Definition:Multiplication/Natural Numbers

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Definition

Let $\N$ be the natural numbers.

Multiplication on $\N$ is the basic operation $\times$ everyone is familiar with.

For example:

$3 \times 4 = 12$
$13 \times 7 = 91$


Every attempt to describe the natural numbers via suitable axioms should reproduce the intuitive behaviour of $\times$.

The same holds for any construction of $\N$ in an ambient theory.


Multiplication in terms of Addition

Let $+$ denote addition.


The binary operation $\times$ is recursively defined on $\N$ as follows:

$\forall m, n \in \N: \begin{cases} m \times 0 & = 0 \\ m \times \left({n + 1}\right) & = m \times n + m \end{cases}$

This operation is called multiplication.


Equivalently, multiplication can be defined as:

$\forall m, n \in \N: m \times n := +^n m$

where $+^n m$ denotes the $n$th power of $m$ under $+$.


Also defined as

Some treatments of Peano's axioms define the non-successor element (or primal element) to be $1$ and not $0$.

The treatments are similar, but the $1$-based system results in an algebraic structure which has no identity element for addition, and so no zero for multiplication.


Also see


Sources