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- Theorem “A7.1.1 no”: ¬ (∃ x : ℤ • ∀ y : ℤ • y + 2 < 3 · x )
- Proof:
- ¬ (∃ x : ℤ • ∀ y : ℤ • y + 2 < 3 · x )
- ≡⟨“Generalised De Morgan”⟩
- (∀ x : ℤ • ¬ ∀ y : ℤ • y + 2 < 3 · x )
- ≡⟨“Generalised De Morgan”⟩
- (∀ x : ℤ • ∃ y : ℤ • ¬ (y + 2 < 3 · x))
- ≡⟨ Subproof:
- For any `x : ℤ`:
- ∃ y : ℤ • ¬ (y + 2 < 3 · x)
- ⇐⟨“∃-Introduction”⟩
- (¬ (y + 2 < 3 · x))[y ≔ 3 · x]
- ≡⟨ Substitution⟩
- (¬ (3 · x + 2 < 3 · x))
- ≡⟨ “Identity of +”⟩
- (¬ (3 · x + 2 < 3 · x + 0))
- ≡⟨ “<-Isotonicity of +”⟩
- (¬ (2 < 0))
- ≡⟨ Evaluation ⟩
- (¬ (false))
- ≡⟨ “Negation of `false`” ⟩
- true
- ⟩
- true
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