Theorem

The empty set Ø is a subset of every set including itself and the universal set S
i.e. Ø ⊆ A ∀A ⊆ S.



Proof

By definition of the subset, Ø ⊆ A is true because all of the elements in Ø (of which there are none) are in A. Thus Ø ⊆ A is vacuously true.
$ \blacksquare $



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Prof. Math. Ohio State and Associate Dean
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Jeff McNeal