Square pyramidal number
From Wikipedia, the free encyclopedia.
The first few pyramidal numbers are
1, 5, 14, 30, 55, 91, 140, 204, 285, 385, 506, 650, 819
Pyramidal numbers can be modelled in physical space with a given number of balls and a square frame that hold in place the number of balls forming the base, that is, n2. Besides 1, there is only one other number that is both a square and a pyramidal number, 4900. This fact was proven by G.N. Watson in 1918.
The sum of two consecutive square pyramidal numbers is an octahedral number.
See also: tetrahedral number
