Editing Polygonal number (section)

== Definition and examples ==

The number 10 for example, can be arranged as a [[triangle]] (see [[triangular number]]):

:{|
| align="center" style="line-height: 0;" | [[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]
|}

But 10 cannot be arranged as a [[square (geometry)|square]]. The number 9, on the other hand, can be (see [[square number]]):

:{|
| align="center" style="line-height: 0;" | [[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]
|}

Some numbers, like 36, can be arranged both as a square and as a triangle (see [[square triangular number]]):

:{| cellpadding="5"
|- align="center" valign="bottom"
| style="line-height: 0; display: inline-block;"|[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]<br>[[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]][[File:GrayDot.svg|16px|*]]
| style="line-height: 0; display: inline-block"|[[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]<br>[[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]][[File:GrayDotX.svg|16px|*]]
|}

By convention, 1 is the first polygonal number for any number of sides.  The rule for enlarging the polygon to the next size is to extend two adjacent arms by one point and to then add the required extra sides between those points.  In the following diagrams, each extra layer is shown as in red.

===Triangular numbers===
:[[File:Polygonal Number 3.gif|500px|none]]

The triangular number sequence is the representation of the numbers in the form of equilateral triangle arranged in a series or sequence. These numbers are in a sequence of 1, 3, 6, 10, 15, 21, 28, 36, 45, and so on.

===Square numbers===

:[[File:Polygonal Number 4.gif|500px|none]]

Polygons with higher numbers of sides, such as pentagons and hexagons, can also be constructed according to this rule, although the dots will no longer form a perfectly regular lattice like above.

===Pentagonal numbers===

:[[File:Polygonal Number 5.gif|500px|none]]

===Hexagonal numbers===

:[[File:Polygonal Number 6.gif|500px|none]]