A polygonal number of the form . The first few are 1, 7, 18, 34, 55, 81, 112, ... (OEIS A000566). The generating function for the heptagonal numbers is
Heptagonal Number
See also
Heptagonal Hexagonal Number, Heptagonal Pentagonal Number, Heptagonal Square Number, Heptagonal Triangular Number, Octagonal Heptagonal NumberExplore with Wolfram|Alpha
References
Sloane, N. J. A. Sequence A000566/M4358 in "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
Heptagonal NumberCite this as:
Weisstein, Eric W. "Heptagonal Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HeptagonalNumber.html