Define natural numbers formally inside chapter on real numbers

[dcernst]

$\mathbb{N}$ denotes the set of natural numbers, so that $\mathbb{N}= {1,2,3,\ldots}$ is the smallest subset of $\mathbb{R}$ satisfying: (a) $1\in\mathbb{N}$ ,and (b) for all $n\in\mathbb{N}$, we have $n+1\in\mathbb{N}$.

[dcernst]

Or define inside chapter on induction if I swap chapters?

[dcernst]

Done.