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- Theorem (11.48): S ∩ T = {} ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- Proof:
- S ∩ T = {} ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Set extensionality” ⟩
- (∀ x • x ∈ (S ∩ T) ≡ x ∈ {}) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Empty set” ⟩
- (∀ x • x ∈ (S ∩ T) ≡ false) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Definition of ¬ from ≡” ⟩
- (∀ x • ¬ (x ∈ (S ∩ T))) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Intersection” ⟩
- (∀ x • ¬ (x ∈ S ∧ x ∈ T)) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “De Morgan” ⟩
- (∀ x • (¬ (x ∈ S) ∨ (¬ (x ∈ T)))) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Complement” ⟩
- (∀ x • (¬ (x ∈ S) ∨ (x ∈ ~ T))) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Definition of ⇒” ⟩
- (∀ x • ((x ∈ S) ⇒ (x ∈ ~ T))) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Complement” ⟩
- (∀ x • ((x ∈ S) ⇒ ¬ (x ∈ T))) ≡ (∀ x • x ∈ S ⇒ ¬ (x ∈ T))
- =⟨ “Reflexivity of ≡” ⟩
- true
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