# Definition:Hexagonal Number

## Definition

Hexagonal numbers are those denumerating a collection of objects which can be arranged in the form of a regular hexagon.

They can be denoted:

$H_1, H_2, H_3, \ldots$

### Definition 1

$H_n = \begin{cases} 0 & : n = 0 \\ H_{n - 1} + 4 \paren {n - 1} + 1 & : n > 0 \end{cases}$

### Definition 2

$\ds H_n = \sum_{i \mathop = 1}^n \paren {4 \paren {i - 1} + 1} = 1 + 5 + \cdots + \paren {4 \paren {n - 2} + 1} + \paren {4 \paren {n - 1} + 1}$

### Definition 3

$\forall n \in \N: H_n = \map P {6, n} = \begin{cases} 0 & : n = 0 \\ \map P {6, n - 1} + 4 \paren {n - 1} + 1 & : n > 0 \end{cases}$

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

## Examples of Hexagonal Numbers

The first few hexagonal numbers are as follows: ### Sequence of Hexagonal Numbers

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

$0, 1, 6, 15, 28, 45, 66, 91, 120, 153, 190, 231, \ldots$

## Also known as

Hexagonal numbers are also known as hexagon numbers.

Or we can just say that a number is hexagonal.

Some sources denote the $n$th hexagonal number as $\map H n$ in preference to $H_n$.

## Also see

• Results about hexagonal numbers can be found here.