User:Dfeuer/Successor of Natural Number is Natural Number
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Theorem
Let $n$ be a natural number.
Then $n^+$ is also a natural number.
Proof
Let $a$ be an User:Dfeuer/Definition:Inductive Set.
Since $n$ is a natural number, $n \in a$.
Thus by the definition of inductive set, $n^+ \in a$.
Since this holds for all inductive sets $a$, $n^+$ is a natural number.
$\blacksquare$