Octonion

The octonions were discovered in 1843 by John T. Graves, inspired by his friend William Rowan Hamilton's discovery of quaternions. Graves called his discovery "octaves", and mentioned them in a letter to Hamilton dated 16 December 1843. He first published his result slightly later than Arthur Cayley's article.[1] The octonions were discovered independently by Cayley[2] and are sometimes referred to as "Cayley numbers" or the "Cayley algebra". Hamilton described the early history of Graves' discovery.[3]

The octonions can be thought of as octets (or 8-tuples) of real numbers. Every octonion is a real linear combination of the unit octonions:

${\displaystyle \{e_{0},e_{1},e_{2},e_{3},e_{4},e_{5},e_{6},e_{7}\},}$

where e₀ is the scalar or real element; it may be identified with the real number 1. That is, every octonion x can be written in the form

${\displaystyle x=x_{0}e_{0}+x_{1}e_{1}+x_{2}e_{2}+x_{3}e_{3}+x_{4}e_{4}+x_{5}e_{5}+x_{6}e_{6}+x_{7}e_{7},\,}$

with real coefficients x_i.

Addition and subtraction of octonions is done by adding and subtracting corresponding terms and hence their coefficients, like quaternions. Multiplication is more complex. Multiplication is distributive over addition, so the product of two octonions can be calculated by summing the products of all the terms, again like quaternions. The product of each pair of terms can be given by multiplication of the coefficients and a multiplication table of the unit octonions, like this one (due to Cayley, 1845, and Graves, 1843):[4]

${\displaystyle e_{i}e_{j}}$		${\displaystyle e_{j}}$
		${\displaystyle e_{0}}$	${\displaystyle e_{1}}$	${\displaystyle e_{2}}$	${\displaystyle e_{3}}$	${\displaystyle e_{4}}$	${\displaystyle e_{5}}$	${\displaystyle e_{6}}$	${\displaystyle e_{7}}$
${\displaystyle e_{i}}$	${\displaystyle e_{0}}$	${\displaystyle e_{0}}$	${\displaystyle e_{1}}$	${\displaystyle e_{2}}$	${\displaystyle e_{3}}$	${\displaystyle e_{4}}$	${\displaystyle e_{5}}$	${\displaystyle e_{6}}$	${\displaystyle e_{7}}$
	${\displaystyle e_{1}}$	${\displaystyle e_{1}}$	${\displaystyle -e_{0}}$	${\displaystyle e_{3}}$	${\displaystyle -e_{2}}$	${\displaystyle e_{5}}$	${\displaystyle -e_{4}}$	${\displaystyle -e_{7}}$	${\displaystyle e_{6}}$
	${\displaystyle e_{2}}$	${\displaystyle e_{2}}$	${\displaystyle -e_{3}}$	${\displaystyle -e_{0}}$	${\displaystyle e_{1}}$	${\displaystyle e_{6}}$	${\displaystyle e_{7}}$	${\displaystyle -e_{4}}$	${\displaystyle -e_{5}}$
	${\displaystyle e_{3}}$	${\displaystyle e_{3}}$	${\displaystyle e_{2}}$	${\displaystyle -e_{1}}$	${\displaystyle -e_{0}}$	${\displaystyle e_{7}}$	${\displaystyle -e_{6}}$	${\displaystyle e_{5}}$	${\displaystyle -e_{4}}$
	${\displaystyle e_{4}}$	${\displaystyle e_{4}}$	${\displaystyle -e_{5}}$	${\displaystyle -e_{6}}$	${\displaystyle -e_{7}}$	${\displaystyle -e_{0}}$	${\displaystyle e_{1}}$	${\displaystyle e_{2}}$	${\displaystyle e_{3}}$
	${\displaystyle e_{5}}$	${\displaystyle e_{5}}$	${\displaystyle e_{4}}$	${\displaystyle -e_{7}}$	${\displaystyle e_{6}}$	${\displaystyle -e_{1}}$	${\displaystyle -e_{0}}$	${\displaystyle -e_{3}}$	${\displaystyle e_{2}}$
	${\displaystyle e_{6}}$	${\displaystyle e_{6}}$	${\displaystyle e_{7}}$	${\displaystyle e_{4}}$	${\displaystyle -e_{5}}$	${\displaystyle -e_{2}}$	${\displaystyle e_{3}}$	${\displaystyle -e_{0}}$	${\displaystyle -e_{1}}$
	${\displaystyle e_{7}}$	${\displaystyle e_{7}}$	${\displaystyle -e_{6}}$	${\displaystyle e_{5}}$	${\displaystyle e_{4}}$	${\displaystyle -e_{3}}$	${\displaystyle -e_{2}}$	${\displaystyle e_{1}}$	${\displaystyle -e_{0}}$

Most off-diagonal elements of the table are antisymmetric, making it almost a skew-symmetric matrix except for the elements on the main diagonal, as well as the row and column for which e₀ is an operand.

