Hep-tāg'o-nal, a.

a. [Fr. heptagonal. See supra.] Having seven angles or sides.

Heptagonal numbers (Arith.), a sort of polygonal numbers, wherein the difference of the terms of the corresponding arithmetical progression is 5; e. g.—

1, 6, 11, 16, &c., arithmetical progression.

1, 7, 18, 34, &c., heptagonal numbers.

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. Math. Dict.