-- Lean 4 proof step from Mathlib.NumberTheory.NumberField.Discriminant.Basic (goal)
Goal:
K : Type u_1
inst : Field K
inst_1 : NumberField K
I : (FractionalIdeal (R := 𝓞 K) (𝓞 K)⁰ K)ˣ
The The₁ The₂ :
  ∃ a ∈ (↑I : FractionalIdeal (𝓞 K)⁰ K),
    a ≠ 0 ∧
      (↑|(Algebra.norm ℚ : (a : K) → ℚ) a| : ℝ) ≤
        (↑((FractionalIdeal.absNorm : (a : FractionalIdeal (𝓞 K)⁰ K) → ℚ) (↑I : FractionalIdeal (𝓞 K)⁰ K)) : ℝ) *
                (4 / π) ^ nrComplexPlaces K *
              (↑(finrank ℚ K).factorial : ℝ) /
            HPow.hPow (α := ℝ) (↑(finrank ℚ K) : ℝ) (finrank ℚ K) *
          √|(↑(discr K) : ℝ)|
⊢ ∃ a ∈ (↑I : FractionalIdeal (𝓞 K)⁰ K),
    a ≠ 0 ∧
      (↑|(Algebra.norm ℚ : (a : K) → ℚ) a| : ℝ) ≤
        (↑((FractionalIdeal.absNorm : (a : FractionalIdeal (𝓞 K)⁰ K) → ℚ) (↑I : FractionalIdeal (𝓞 K)⁰ K)) : ℝ) *
                (4 / π) ^ nrComplexPlaces K *
              (↑(finrank ℚ K).factorial : ℝ) /
            HPow.hPow (α := ℝ) (↑(finrank ℚ K) : ℝ) (finrank ℚ K) *
          √|(↑(discr K) : ℝ)|
have The₃ : let B := (minkowskiBound K I * (↑(convexBodySumFactor K)⁻¹ : ℝ≥0∞)).toReal ^ HDiv.hDiv (α := ℝ) 1 (↑(finrank ℚ K) : ℝ);
∀
  (h_le :
    minkowskiBound K I ≤ ((volume : Measure (mixedSpace K)) : (a : Set (mixedSpace K)) → ℝ≥0∞) (convexBodySum K B)),
  ∃ a ∈ (↑I : FractionalIdeal (𝓞 K)⁰ K),
    a ≠ 0 ∧
      (↑|(Algebra.norm ℚ : (a : K) → ℚ) a| : ℝ) ≤
        (↑((FractionalIdeal.absNorm : (a : FractionalIdeal (𝓞 K)⁰ K) → ℚ) (↑I : FractionalIdeal (𝓞 K)⁰ K)) : ℝ) *
                (4 / π) ^ nrComplexPlaces K *
              (↑(finrank ℚ K).factorial : ℝ) /
            HPow.hPow (α := ℝ) (↑(finrank ℚ K) : ℝ) (finrank ℚ K) *
          √|(↑(discr K) : ℝ)|
Tactic: classical
  -- The smallest possible value for `exists_ne_zero_mem_ideal_of_norm_le`
  let B := (minkowskiBound K I * (convexBodySumFactor K)⁻¹).toReal ^ (1 / (finrank ℚ K : ℝ))
  have h_le : (minkowskiBound K I) ≤ volume (convexBodySum K B) := by
    refine le_of_eq ?_
    rw [convexBodySum_volume, ← ENNReal.ofReal_pow (by positivity), ← Real.rpow_natCast,
      ← Real.rpow_mul toReal_nonneg, div_mul_cancel₀, Real.rpow_one, ofReal_toReal, mul_comm,
      mul_assoc, ← coe_mul, inv_mul_cancel₀ (convexBodySumFactor_ne_zero K), ENNReal.coe_one,
      mul_one]
    · exact mul_ne_top (ne_of_lt (minkowskiBound_lt_top K I)) coe_ne_top
    · exact (Nat.cast_ne_zero.mpr (ne_of_gt finrank_pos))
  convert! exists_ne_zero_mem_ideal_of_norm_le K I h_le
  rw [div_pow B, ← Real.rpow_natCast B, ← Real.rpow_mul (by positivity), div_mul_cancel₀ _
    (Nat.cast_ne_zero.mpr <| ne_of_gt finrank_pos), Real.rpow_one, mul_comm_div, mul_div_assoc']
  congr 1
  rw [eq_comm]
  calc
    _ = FractionalIdeal.absNorm I.1 * (2 : ℝ)⁻¹ ^ nrComplexPlaces K * sqrt ‖discr K‖₊ *
          (2 : ℝ) ^ finrank ℚ K * ((2 : ℝ) ^ nrRealPlaces K * (π / 2) ^ nrComplexPlaces K /
            (Nat.factorial (finrank ℚ K)))⁻¹ := by
      simp_rw [minkowskiBound, convexBodySumFactor,
        volume_fundamentalDomain_fractionalIdealLatticeBasis,
        volume_fundamentalDomain_latticeBasis, toReal_mul, toReal_pow, toReal_inv, coe_toReal,
        toReal_ofNat, mixedEmbedding.finrank, mul_assoc]
      rw [ENNReal.toReal_ofReal (Rat.cast_nonneg.mpr (FractionalIdeal.absNorm_nonneg I.1))]
      simp_rw [NNReal.coe_inv, NNReal.coe_div, NNReal.coe_mul, NNReal.coe_pow, NNReal.coe_div,
        coe_real_pi, NNReal.coe_ofNat, NNReal.coe_natCast]
    _ = FractionalIdeal.absNorm I.1 * (2 : ℝ) ^ (finrank ℚ K - nrComplexPlaces K - nrRealPlaces K +
          nrComplexPlaces K : ℤ) * Real.sqrt ‖discr K‖ * Nat.factorial (finrank ℚ K) *
            π⁻¹ ^ (nrComplexPlaces K) := by
      simp_rw [inv_div, div_eq_mul_inv, mul_inv, ← zpow_neg_one, ← zpow_natCast, mul_zpow,
        ← zpow_mul, neg_one_mul, mul_neg_one, neg_neg, Real.coe_sqrt, coe_nnnorm, sub_eq_add_neg,
        zpow_add₀ (two_ne_zero : (2 : ℝ) ≠ 0)]
      ring
    _ = FractionalIdeal.absNorm I.1 * (2 : ℝ) ^ (2 * nrComplexPlaces K : ℤ) * Real.sqrt ‖discr K‖ *
          Nat.factorial (finrank ℚ K) * π⁻¹ ^ (nrComplexPlaces K) := by
      congr
      rw [← card_add_two_mul_card_eq_rank, Nat.cast_add, Nat.cast_mul, Nat.cast_ofNat]
      ring
    _ = FractionalIdeal.absNorm I.1 * (4 / π) ^ nrComplexPlaces K * (finrank ℚ K).factorial *
          Real.sqrt |discr K| := by
      rw [Int.norm_eq_abs, zpow_mul, show (2 : ℝ) ^ (2 : ℤ) = 4 by norm_cast, div_pow,
        inv_eq_one_div, div_pow, one_pow, zpow_natCast]
      ring