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Webster 1913 Edition

Heptagonal

Hep-tag′o-nal

,
Adj.
[Cf. F.
heptagonal
.]
Having seven angles or sides.
Heptagonal numbers
(Arith.)
,
the numbers of the series 1, 7, 18, 34, 55, etc., being figurate numbers formed by adding successively the terms of the arithmetical series 1, 6, 11, 16, 21, etc.

Webster 1828 Edition

Heptagonal

HEPTAG'ONAL

,
Adj.
Having seven angles or sides. Heptagonal numbers, in arithmetic, a sort of polygonal numbers, wherein the difference of the terms of the corresponding arithmetical progression is 5. One of the properties of these numbers is, that if they are multiplied by 40, and 9 is added to the product, the sum will be a square number.

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