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NormResidueBoundary.lean

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1 import PaperX.NormResidueArithmetic
2 
3 /-!
4 # Why the norm sign is part of a residue filter
5 
6 The rational integers `-1` and `2` agree modulo `3`. Their signed norms agree
7 modulo `3`, but their principal ideal norms do not. This is an actual ideal
8 counterexample to omitting the sign hypothesis.
9 -/
10 
11 namespace PaperX.NormResidue
12 
13 theorem opposite_sign_elements_congruent :
14     (-1 : ℤ) - 2 ∈ modulusIdeal ℤ 3 := by
15   change (-1 : ℤ) - 2 ∈ Ideal.span {(3 : ℤ)}
16   exact Ideal.mem_span_singleton.mpr ⟨-1, by norm_num⟩
17 
18 theorem opposite_sign_signed_norms_congruent :
19     (Algebra.norm ℤ (-1 : ℤ) : ZMod 3) = (Algebra.norm ℤ (2 : ℤ) : ZMod 3) := by
20   norm_num
21   decide
22 
23 theorem opposite_sign_principal_norms_differ :
24     (Ideal.absNorm (Ideal.span {(-1 : ℤ)}) : ZMod 3) ≠
25       (Ideal.absNorm (Ideal.span {(2 : ℤ)}) : ZMod 3) := by
26   norm_num [Ideal.absNorm_span_singleton]
27   decide
28 
29 end PaperX.NormResidue

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