# Hypercomplex number

In mathematics, a hypercomplex number is a traditional term for an element of an algebra over a field where the field is the real numbers or the complex numbers. In the nineteenth century number systems called quaternions, tessarines, coquaternions, biquaternions, and octonions became established concepts in mathematical literature. The concept of a hypercomplex number covered them all, and called for a science to explain and classify them.

The cataloguing project began in 1872 when Benjamin Peirce first published his Linear Associative Algebra, and was carried forward by his son Charles Sanders Peirce.[1] Most significantly, they identified the nilpotent and the idempotent elements as useful hypercomplex numbers for classifications. The Cayley–Dickson construction used involutions to generate complex numbers, quaternions, and octonions out of the real number system. Hurwitz and Frobenius proved theorems that put limits on hypercomplexity: Hurwitz's theorem (normed division algebras), and Frobenius theorem (real division algebras).

It was matrix algebra that harnessed the hypercomplex systems. First, matrices contributed new hypercomplex numbers like 2 × 2 real matrices. Soon the matrix paradigm began to explain the others as they became represented by matrices and their operations. In 1907 Joseph Wedderburn showed that associative hypercomplex systems could be represented by matrices, or direct sums of systems of matrices. From that date the preferred term for a hypercomplex system became associative algebra as seen in the title of Wedderburn’s thesis at University of Edinburgh. Note however, that non-associative systems like octonions and hyperbolic quaternions represent another type of hypercomplex number.

As Hawkins (1972) explains, the hypercomplex numbers are stepping stones to learning about Lie groups and group representation theory. For instance, in 1929 Emmy Noether at Bryn Mawr wrote on "hypercomplex quantities and representation theory". Review of the historic particulars gives body to the generalities of modern theory. In 1973 Kantor and Soldovnikov published a textbook on hypercomplex numbers which was translated in 1989; a reviewer says it has a "highly classical flavour". See Karen Parshall (1985) for a detailed exposition of the heyday of hypercomplex numbers, including the role of such luminaries as Theodor Molien and Eduard Study. For the transition to modern algebra, Bartel van der Waerden devotes thirty pages to hypercomplex numbers in his History of Algebra (1985).

## Catalogue

A definition of hypercomplex number is given by Kantor & Solodovnikov (1989) as finite dimensional algebras over the reals that are unital and distributive (but not necessarily associative). Elements are generated with real number coefficients $(a_0, \dots, a_n)$ for a basis $\{ 1, i_1, \dots, i_n \}$. If possible, it is conventional to "orthonormalize" the basis so that the product of two basis elements is again a single basis element (or its negative or zero), and especially, if possible, so that $i_k^2 \in \{ -1, 0, +1 \}$ (which is certainly impossible if the algebra contains a purely inaginary subalgebra or ideal whos basis elements don't all sqare to zero). A technical approach to hypercomplex numbers directs attention first to those of dimension two. Higher dimensions are configured as Cliffordian or algebraic sums of other algebras.

## Two-dimensional real algebras

Theorem:[2] [3][4] Up to isomorphism, there are exactly three 2-dimensional unital algebras over the reals: the ordinary complex numbers, the split-complex numbers, and the dual numbers.

proof: Since the algebra is closed under squaring, and it has but two dimensions, the non-real basis element u squares to an arbitrary linear combination of 1 and u:
$u^2=a_0+a_1u$

with arbitrary real numbers a0 and a1. Using the common method of completing the square by subtracting a1u and adding the quadratic complement a1²/4 to both sides yields

$u^2-a_1u+\frac{a_1^2}{4}=a_0+\frac{a_1^2}{4} .$
$u^2-a_1u+\frac{a_1^2}{4}=\left(u-\frac{a_1}{2}\right)^2= \tilde{u}^2$  so that
$\tilde{u}^2~=a_0+\frac{a_1^2}{4} .$

The three cases depend on this real value:

• If 4a0 = −a 2
1

, the above formula yields ũ2 = 0. Hence, ũ can directly be identified with the nilpotent element $\epsilon$ of the Dual numbers' basis $\{ 1, ~\epsilon \}$.
• If 4a0 > −a 2
1

, the above formula yields ũ2 > 0. This leads to the split-complex numbers which have normalized basis $\{ 1 , ~j \}$ with $j^2 = +1$. To obtain j from ũ, the latter must be divided by the positive real number $a:=\sqrt{a_0+\frac{a_1^2}{4}}$ which has the same square as ũ.
• If 4a0 < −a 2
1

, the above formula yields ũ2 < 0. This leads to the complex numbers which have normalized basis $\{ 1 , ~i \}$ with $i^2 = -1$. To yield i from ũ, the latter has to be divided by a positive real number $a:=\sqrt{\frac{a_1^2}{4}-a_0}$ which squares to the negative of ũ2.

The complex numbers are the only two-dimensional hypercomplex algebra that is a field. Algebras such as the split-complex numbers that include non-real roots of 1 also contain idempotents $\tfrac{1}{2} (1 \pm j)$ and zero divisors $(1 + j)(1 - j) = 0$, so such algebras cannot be division algebras. However, these properties can turn out to be very meaningful, for instance in describing the Lorentz transformations of special relativity.

