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{{refimprove|date=August 2012}}
AAn '''enneagonal number''' is a [[figurate number]] that extends the concept of [[triangular number|triangular]] and [[square number]]s to the [[enneagon]] (a nine-sided polygon). However, unlike the triangular and square numbers, the patterns involved in the construction of enneagonal numbers are not rotationally symmetrical. Specifically, the ''n''th enneagonal numbers counts the number of dots in a pattern of ''n'' nested enneagons, all sharing a common corner, where the ''i''th enneagon in the pattern has sides made of ''i'' dots spaced one unit apart from each other. The enneagonal number for ''n'' is given by the formula:
:<math>\frac {n(7n - 5)}{2}.</math>
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