# Split-complex number

In algebra, a **split complex number** (or **hyperbolic number**, also **perplex number**, **double number**) has two real number components *x* and *y*, and is written *z* = *x* + *y j*, where *j*^{2} = 1. The *conjugate* of *z* is *z*^{∗} = *x* − *y j*. Since *j*^{2} = 1, the product of a number *z* with its conjugate is *zz*^{∗} = *x*^{2} − *y*^{2}, an isotropic quadratic form, *N*(*z*) = *x*^{2} − *y*^{2}.

The collection *D* of all split complex numbers *z* = *x* + *y j* for *x*, *y* ∈ **R** forms an algebra over the field of real numbers. Two split-complex numbers *w* and *z* have a product *wz* that satisfies *N*(*wz*) = *N*(*w*)*N*(*z*). This composition of *N* over the algebra product makes (*D*, +, ×, *) a composition algebra.

A similar algebra based on **R**^{2} and component-wise operations of addition and multiplication, (**R**^{2}, +, ×, *xy*), where *xy* is the quadratic form on **R**^{2}, also forms a quadratic space. The ring isomorphism

relates proportional quadratic forms, but the mapping is *not* an isometry since the multiplicative identity (1, 1) of **R**^{2} is at a distance √2 from 0, which is normalized in *D*.

Split-complex numbers have many other names; see *§ Synonyms* below. See the article *Motor variable* for functions of a split-complex number.

A **split-complex number** is an ordered pair of real numbers, written in the form

The collection of all such *z* is called the **split-complex plane**. Addition and multiplication of split-complex numbers are defined by

This multiplication is commutative, associative and distributes over addition.

Just as for complex numbers, one can define the notion of a **split-complex conjugate**. If

The conjugate satisfies similar properties to usual complex conjugate. Namely,

These three properties imply that the split-complex conjugate is an automorphism of order 2.

The **modulus** of a split-complex number *z* = *x* + *j* *y* is given by the isotropic quadratic form

However, this quadratic form is not positive-definite but rather has signature (1, −1), so the modulus is *not* a norm.

where *z* = *x* + *j* *y* and *w* = *u* + *j* *v*. Another expression for the modulus is then

Since it is not positive-definite, this bilinear form is not an inner product; nevertheless the bilinear form is frequently referred to as an *indefinite inner product*. A similar abuse of language refers to the modulus as a norm.

Split-complex numbers which are not invertible are called null vectors. These are all of the form (*a* ± *j* *a*) for some real number *a*.

There are two nontrivial idempotent elements given by *e* = (1 − *j*)/2 and *e*^{∗} = (1 + *j*)/2. Recall that idempotent means that *ee* = *e* and *e*^{∗}*e*^{∗} = *e*^{∗}. Both of these elements are null:

It is often convenient to use *e* and *e*^{∗} as an alternate basis for the split-complex plane. This basis is called the **diagonal basis** or **null basis**. The split-complex number *z* can be written in the null basis as

If we denote the number *z* = *ae* + *be*^{∗} for real numbers *a* and *b* by (*a*, *b*), then split-complex multiplication is given by

In this basis, it becomes clear that the split-complex numbers are ring-isomorphic to the direct sum **R** ⊕ **R** with addition and multiplication defined pairwise.

A two-dimensional real vector space with the Minkowski inner product is called (1 + 1)-dimensional Minkowski space, often denoted **R**^{1,1}. Just as much of the geometry of the Euclidean plane **R**^{2} can be described with complex numbers, the geometry of the Minkowski plane **R**^{1,1} can be described with split-complex numbers.

is a hyperbola for every nonzero *a* in **R**. The hyperbola consists of a right and left branch passing through (*a*, 0) and (−*a*, 0). The case *a* = 1 is called the unit hyperbola. The conjugate hyperbola is given by

with an upper and lower branch passing through (0, *a*) and (0, −*a*). The hyperbola and conjugate hyperbola are separated by two diagonal asymptotes which form the set of null elements:

These two lines (sometimes called the **null cone**) are perpendicular in **R**^{2} and have slopes ±1.

Split-complex numbers *z* and *w* are said to be hyperbolic-orthogonal if ⟨*z*, *w*⟩ = 0. While analogous to ordinary orthogonality, particularly as it is known with ordinary complex number arithmetic, this condition is more subtle. It forms the basis for the simultaneous hyperplane concept in spacetime.

This can be derived from a power series expansion using the fact that cosh has only even powers while that for sinh has odd powers. For all real values of the hyperbolic angle *θ* the split-complex number *λ* = exp(*jθ*) has norm 1 and lies on the right branch of the unit hyperbola. Numbers such as λ have been called hyperbolic versors.

Since λ has modulus 1, multiplying any split-complex number *z* by *λ* preserves the modulus of *z* and represents a *hyperbolic rotation* (also called a Lorentz boost or a squeeze mapping). Multiplying by *λ* preserves the geometric structure, taking hyperbolas to themselves and the null cone to itself.

