/-- *Dirichlet's approximation theorem:*
For any real number `ξ` and positive natural `n`, there are integers `j` and `k`,
with `0 < k ≤ n` and `|k*ξ - j| ≤ 1/(n+1)`.

See also `Real.exists_nat_abs_mul_sub_round_le`. -/
theorem exists_int_int_abs_mul_sub_le (ξ : ℝ) {n : ℕ} (n_pos : 0 < n) :
    ∃ j k : ℤ, 0 < k ∧ k ≤ n ∧ |↑k * ξ - j| ≤ 1 / (n + 1) := by
  let f : ℤ → ℤ := fun m => ⌊fract (ξ * m) * (n + 1)⌋
  have hn : 0 < (n : ℝ) + 1 := mod_cast Nat.succ_pos _
  have hfu := fun m : ℤ => mul_lt_of_lt_one_left hn <| fract_lt_one (ξ * ↑m)
  conv in |_| ≤ _ => rw [mul_comm, le_div_iff hn, ← abs_of_pos hn, ← abs_mul]
  let D := Icc (0 : ℤ) n
  by_cases H : ∃ m ∈ D, f m = n
  · obtain ⟨m, hm, hf⟩ := H
    have hf' : ((n : ℤ) : ℝ) ≤ fract (ξ * m) * (n + 1) := hf ▸ floor_le (fract (ξ * m) * (n + 1))
    have hm₀ : 0 < m := by
      have hf₀ : f 0 = 0 := by
        -- Porting note: was
        -- simp only [floor_eq_zero_iff, algebraMap.coe_zero, mul_zero, fract_zero,
        --   zero_mul, Set.left_mem_Ico, zero_lt_one]
        simp only [cast_zero, mul_zero, fract_zero, zero_mul, floor_zero]
      refine' Ne.lt_of_le (fun h => n_pos.ne _) (mem_Icc.mp hm).1
      exact mod_cast hf₀.symm.trans (h.symm ▸ hf : f 0 = n)
    refine' ⟨⌊ξ * m⌋ + 1, m, hm₀, (mem_Icc.mp hm).2, _⟩
    rw [cast_add, ← sub_sub, sub_mul, cast_one, one_mul, abs_le]
    refine'
      ⟨le_sub_iff_add_le.mpr _, sub_le_iff_le_add.mpr <| le_of_lt <| (hfu m).trans <| lt_one_add _⟩
    simpa only [neg_add_cancel_comm_assoc] using hf'
  · -- Porting note(https://github.com/leanprover-community/mathlib4/issues/5127): added `not_and`
    simp_rw [not_exists, not_and] at H
    have hD : (Ico (0 : ℤ) n).card < D.card := by rw [card_Icc, card_Ico]; exact lt_add_one n
    have hfu' : ∀ m, f m ≤ n := fun m => lt_add_one_iff.mp (floor_lt.mpr (mod_cast hfu m))
    have hwd : ∀ m : ℤ, m ∈ D → f m ∈ Ico (0 : ℤ) n := fun x hx =>
      mem_Ico.mpr
        ⟨floor_nonneg.mpr (mul_nonneg (fract_nonneg (ξ * x)) hn.le), Ne.lt_of_le (H x hx) (hfu' x)⟩
    have : ∃ (x : ℤ) (_ : x ∈ D) (y : ℤ) (_ : y ∈ D), x < y ∧ f x = f y := by
      obtain ⟨x, hx, y, hy, x_ne_y, hxy⟩ := exists_ne_map_eq_of_card_lt_of_maps_to hD hwd
      rcases lt_trichotomy x y with (h | h | h)
      exacts [⟨x, hx, y, hy, h, hxy⟩, False.elim (x_ne_y h), ⟨y, hy, x, hx, h, hxy.symm⟩]
    obtain ⟨x, hx, y, hy, x_lt_y, hxy⟩ := this
    refine'
      ⟨⌊ξ * y⌋ - ⌊ξ * x⌋, y - x, sub_pos_of_lt x_lt_y,
        sub_le_iff_le_add.mpr <| le_add_of_le_of_nonneg (mem_Icc.mp hy).2 (mem_Icc.mp hx).1, _⟩
    convert_to |fract (ξ * y) * (n + 1) - fract (ξ * x) * (n + 1)| ≤ 1
    · congr; push_cast; simp only [fract]; 

Proof state:
case h.e'_3.e_a
ξ : ℝ
n : ℕ
n_pos : 0 < n
f : ℤ → ℤ := fun m => ⌊fract (ξ * ↑m) * (↑n + 1)⌋
hn : 0 < ↑n + 1
hfu : ∀ (m : ℤ), fract (ξ * ↑m) * (↑n + 1) < ↑n + 1
D : Finset ℤ := Icc 0 ↑n
H : ∀ x ∈ Icc 0 ↑n, ¬⌊fract (ξ * ↑x) * (↑n + 1)⌋ = ↑n
hD : card (Ico 0 ↑n) < card D
hfu' : ∀ (m : ℤ), f m ≤ ↑n
hwd : ∀ m ∈ D, f m ∈ Ico 0 ↑n
x : ℤ
hx : x ∈ D
y : ℤ
hy : y ∈ D
x_lt_y : x < y
hxy : f x = f y
⊢ (ξ * (↑y - ↑x) - (↑⌊ξ * ↑y⌋ - ↑⌊ξ * ↑x⌋)) * (↑n + 1) =
    (ξ * ↑y - ↑⌊ξ * ↑y⌋) * (↑n + 1) - (ξ * ↑x - ↑⌊ξ * ↑x⌋) * (↑n + 1)

Next tactic:
ring