-- 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:
case intro.mk.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
x : mixedSpace K
hx : x ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
h_nz : ⟨x, hx⟩ ≠ 0
h_mem : ↑⟨x, hx⟩ ∈ convexBodySum K B
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
rw [mem_toAddSubgroup, mem_span_fractionalIdealLatticeBasis] at hx

Result:
case intro.mk.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
x : mixedSpace K
hx✝ : x ∈ (span ℤ (Set.range ⇑(fractionalIdealLatticeBasis K I))).toAddSubgroup
hx : x ∈ ⇑(mixedEmbedding K) '' ↑↑I
h_nz : ⟨x, hx✝⟩ ≠ 0
h_mem : ↑⟨x, hx✝⟩ ∈ convexBodySum K B
⊢ ∃ 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
⊢ ∃ a ∈ ↑I, a ≠ 0 ∧ ↑|(Algebra.norm ℚ) a| ≤ (B / ↑(finrank ℚ K)) ^ finrank ℚ K

Tactic:
refine ⟨a, ha, by simpa using h_nz, ?_⟩

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
⊢ ↑|(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
⊢ ↑(finrank ℚ K) * ↑|(Algebra.norm ℚ) a| ^ (↑(finrank ℚ K))⁻¹ ≤ B

Tactic:
refine le_trans ?_ ((convexBodySum_mem K B).mp h_mem)

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 ≤ ∑ w : InfinitePlace K, ↑w.mult * ↑w a

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
⊢ ↑(finrank ℚ K) * ↑|(Algebra.norm ℚ) a| ^ (↑(finrank ℚ K))⁻¹ ≤ ∑ w : InfinitePlace K, ↑w.mult * ↑w a

Tactic:
rw [← le_div_iff₀' (Nat.cast_pos.mpr finrank_pos), ← sum_mult_eq, Nat.cast_sum]

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
⊢ ↑|(Algebra.norm ℚ) a| ^ (∑ x : InfinitePlace K, ↑x.mult)⁻¹ ≤
    (∑ w : InfinitePlace K, ↑w.mult * ↑w a) / ∑ x : InfinitePlace K, ↑x.mult