Heptagonal pyramidal number

In mathematics, a heptagonal pyramidal number is a figurate number representing the number of dots in a three-dimensional pattern in the shape of a heptagonal pyramid.[1]

The first few heptagonal pyramidal numbers are:[2]

1, 8, 26, 60, 115, 196, 308, 456, 645, 880, 1166, 1508, 1911, ... (sequence A002413 in the OEIS)

The nth heptagonal number can be calculated by adding up the first n heptagonal numbers, or more directly by using the formula[1][2]

{\frac {n(n+1)(5n-2)}{6}}.

References

Deza, Elena; Deza, M. (2012), Figurate Numbers, World Scientific, p. 92, ISBN 9789814355483.
Beiler, Albert H. (1966), Recreations in the Theory of Numbers: The Queen of Mathematics Entertains, Courier Dover Publications, p. 194, ISBN 9780486210964.

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[dd-1] Deza, Elena; Deza, M. (2012), Figurate Numbers, World Scientific, p. 92, ISBN 9789814355483.

[b-2] Beiler, Albert H. (1966), Recreations in the Theory of Numbers: The Queen of Mathematics Entertains, Courier Dover Publications, p. 194, ISBN 9780486210964.