The table can be summarized as follows:[5]

${\displaystyle e_{i}e_{j}={\begin{cases}e_{j},&{\text{if }}i=0\\e_{i},&{\text{if }}j=0\\-\delta _{ij}e_{0}+\varepsilon _{ijk}e_{k},&{\text{otherwise}}\end{cases}}}$

where δ_ij is the Kronecker delta (equal to 1 if and only if i = j), and ε_ijk is a completely antisymmetric tensor with value 1 when ijk = 123, 145, 176, 246, 257, 347, 365.

The above definition though is not unique, but is only one of 480 possible definitions for octonion multiplication with e₀ = 1. The others can be obtained by permuting and changing the signs of the non-scalar basis elements {e₁, e₂, e₃, e₄, e₅, e₆, e₇}. The 480 different algebras are isomorphic, and there is rarely a need to consider which particular multiplication rule is used.

Each of these 480 definitions is invariant up to signs under some 7-cycle of the points (1234567), and for each 7-cycle there are four definitions, differing by signs and reversal of order. A common choice is to use the definition invariant under the 7-cycle (1234567) with e₁e₂ = e₄ — by using the triangular multiplication diagram, or Fano plane, below (and instead of the multiplication table, above) — as it is particularly easy to remember the multiplication.

A variation of this sometimes used is to label the elements of the basis by the elements ∞, 0, 1, 2, ..., 6, of the projective line over the finite field of order 7. The multiplication is then given by e_∞ = 1 and e₁e₂ = e₄, and all expressions obtained from this by adding a constant (modulo 7) to all subscripts: in other words using the seven triples (124) (235) (346) (450) (561) (602) (013). These are the nonzero codewords of the quadratic residue code of length 7 over the Galois field of two elements, GF(2). There is a symmetry of order 7 given by adding a constant mod 7 to all subscripts, and also a symmetry of order 3 given by multiplying all subscripts by one of the quadratic residues 1, 2, 4 mod 7.[6][7]

The multiplication table for a geometric algebra of signature (−−−−) can be given in terms of the following 7 quaternionic triples (omitting the identity element):

(I, j, k), (i, J, k), (i, j, K), (I, J, K), (∗I, i, m), (∗J, j, m), (∗K, k, m)

in which the lowercase items are vectors and the uppercase ones are bivectors and ∗ = mijk (which is the Hodge star operator). If the ∗ is forced to be equal to the identity then the multiplication ceases to be associative, but the ∗ may be removed from the multiplication table resulting in an octonion multiplication table.

In keeping ∗ = mijk associative and thus not reducing the 4-dimensional geometric algebra to an octonion one, the whole multiplication table can be derived from the equation for ∗. Consider the gamma matrices. The formula defining the fifth gamma matrix shows that it is the ∗ of a four-dimensional geometric algebra of the gamma matrices.

Fano plane mnemonic

A mnemonic for the products of the unit octonions.[8]

A 3D mnemonic visualization showing the 7 triads as hyperplanes through the real (e₀) vertex of the octonion example given above.[8]

A convenient mnemonic for remembering the products of unit octonions is given by the diagram, which represents the multiplication table of Cayley and Graves.[4][9] This diagram with seven points and seven lines (the circle through 1, 2, and 3 is considered a line) is called the Fano plane. The lines are directional. The seven points correspond to the seven standard basis elements of Im(O) (see definition below). Each pair of distinct points lies on a unique line and each line runs through exactly three points.

Let (a, b, c) be an ordered triple of points lying on a given line with the order specified by the direction of the arrow. Then multiplication is given by

ab = c and ba = −c

together with cyclic permutations. These rules together with

• 1 is the multiplicative identity,
• e²
  _i = −1 for each point in the diagram

completely defines the multiplicative structure of the octonions. Each of the seven lines generates a subalgebra of O isomorphic to the quaternions H.

Octonionic multiplication is neither commutative:

e_ie_j = −e_je_i ≠ e_je_i if i, j are distinct and non-zero,

nor associative:

(e_ie_j)e_k = −e_i(e_je_k) ≠ e_i(e_je_k) if i, j, k are distinct, non-zero and e_ie_j ≠ ±e_k.

The octonions do satisfy a weaker form of associativity: they are alternative. This means that the subalgebra generated by any two elements is associative. Actually, one can show that the subalgebra generated by any two elements of O is isomorphic to R, C, or H, all of which are associative. Because of their non-associativity, octonions cannot be represented by a subalgebra of a matrix ring over ${\displaystyle \mathbb {R} }$, unlike the real numbers, complex numbers and quaternions.

