-- Mathlib/NumberTheory/NumberField/CanonicalEmbedding/ConvexBody.lean
theorem exists_ne_zero_mem_ideal_of_norm_le {B : ℝ}
    (h : (minkowskiBound K I) ≤ volume (convexBodySum K B)) :
    ∃ a ∈ (I : FractionalIdeal (𝓞 K)⁰ K), a ≠ 0 ∧
      |Algebra.norm ℚ (a : K)| ≤ (B / finrank ℚ K) ^ finrank ℚ K

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
have h1 : 0 < (finrank ℚ K : ℝ)⁻¹ := inv_pos.mpr (Nat.cast_pos.mpr finrank_pos)

Result:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Proof state:
case intro.mk.intro.intro.intro
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
a : K
ha : a ∈ ↑↑I
hx : (mixedEmbedding K) a ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨(mixedEmbedding K) a, hx⟩ ≠ 0
h_mem : ↑⟨(mixedEmbedding K) a, hx⟩ ∈ convexBodySum K B
⊢ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
rw [← rpow_natCast, ← rpow_le_rpow_iff (by simp only [Rat.cast_abs, abs_nonneg])
    (rpow_nonneg h2 _) h1, ← rpow_mul h2,  mul_inv_cancel₀ (Nat.cast_ne_zero.mpr
    (ne_of_gt finrank_pos)), rpow_one, le_div_iff₀' (Nat.cast_pos.mpr finrank_pos)]

Result:
case intro.mk.intro.intro.intro
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
a : K
ha : a ∈ ↑↑I
hx : (mixedEmbedding K) a ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨(mixedEmbedding K) a, hx⟩ ≠ 0
h_mem : ↑⟨(mixedEmbedding K) a, hx⟩ ∈ convexBodySum K B
⊢ ↑(finrank ℚ K) * ↑|(Algebra.norm ℚ) a| ^ (↑(finrank ℚ K))⁻¹ ≤ B

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
have h_fund := ZSpan.isAddFundamentalDomain' (fractionalIdealLatticeBasis K I) volume

Result:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
have : Countable (span ℤ (Set.range (fractionalIdealLatticeBasis K I))).toAddSubgroup := by
  change Countable (span ℤ (Set.range (fractionalIdealLatticeBasis K I)))
  infer_instance

Result:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Proof state:
case intro.mk.intro.intro.intro.refine_2
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
a : K
ha : a ∈ ↑↑I
hx : (mixedEmbedding K) a ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨(mixedEmbedding K) a, hx⟩ ≠ 0
h_mem : ↑⟨(mixedEmbedding K) a, hx⟩ ∈ convexBodySum K B
⊢ 0 < ∑ i : InfinitePlace K, ↑i.mult

Tactic:
rw [← Nat.cast_sum, sum_mult_eq, Nat.cast_pos]

Result:
case intro.mk.intro.intro.intro.refine_2
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
a : K
ha : a ∈ ↑↑I
hx : (mixedEmbedding K) a ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨(mixedEmbedding K) a, hx⟩ ≠ 0
h_mem : ↑⟨(mixedEmbedding K) a, hx⟩ ∈ convexBodySum K B
⊢ 0 < finrank ℚ K

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
I : (FractionalIdeal (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ volume (convexBodySum K B)
hB : 0 ≤ B
h1 : 0 < (↑(finrank ℚ K))⁻¹
h2 : 0 ≤ B / ↑(finrank ℚ K)
h_fund :
  IsAddFundamentalDomain (↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup)
    (fundamentalDomain (fractionalIdealLatticeBasis K I)) volume
this : Countable ↥(span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
a : K
ha : a ∈ ↑↑I
hx : (mixedEmbedding K) a ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨(mixedEmbedding K) a, hx⟩ ≠ 0
h_mem : ↑⟨(mixedEmbedding K) a, hx⟩ ∈ convexBodySum K B
⊢ 0 ≤ ↑|(Algebra.norm ℚ) a|

Tactic:
simp only [Rat.cast_abs, abs_nonneg]

Result:
no goals