# natural number

The first few natural numbers are:

• $0:=\emptyset$

• $1:=0^{\prime}=\{0\}=\{\emptyset\}$

• $2:=1^{\prime}=\{0,1\}=\{\emptyset,\{\emptyset\}\}$

• $3:=2^{\prime}=\{0,1,2\}=\{\emptyset,\{\emptyset\},\{\emptyset,\{\emptyset\}\}\}$

Note that the set $0$ has zero elements, the set $1$ has one element, the set $2$ has two elements, etc. Informally, the set $n$ is the set consisting of the $n$ elements $0,1,\dots,n-1$, and $n$ is both a subset of $\mathbb{N}$ and an element of $\mathbb{N}$.

In some contexts (most notably, in number theory   ), it is more convenient to exclude $0$ from the set of natural numbers, so that $\mathbb{N}=\{1,2,3,\dots\}$. When it is not explicitly specified, one must determine from context whether $0$ is being considered a natural number or not.

Addition  of natural numbers is defined inductively as follows:

• $a+0:=a$ for all $a\in\mathbb{N}$

• $a+b^{\prime}:=(a+b)^{\prime}$ for all $a,b\in\mathbb{N}$

Multiplication of natural numbers is defined inductively as follows:

• $a\cdot 0:=0$ for all $a\in\mathbb{N}$

• $a\cdot b^{\prime}:=(a\cdot b)+a$ for all $a,b\in\mathbb{N}$

The natural numbers form a monoid under either addition or multiplication. There is an ordering relation on the natural numbers, defined by: $a\leq b$ if $a\subseteq b$.

Title natural number NaturalNumber 2013-03-22 11:50:05 2013-03-22 11:50:05 djao (24) djao (24) 16 djao (24) Definition msc 03E10 msc 74D99 $\mathbb{N}$ InductiveSet Successor    PeanoArithmetic VonNeumannInteger