The octonions do retain one important property shared by R, C, and H: the norm on O satisfies

${\displaystyle \|xy\|=\|x\|\|y\|}$

This equation means that the octonions form a composition algebra. The higher-dimensional algebras defined by the Cayley–Dickson construction (starting with the sedenions) all fail to satisfy this property. They all have zero divisors.

Wider number systems exist which have a multiplicative modulus (for example, 16-dimensional conic sedenions). Their modulus is defined differently from their norm, and they also contain zero divisors.

As shown by Hurwitz, R, C, H, and O are the only normed division algebras over the real numbers. These four algebras also form the only alternative, finite-dimensional division algebras over the real numbers (up to isomorphism).

Not being associative, the nonzero elements of O do not form a group. They do, however, form a loop, specifically a Moufang loop.

The octonions play a significant role in the classification and construction of other mathematical entities. For example, the exceptional Lie group G₂ is the automorphism group of the octonions, and the other exceptional Lie groups F₄, E₆, E₇ and E₈ can be understood as the isometries of certain projective planes defined using the octonions.[13] The set of self-adjoint 3 × 3 octonionic matrices, equipped with a symmetrized matrix product, defines the Albert algebra. In discrete mathematics, the octonions provide an elementary derivation of the Leech lattice, and thus they are closely related to the sporadic simple groups.[14][15]

Applications of the octonions to physics have largely been conjectural. For example, in the 1970s, attempts were made to understand quarks by way of an octonionic Hilbert space.[16] It is known that the octonions, and the fact that only four normed division algebras can exist, relates to the spacetime dimensions in which supersymmetric quantum field theories can be constructed.[17][18] Also, attempts have been made to obtain the Standard Model of elementary particle physics from octonionic constructions, for example using the "Dixon algebra" C ⊗ H ⊗ O.[19][20]

Octonions have also arisen in the study of black hole entropy and quantum information science.[21][22]

Octonions have been used in solutions to the hand eye calibration problem in robotics.[23]

Deep octonion networks provide a means of efficient and compact expression in machine learning applications.[24]

There are several natural ways to choose an integral form of the octonions. The simplest is just to take the octonions whose coordinates are integers. This gives a nonassociative algebra over the integers called the Gravesian octonions. However it is not a maximal order (in the sense of ring theory); there are exactly seven maximal orders containing it. These 7=seven maximal orders are all equivalent under automorphisms. The phrase "integral octonions" usually refers to a fixed choice of one of these seven orders.

These maximal orders were constructed by Kirmse (1925) harvtxt error: no target: CITEREFKirmse1925 (help), Dickson and Bruck as follows. Label the eight basis vectors by the points of the projective line over the field with seven elements. First form the "Kirmse integers" : these consist of octonions whose coordinates are integers or half integers, and that are half integers (that is, halves of odd integers) on one of the 16 sets

∅ (∞124) (∞235) (∞346) (∞450) (∞561) (∞602) (∞013) (∞0123456) (0356) (1460) (2501) (3612) (4023) (5134) (6245)

of the extended quadratic residue code of length 8 over the field of two elements, given by ∅, (∞124) and its images under adding a constant modulo 7, and the complements of these eight sets. Then switch infinity and any one other coordinate; this operation creates a bijection of the Kirmse integers onto a different set, which is a maximal order. There are seven ways to do this, giving seven maximal orders, which are all equivalent under cyclic permutations of the seven coordinates 0123456. (Kirmse incorrectly claimed that the Kirmse integers also form a maximal order, so he thought there were eight maximal orders rather than seven, but as Coxeter (1946) pointed out they are not closed under multiplication; this mistake occurs in several published papers.)

The Kirmse integers and the seven maximal orders are all isometric to the E₈ lattice rescaled by a factor of ​¹⁄_√2. In particular there are 240 elements of minimum nonzero norm 1 in each of these orders, forming a Moufang loop of order 240.

The integral octonions have a "division with remainder" property: given integral octonions a and b ≠ 0, we can find q and r with a = qb + r, where the remainder r has norm less than that of b.

In the integral octonions, all left ideals and right ideals are 2-sided ideals, and the only 2-sided ideals are the principal ideals nO where n is a non-negative integer.

The integral octonions have a version of factorization into primes, though it is not straightforward to state because the octonions are not associative so the product of octonions depends on the order in which one does the products. The irreducible integral octonions are exactly those of prime norm, and every integral octonion can be written as a product of irreducible octonions. More precisely an integral octonion of norm mn can be written as a product of integral octonions of norms m and n.

The automorphism group of the integral octonions is the group G₂(F₂) of order 12096, which has a simple subgroup of index 2 isomorphic to the unitary group ²A₂(3²). The isotopy group of the integral octonions is the perfect double cover of the group of rotations of the E₈ lattice.

• "Cayley numbers", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
• Wilson, R. A. (2008), Octonions (PDF), Pure Mathematics Seminar notes