User:Dfeuer/Zero is Natural Number
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Theorem
User:Dfeuer/Definition:Zero is a User:Dfeuer/Definition:Natural Number.
Proof
By the definition of zero, $0 = \varnothing$.
Let $a$ be any User:Dfeuer/Definition:Inductive Set.
Then by the definition of inductive set, $0 \in a$.
Since this holds for every inductive set $a$, $0$ is a natural number.
$\blacksquare$