# Principle of Mathematical Induction for Natural Numbers in Real Numbers

## Theorem

Let $\left({\R, +, \times, \le}\right)$ be the field of real numbers.

Let $\N$ be the natural numbers in $\R$.

Suppose that $A \subseteq \N$ is an inductive set.

Then $A = \N$.

## Proof

By definition of the natural numbers in $\R$:

- $\N = \displaystyle \bigcap \mathcal I$

where $\mathcal I$ is the set of inductive sets in $\R$.

Since $A$ was supposed to be inductive, it follows that:

- $\N \subseteq A$

from Intersection is Subset: General Result.

Hence by definition of set equality:

- $A = \N$.

$\blacksquare$

## Sources

- 2000: James R. Munkres:
*Topology*(2nd ed.) ... (previous) ... (next): $1$: Set Theory and Logic: $\S 4$: The Integers and the Real Numbers