The Collaborative International Dictionary
Heptagonal numbers
Heptagonal \Hep*tag"o*nal\, a. [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.