Octahedral number

146 magnetic balls, packed in the form of an octahedron

In number theory, an octahedral number is a figurate number that represents the number of spheres in an octahedron formed from close-packed spheres. The nth octahedral number {displaystyle O_{n}} can be obtained by the formula:^[1]

{displaystyle O_{n}={n(2n^{2}+1) over 3}.}

The first few octahedral numbers are:

1, 6, 19, 44, 85, 146, 231, 344, 489, 670, 891 (sequence A005900 in OEIS).

Properties and applications[edit]

The octahedral numbers have a generating function

{displaystyle {frac {z(z+1)^{2}}{(z-1)^{4}}}=sum _{n=1}^{infty }O_{n}z^{n}=z+6z^{2}+19z^{3}+cdots .}

Sir Frederick Pollock conjectured in 1850 that every number is the sum of at most 7 octahedral numbers: see Pollock octahedral numbers conjecture.^[2]

In chemistry, octahedral numbers may be used to describe the numbers of atoms in octahedral clusters; in this context they are called magic numbers.^[3][4]

Relation to other figurate numbers[edit]

Square pyramids[edit]

Square pyramids in which each layer has a centered square number of cubes. The total number of cubes in each pyramid is an octahedral number.

An octahedral packing of spheres may be partitioned into two square pyramids, one upside-down underneath the other, by splitting it along a square cross-section. Therefore, the nth octahedral number {displaystyle O_{n}}can be obtained by adding two consecutive square pyramidal numbers together:^[1]

{displaystyle O_{n}=P_{n-1}+P_{n}.}

Tetrahedra[edit]

If {displaystyle O_{n}} is the nth octahedral number and {displaystyle T_{n}} is the nth tetrahedral number then

{displaystyle O_{n}+4T_{n-1}=T_{2n-1}.}

This represents the geometric fact that gluing a tetrahedron onto each of four non-adjacent faces of an octahedron produces a tetrahedron of twice the size. Another relation between octahedral numbers and tetrahedral numbers is also possible, based on the fact that an octahedron may be divided into four tetrahedra each having two adjacent original faces (or alternatively, based on the fact that each square pyramidal number is the sum of two tetrahedral numbers):

{displaystyle O_{n}=T_{n}+2T_{n-1}+T_{n-2}.}

Cubes[edit]

If two tetrahedra are attached to opposite faces of an octahedron, the result is a rhombohedron.^[5] The number of close-packed spheres in the rhombohedron is a cube, justifying the equation

{displaystyle O_{n}+2T_{n-1}=n^{3}.}

Centered squares[edit]

The difference between two consecutive octahedral numbers is a centered square number:^[1]

{displaystyle O_{n}-O_{n-1}=C_{4,n}=n^{2}+(n-1)^{2}.}

Therefore, an octahedral number also represents the number of points in a square pyramid formed by stacking centered squares; for this reason, in his book Arithmeticorum libri duo (1575), Francesco Maurolico called these numbers "pyramides quadratae secundae".^[6]

The number of cubes in an octahedron formed by stacking centered squares is a centered octahedral number, the sum of two consecutive octahedral numbers. These numbers are

1, 7, 25, 63, 129, 231, 377, 575, 833, 1159, 1561, 2047, 2625, ... (sequence A001845 in OEIS)

given by the formula

{displaystyle O_{n}+O_{n-1}={frac {(2n-1)(2n^{2}-2n+3)}{3}}} for n = 1, 2, 3, ...

References[edit]