/-- We can approximate `a / b : L` with `q / r`, where `r` has finitely many options for `L`. -/
theorem exists_mem_finsetApprox (a : S) {b} (hb : b ≠ (0 : R)) :
    ∃ q : S,
      ∃ r ∈ finsetApprox bS adm, abv (Algebra.norm R (r • a - b • q)) <
      abv (Algebra.norm R (algebraMap R S b)) := by
  have dim_pos := Fintype.card_pos_iff.mpr bS.index_nonempty
  set ε : ℝ := normBound abv bS ^ (-1 / Fintype.card ι : ℝ) with ε_eq
  have hε : 0 < ε := Real.rpow_pos_of_pos (Int.cast_pos.mpr (normBound_pos abv bS)) _
  have ε_le : (normBound abv bS : ℝ) * (abv b • ε) ^ (Fintype.card ι : ℝ)
                ≤ abv b ^ (Fintype.card ι : ℝ) := by
    have := normBound_pos abv bS
    have := abv.nonneg b
    rw [ε_eq, Algebra.smul_def, eq_intCast, mul_rpow, ← rpow_mul, div_mul_cancel, rpow_neg_one,
      mul_left_comm, mul_inv_cancel, mul_one, rpow_nat_cast] <;>
      try norm_cast; linarith
    · exact Iff.mpr Int.cast_nonneg this
    · linarith
  set μ : Fin (cardM bS adm).succ ↪ R := distinctElems bS adm with hμ
  let s : ι →₀ R := bS.repr a
  have s_eq : ∀ i, s i = bS.repr a i := fun i => rfl
  let qs : Fin (cardM bS adm).succ → ι → R := fun j i => μ j * s i / b
  let rs : Fin (cardM bS adm).succ → ι → R := fun j i => μ j * s i % b
  have r_eq : ∀ j i, rs j i = μ j * s i % b := fun i j => rfl
  have μ_eq : ∀ i j, μ j * s i = b * qs j i + rs j i := by
    intro i j
    rw [r_eq, EuclideanDomain.div_add_mod]
  have μ_mul_a_eq : ∀ j, μ j • a = b • ∑ i, qs j i • bS i + ∑ i, rs j i • bS i := by
    intro j
    rw [← bS.sum_repr a]
    simp only [Finset.smul_sum, ← Finset.sum_add_distrib]
    refine'
      Finset.sum_congr rfl fun i _ => _
-- Porting note `← hμ, ← r_eq` and the final `← μ_eq` were not needed.
    rw [← hμ, ← r_eq, ← s_eq, ← mul_smul, μ_eq, add_smul, mul_smul, ← μ_eq]
  obtain ⟨j, k, j_ne_k, hjk⟩ := adm.exists_approx hε hb fun j i => μ j * s i
  have hjk' : ∀ i, (abv (rs k i - rs j i) : ℝ) < abv b • ε := by simpa only [r_eq] using hjk
  let q := ∑ i, (qs k i - qs j i) • bS i
  set r := μ k - μ j with r_eq
  refine' ⟨q, r, (mem_finsetApprox bS adm).mpr _, _⟩
  · exact ⟨k, j, j_ne_k.symm, rfl⟩
  have : r • a - b • q = ∑ x : ι, (rs k x • bS x - rs j x • bS x) := by
    simp only [r_eq, sub_smul, μ_mul_a_eq, Finset.smul_sum, ← Finset.sum_add_distrib,
      ← Finset.sum_sub_distrib, smul_sub]
    

Proof state:
R : Type u_1
S : Type u_2
K : Type u_3
L : Type u_4
inst✝¹⁶ : EuclideanDomain R
inst✝¹⁵ : CommRing S
inst✝¹⁴ : IsDomain S
inst✝¹³ : Field K
inst✝¹² : Field L
inst✝¹¹ : Algebra R K
inst✝¹⁰ : IsFractionRing R K
inst✝⁹ : Algebra K L
inst✝⁸ : FiniteDimensional K L
inst✝⁷ : IsSeparable K L
algRL : Algebra R L
inst✝⁶ : IsScalarTower R K L
inst✝⁵ : Algebra R S
inst✝⁴ : Algebra S L
ist : IsScalarTower R S L
iic : IsIntegralClosure S R L
abv : AbsoluteValue R ℤ
ι : Type u_5
inst✝³ : DecidableEq ι
inst✝² : Fintype ι
bS : Basis ι R S
adm : AbsoluteValue.IsAdmissible abv
inst✝¹ : Infinite R
inst✝ : DecidableEq R
a : S
b : R
hb : b ≠ 0
dim_pos : 0 < Fintype.card ι
ε : ℝ := ↑(normBound abv bS) ^ (-1 / ↑(Fintype.card ι))
ε_eq : ε = ↑(normBound abv bS) ^ (-1 / ↑(Fintype.card ι))
hε : 0 < ε
ε_le : ↑(normBound abv bS) * (abv b • ε) ^ ↑(Fintype.card ι) ≤ ↑(abv b) ^ ↑(Fintype.card ι)
μ : Fin (Nat.succ (cardM bS adm)) ↪ R := distinctElems bS adm
hμ : μ = distinctElems bS adm
s : ι →₀ R := bS.repr a
s_eq : ∀ (i : ι), s i = (bS.repr a) i
qs : Fin (Nat.succ (cardM bS adm)) → ι → R := fun j i => μ j * s i / b
rs : Fin (Nat.succ (cardM bS adm)) → ι → R := fun j i => μ j * s i % b
r_eq✝ : ∀ (j : Fin (Nat.succ (cardM bS adm))) (i : ι), rs j i = μ j * s i % b
μ_eq : ∀ (i : ι) (j : Fin (Nat.succ (cardM bS adm))), μ j * s i = b * qs j i + rs j i
μ_mul_a_eq : ∀ (j : Fin (Nat.succ (cardM bS adm))), μ j • a = b • ∑ i : ι, qs j i • bS i + ∑ i : ι, rs j i • bS i
j k : Fin (Nat.succ (AbsoluteValue.IsAdmissible.card adm ε ^ Fintype.card ι))
j_ne_k : j ≠ k
hjk : ∀ (k_1 : ι), ↑(abv (μ k * s k_1 % b - μ j * s k_1 % b)) < abv b • ε
hjk' : ∀ (i : ι), ↑(abv (rs k i - rs j i)) < abv b • ε
q : S := ∑ i : ι, (qs k i - qs j i) • bS i
r : R := μ k - μ j
r_eq : r = μ k - μ j
⊢ ∑ x : ι,
      (b • ((distinctElems bS adm) k * (bS.repr a) x / b) • bS x +
            ((distinctElems bS adm) k * (bS.repr a) x % b) • bS x -
          (b • ((distinctElems bS adm) j * (bS.repr a) x / b) • bS x +
            ((distinctElems bS adm) j * (bS.repr a) x % b) • bS x) -
        (b • ((distinctElems bS adm) k * (bS.repr a) x / b) • bS x -
          b • ((distinctElems bS adm) j * (bS.repr a) x / b) • bS x)) =
    ∑ x : ι,
      (((distinctElems bS adm) k * (bS.repr a) x % b) • bS x - ((distinctElems bS adm) j * (bS.repr a) x % b) • bS x)

Next tactic:
refine' Finset.sum_congr rfl fun x _ => _