A pyramidal number of the form 
,
The first few are 1, 8, 26, 60, 115, ... (OEIS A002413).
The generating function for the heptagonal
pyramidal numbers is
 |
Heptagonal Pyramidal Number
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References
Sloane, N. J. A. Sequence A002413/M4498 in "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
Heptagonal Pyramidal NumberCite this as:
Weisstein, Eric W. "Heptagonal Pyramidal Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HeptagonalPyramidalNumber.html