Heptagonal number

A heptagonal number is a figurate number that represents a heptagon. The heptagonal number for n is given by the formula n(5n – 3)/2, with n > 0. The first few heptagonal numbers are

1, 7, 18, 34, 55, 81, 112, 148, 189, 235, 286, 342, 403, 469, 540, 616, 697, 783, 874, 970

The parity of heptagonal numbers follows the pattern odd-odd-even-even. Like square numbers, the digital root in base 10 of a heptagonal number can only be 1, 4, 7 or 9.

Five times a heptagonal number, plus 1 equals a triangular number.