# Definition:Square Number

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This page is about Square Number. For other uses, see Square.

## Definition

Square numbers are those denumerating a collection of objects which can be arranged in the form of a square.

They can be denoted:

$S_1, S_2, S_3, \ldots$

### Definition 1

An integer $n$ is classified as a square number if and only if:

$\exists m \in \Z: n = m^2$

where $m^2$ denotes the integer square function.

#### Euclid's Definition

In the words of Euclid:

A square number is equal multiplied by equal, or a number which is contained by two equal numbers.

### Definition 2

$S_n = \begin {cases} 0 & : n = 0 \\ S_{n - 1} + 2 n - 1 & : n > 0 \end {cases}$

### Definition 3

$\ds S_n = \sum_{i \mathop = 1}^n \paren {2 i - 1} = 1 + 3 + 5 + \cdots + \paren {2 n - 1}$

### Definition 4

$\forall n \in \N: S_n = \map P {4, n} = \begin{cases} 0 & : n = 0 \\ \map P {4, n - 1} + 2 \paren {n - 1} + 1 & : n > 0 \end{cases}$

where $\map P {k, n}$ denotes the $k$-gonal numbers.

## Examples of Square Numbers

The first few square numbers are as follows:

### Sequence of Square Numbers

The sequence of square numbers, for $n \in \Z_{\ge 0}$, begins:

$0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, \ldots$

## Also known as

A square number is often referred to as a square.

A square number is also often referred to as a perfect square, but this could cause confusion with the concept of perfect number, so its use is discouraged on $\mathsf{Pr} \infty \mathsf{fWiki}$.

In fact it is prime.mover's opinion that perfect square is so utterly bletheringly pointlessly stupid that he has difficulty wondering whether it's worth carrying on sharing a universe with the utter imbeciles who continue to think it's worthwhile to try and defend its use.

This usage may in fact be regional.

## Also see

• Results about square numbers can be found here.

## Historical Note

Figurate numbers, that is:

triangular numbers
square numbers
pentagonal numbers
hexagonal numbers

and so on, were classified and investigated by the Pythagorean school in the $6$th century BCE. This was possibly the first time this had ever been done.