## Higher dimensional examples (more than one non-real axis)

### Clifford algebras

Clifford algebra is the unital associative algebra generated over an underlying vector space equipped with a quadratic form. Over the real numbers this is equivalent to being able to define a symmetric scalar product, uv = ½(uv + vu) that can be used to orthogonalise the quadratic form, to give a set of bases {e1, ..., ek} such that:

$\tfrac{1}{2} (e_i e_j + e_j e_i) = \Bigg\{ \begin{matrix} -1, 0, +1 & i=j, \\ 0 & i \not = j. \end{matrix}$

Imposing closure under multiplication now generates a multivector space spanned by 2k bases, {1, e1, e2, e3, ..., e1e2, ..., e1e2e3, ...}. These can be interpreted as the bases of a hypercomplex number system. Unlike the bases {e1, ..., ek}, the remaining bases may or may not anti-commute, depending on how many simple exchanges must be carried out to swap the two factors. So e1e2 = −e2e1; but e1(e2e3) = +(e2e3)e1.

Putting aside the bases for which ei2 = 0 (i.e. directions in the original space over which the quadratic form was degenerate), the remaining Clifford algebras can be identified by the label Cp,q(R) indicating that the algebra is constructed from p simple bases with ei2 = +1, q with ei2 = −1, and where R indicates that this is to be a Clifford algebra over the reals—i.e. coefficients of elements of the algebra are to be real numbers.

These algebras, called geometric algebras, form a systematic set which turn out to be very useful in physics problems which involve rotations, phases, or spins, notably in classical and quantum mechanics, electromagnetic theory and relativity.

Examples include: the complex numbers C0,1(R); split-complex numbers C1,0(R); quaternions C0,2(R); split-biquaternions C0,3(R); coquaternions C1,1(R) ≈ C2,0(R) (the natural algebra of 2d space); C3,0(R) (the natural algebra of 3d space, and the algebra of the Pauli matrices); and C1,3(R) the spacetime algebra.

The elements of the algebra Cp,q(R) form an even subalgebra C0q+1,p(R) of the algebra Cq+1,p(R), which can be used to parametrise rotations in the larger algebra. There is thus a close connection between complex numbers and rotations in 2D space; between quaternions and rotations in 3D space; between split-complex numbers and (hyperbolic) rotations (Lorentz transformations) in 1+1 D space, and so on.

Whereas Cayley–Dickson and split-complex constructs with eight or more dimensions are not associative anymore with respect to multiplication, Clifford algebras retain associativity at any dimensionality.

In 1995 Ian R. Porteous wrote on "The recognition of subalgebras" in his book on Clifford algebras. His Proposition 11.4 summarizes the hypercomplex cases:

Let A be a real associative algebra with unit element 1. Then
• any two-dimensional subalgebra generated by an element e0 of A such that e02 = −1 is isomorphic to C (algebra of complex numbers),
• any four-dimensional subalgebra generated by a set {e0, e1} of mutually anti-commuting elements of A such that $e_0 ^2 = e_1 ^2 = -1$ is isomorphic to H (algebra of quaternions),
• any four-dimensional subalgebra generated by a set {e0, e1} of mutually anti-commuting elements of A such that $e_0 ^2 = e_1 ^2 = 1$ is isomorphic to R(2) (2 × 2 real matrices, coquaternions),
• any eight-dimensional subalgebra generated by a set {e0, e1, e2} of mutually anti-commuting elements of A such that $e_0 ^2 = e_1 ^2 = e_2 ^2 = -1$ is isomorphic to 2H (split-biquaternions),
• any eight-dimensional subalgebra generated by a set {e0, e1, e2} of mutually anti-commuting elements of A such that $e_0 ^2 = e_1 ^2 = e_2 ^2 = 1$ is isomorphic to C(2) (biquaternions, Pauli algebra, 2 × 2 complex matrices).

For extension beyond the classical algebras, see Classification of Clifford algebras.

### Cayley–Dickson construction

All of the Clifford algebras Cp,q(R) apart from the complex numbers and the quaternions contain non-real elements j that square to 1; and so cannot be division algebras. A different approach to extending the complex numbers is taken by the Cayley–Dickson construction. This generates number systems of dimension 2n, n in {2, 3, 4, ...}, with bases $\{1, i_1, \dots, i_{2^n-1}\}$, where all the non-real bases anti-commute and satisfy $i_m^2 = -1$. In eight or more dimensions these algebras are non-associative.

The first algebras in this sequence are the four-dimensional quaternions, eight-dimensional octonions, and 16-dimensional sedenions. However, satisfying these requirements comes at a price: Each increase in dimensionality involves a loss of algebraic symmetry: Quaternion multiplication is not commutative, octonion multiplication is non-associative, and the norm of sedenions is not multiplicative.

The Cayley–Dickson construction can be modified by inserting an extra sign at some stages. It then generates two of the "split algebras" in the collection of composition algebras:

split-quaternions with basis $\{ 1, i_1, i_2, i_3 \}$ satisfying $\ i_1^2 = -1, i_2^2 = i_3^2 = +1$, ) and
split-octonions with basis $\{ 1, i_1, \dots, i_7 \}$ satisfying $\ i_1^2 = i_2^2 = i_3^2 = -1$, $\ i_4^2 = \cdots = i_7^2 = +1 .$

The split-quaternions contain nilpotents, have a non-commutative multiplication, and are isomorphic to the 2 × 2 real matrices. Split-octonions are non-associative.

### Tensor products

The tensor product of any two algebras is another algebra, which can be used to produce many more examples of hypercomplex number systems.

In particular taking tensor products with the complex numbers (considered as algebras over the reals) leads to four-dimensional tessarines $\mathbb C\otimes_\mathbb{R}\mathbb C$, eight-dimensional biquaternions $\mathbb C\otimes_\mathbb{R}\mathbb H$, and 16-dimensional complex octonions $\mathbb C\otimes_\mathbb{R}\mathbb O$.