]
      infer_instance
    haveI : Infinite K :=
      Infinite.of_injective (algebraMap (Polynomial (ZMod p)) (FractionRing (Polynomial (ZMod p))))
        (IsFractionRing.injective _ _)
    refine' ⟨K, _, _, _⟩ <;> infer_instance
  apply map_injective (ZMod.castHom (dvd_refl p) K) (RingHom.injective _)
  rw [map_dickson, Polynomial.map_pow, map_X]
  apply eq_of_infinite_eval_eq
  -- The two polynomials agree on all `x` of the form `x = y + y⁻¹`.
  apply @Set.Infinite.mono _ { x : K | ∃ y, x = y + y⁻¹ ∧ y ≠ 0 }
  · rintro _ ⟨x, rfl, hx⟩
    simp only [eval_X, eval_pow, Set.mem_setOf_eq, @add_pow_char K _ p,
      dickson_one_one_eval_add_inv _ _ (mul_inv_cancel hx), inv_pow, ZMod.castHom_apply,
      ZMod.cast_one']
  -- Now we need to show that the set of such `x` is infinite.
  -- If the set is finite, then we will show that `K` is also finite.
  · intro h
    rw [← Set.infinite_univ_iff] at H
    apply H
    -- To each `x` of the form `x = y + y⁻¹`
    -- we `bind` the set of `y` that solve the equation `x = y + y⁻¹`.
    -- For every `x`, that set is finite (since it is governed by a quadratic equation).
    -- For the moment, we claim that all these sets together cover `K`.
    suffices (Set.univ : Set K) =
        ⋃ x ∈ { x : K | ∃ y : K, x = y + y⁻¹ ∧ y ≠ 0 }, { y | x = y + y⁻¹ ∨ y = 0 }  by
      rw [this]
      clear this
      refine' h.biUnion fun x _ => _
      -- The following quadratic polynomial has as solutions the `y` for which `x = y + y⁻¹`.
      let φ : K[X] := X ^ 2 - C x * X + 1
      have hφ : φ ≠ 0 := by
        intro H
        have : φ.eval 0 = 0 := by rw [H, eval_zero]
        simpa [eval_X, eval_one, eval_pow, eval_sub, sub_zero, eval_add, eval_mul,
          mul_zero, sq, zero_add, one_ne_zero]
      classical
        convert (φ.roots ∪ {0}).toFinset.finite_toSet using 1
        ext1 y
        simp only [Multiset.mem_toFinset, Set.mem_setOf_eq, Finset.mem_coe, Multiset.mem_union,
          mem_roots hφ, IsRoot, eval_add, eval_sub, eval_pow, eval_mul, eval_X, eval_C, eval_one,
          Multiset.mem_singleton]
        by_cases hy : y = 0
        · simp only [hy, eq_self_iff_true, or_true_iff]
        apply or_congr _ Iff.rfl
        

Proof state:
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
k : ℕ
a : R
p : ℕ
inst✝ : Fact (Nat.Prime p)
K : Type
w✝¹ : Field K
w✝ : CharP K p
H : Set.Infinite Set.univ
h : Set.Finite {x | ∃ y, x = y + y⁻¹ ∧ y ≠ 0}
x : K
x✝ : x ∈ {x | ∃ y, x = y + y⁻¹ ∧ y ≠ 0}
φ : K[X] := X ^ 2 - C x * X + 1
hφ : φ ≠ 0
y : K
hy : ¬y = 0
⊢ x = y + y⁻¹ ↔ y ^ 2 - x * y + 1 = 0

Next tactic:
rw [← mul_left_inj' hy, eq_comm, ← sub_eq_zero, add_mul, inv_mul_cancel hy]