The set of all transformations of the split-complex plane which preserve the modulus (or equivalently, the inner product) forms a group called the generalized orthogonal group O(1, 1). This group consists of the hyperbolic rotations, which form a subgroup denoted SO^{+}(1, 1), combined with four discrete reflections given by

sending *θ* to rotation by exp(*jθ*) is a group isomorphism since the usual exponential formula applies:

If a split-complex number *z* does not lie on one of the diagonals, then *z* has a polar decomposition.

In abstract algebra terms, the split-complex numbers can be described as the quotient of the polynomial ring **R**[*x*] by the ideal generated by the polynomial *x*^{2} − 1,

The image of *x* in the quotient is the "imaginary" unit *j*. With this description, it is clear that the split-complex numbers form a commutative algebra over the real numbers. The algebra is *not* a field since the null elements are not invertible. All of the nonzero null elements are zero divisors.

The split-complex numbers are isomorphic as an algebra to the direct product of **R** by itself. It is thus a reduced Artinian ring.

Since addition and multiplication are continuous operations with respect to the usual topology of the plane, the split-complex numbers form a topological ring.

The algebra of split-complex numbers forms a composition algebra since

From the definition it is apparent that the ring of split-complex numbers is isomorphic to the group ring **R**[C_{2}] of the cyclic group C_{2} over the real numbers **R**.

One can easily represent split-complex numbers by matrices. The split-complex number

Addition and multiplication of split-complex numbers are then given by matrix addition and multiplication. The modulus of *z* is given by the determinant of the corresponding matrix. In this representation, split-complex conjugation corresponds to multiplying on both sides by the matrix

For any real number *a*, a hyperbolic rotation by a hyperbolic angle *a* corresponds to multiplication by the matrix

so the two parametrized hyperbolas are brought into correspondence with *S*.

There is a great number of different representations of split-complex numbers into the 2×2 real matrices. In fact, every matrice whose square is the identity matrix gives such a representation.

The above diagonal representation represents the Jordan canonical form of the matrix representation of the split-complex numbers. For a split-complex number *z* = (*x*, *y*) given by the following matrix representation:

The use of split-complex numbers dates back to 1848 when James Cockle revealed his tessarines.^{[1]} William Kingdon Clifford used split-complex numbers to represent sums of spins. Clifford introduced the use of split-complex numbers as coefficients in a quaternion algebra now called split-biquaternions. He called its elements "motors", a term in parallel with the "rotor" action of an ordinary complex number taken from the circle group. Extending the analogy, functions of a motor variable contrast to functions of an ordinary complex variable.

expressing products on the unit hyperbola illustrates the additivity of rapidities for collinear velocities. Simultaneity of events depends on rapidity *a*;

is the line of events simultaneous with the origin in the frame of reference with rapidity *a*.

Two events *z* and *w* are hyperbolic-orthogonal when *z*^{∗}*w* + *zw*^{∗} = 0. Canonical events exp(*aj*) and *j* exp(*aj*) are hyperbolic orthogonal and lie on the axes of a frame of reference in which the events simultaneous with the origin are proportional to *j* exp(*aj*).

In 1933 Max Zorn was using the split-octonions and noted the composition algebra property. He realized that the Cayley–Dickson construction, used to generate division algebras, could be modified (with a factor gamma (γ)) to construct other composition algebras including the split-octonions. His innovation was perpetuated by Adrian Albert, Richard D. Schafer, and others.^{[8]} The gamma factor, with ℝ as base field, builds split-complex numbers as a composition algebra. Reviewing Albert for Mathematical Reviews, N. H. McCoy wrote that there was an "introduction of some new algebras of order 2^{e} over *F* generalizing Cayley–Dickson algebras."^{[9]} Taking *F* = ℝ and *e* = 1 corresponds to the algebra of this article.

In 1935 J.C. Vignaux and A. Durañona y Vedia developed the split-complex geometric algebra and function theory in four articles in *Contribución a las Ciencias Físicas y Matemáticas*, National University of La Plata, República Argentina (in Spanish). These expository and pedagogical essays presented the subject for broad appreciation.^{[10]}

In 1941 E.F. Allen used the split-complex geometric arithmetic to establish the nine-point hyperbola of a triangle inscribed in *zz*^{∗} = 1.^{[11]}

In 1956 Mieczyslaw Warmus published "Calculus of Approximations" in *Bulletin de l’Académie polonaise des sciences* (see link in References). He developed two algebraic systems, each of which he called "approximate numbers", the second of which forms a real algebra.^{[12]} D. H. Lehmer reviewed the article in Mathematical Reviews and observed that this second system was isomorphic to the "hyperbolic complex" numbers, the subject of this article.

In 1961 Warmus continued his exposition, referring to the components of an approximate number as midpoint and radius of the interval denoted.

Different authors have used a great variety of names for the split-complex numbers. Some of these include:

Split-complex numbers and their higher-dimensional relatives (split-quaternions / coquaternions and split-octonions) were at times referred to as "Musean numbers", since they are a subset of the hypernumber program developed by Charles Musès.