Previously, several well-known sequences (and many new sequences), were
derived from tetrahedral (three-sided) [2], square (four-sided) [3], and pentagonal
(five-sided) [5] pyramids of integers. For example, the number of elements
in the square pyramid is
sn = 12 + 22 + 32 + 42 + 52 + . . . + n2 =
n
i=1
i2 =
1
6
n(n + 1)(2n + 1),