Split-complex number


In abstract algebra, a split complex number has two real number components x and y, and is written, where. The conjugate of z is. Since, the product of a number z with its conjugate is, an isotropic quadratic form,.
The collection D of all split complex numbers for forms an algebra over the field of real numbers. Two split-complex numbers w and z have a product wz that satisfies. This composition of N over the algebra product makes a composition algebra.
A similar algebra based on R2 and component-wise operations of addition and multiplication,, where xy is the quadratic form on R2, also forms a quadratic space. The ring isomorphism
relates proportional quadratic forms, but the mapping is an isometry since the multiplicative identity of R2 is at a distance from 0, which is normalized in D.
Split-complex numbers have many other names; see below. See the article Motor variable for functions of a split-complex number.

Definition

A split-complex number is an ordered pair of real numbers, written in the form
where x and y are real numbers and the quantity j satisfies
Choosing results in the complex numbers. It is this sign change which distinguishes the split-complex numbers from the ordinary complex ones. The quantity j here is not a real number but an independent quantity.
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.

Conjugate, modulus, and bilinear form

Just as for complex numbers, one can define the notion of a split-complex conjugate. If
the conjugate of z is defined as
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 is given by the isotropic quadratic form
It has the composition algebra property:
However, this quadratic form is not positive-definite but rather has signature, so the modulus is not a norm.
The associated bilinear form is given by
where and. 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.
A split-complex number is invertible if and only if its modulus is nonzero, thus x ± j x have no inverse. The multiplicative inverse of an invertible element is given by
Split-complex numbers which are not invertible are called null vectors. These are all of the form for some real number a.

The diagonal basis

There are two nontrivial idempotent elements given by and . Recall that idempotent means that and. 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 for real numbers a and b by, 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 with addition and multiplication defined pairwise.
The split-complex conjugate in the diagonal basis is given by
and the modulus by
Though lying in the same isomorphism class in the category of rings, the split-complex plane and the direct sum of two real lines differ in their layout in the Cartesian plane. The isomorphism, as a planar mapping, consists of a counter-clockwise rotation by 45° and a dilation by. The dilation in particular has sometimes caused confusion in connection with areas of a hyperbolic sector. Indeed, hyperbolic angle corresponds to area of a sector in the plane with its "unit circle" given by. The contracted "unit circle" of the split-complex plane has only half the area in the span of a corresponding hyperbolic sector. Such confusion may be perpetuated when the geometry of the split-complex plane is not distinguished from that of.

Geometry

A two-dimensional real vector space with the Minkowski inner product is called -dimensional Minkowski space, often denoted R1,1. Just as much of the geometry of the Euclidean plane R2 can be described with complex numbers, the geometry of the Minkowski plane R1,1 can be described with split-complex numbers.
The set of points
is a hyperbola for every nonzero a in R. The hyperbola consists of a right and left branch passing through and. The case is called the unit hyperbola. The conjugate hyperbola is given by
with an upper and lower branch passing through and. The hyperbola and conjugate hyperbola are separated by two diagonal asymptotes which form the set of null elements:
These two lines are perpendicular in R2 and have slopes ±1.
Split-complex numbers z and w are said to be hyperbolic-orthogonal if. 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.
The analogue of Euler's formula for the split-complex numbers is
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 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. 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 forms a group called the generalized orthogonal group. This group consists of the hyperbolic rotations, which form a subgroup denoted, combined with four discrete reflections given by
The exponential map
sending θ to rotation by exp 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.

Algebraic properties

In abstract algebra terms, the split-complex numbers can be described as the quotient of the polynomial ring R by the ideal generated by the polynomial,
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 ring with characteristic 0. Moreover, if we define scalar multiplication in the obvious manner, the split-complex numbers form a commutative and associative algebra of dimension two over the reals. The algebra is not a division algebra or field since the null elements are not invertible. All of the nonzero null elements are zero divisors.
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 of the cyclic group C2 over the real numbers R.

Matrix representations

One can easily represent split-complex numbers by matrices. The split-complex number
can be represented by the matrix
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
relates the action of the hyperbolic versor on D to squeeze mapping σ applied to R2
The diagonal basis for the split-complex number plane can be invoked by using an ordered pair for and making the mapping
Now the quadratic form is Furthermore,
so the two parametrized hyperbolas are brought into correspondence with S.
The action of hyperbolic versor then corresponds under this linear transformation to a squeeze mapping
Note that in the context of 2 × 2 real matrices there are in fact a great number of different representations of split-complex numbers. The above diagonal representation represents the Jordan canonical form of the matrix representation of the split-complex numbers. For a split-complex number given by the following matrix representation:
its Jordan canonical form is given by:
where and

History

The use of split-complex numbers dates back to 1848 when James Cockle revealed his tessarines. 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.
Since the late twentieth century, the split-complex multiplication has commonly been seen as a Lorentz boost of a spacetime plane. In that model, the number represents an event in a spacio-temporal plane, where x is measured in nanoseconds and y in Mermin's feet. The future corresponds to the quadrant of events which has the split-complex polar decomposition. The model says that z can be reached from the origin by entering a frame of reference of rapidity a and waiting ρ nanoseconds. The split-complex equation
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. Canonical events exp and are hyperbolic orthogonal and lie on the axes of a frame of reference in which the events simultaneous with the origin are proportional to.
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 to construct other composition algebras including the split-octonions. His innovation was perpetuated by Adrian Albert, Richard D. Schafer, and others. 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 2e over F generalizing Cayley–Dickson algebras." Taking and 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. These expository and pedagogical essays presented the subject for broad appreciation.
In 1941 E.F. Allen used the split-complex geometric arithmetic to establish the nine-point hyperbola of a triangle inscribed in .
In 1956 Mieczyslaw Warmus published "Calculus of Approximations" in Bulletin de l’Académie polonaise des sciences. He developed two algebraic systems, each of which he called "approximate numbers", the second of which forms a real algebra. 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.

Synonyms

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 were at times referred to as "Musean numbers", since they are a subset of the hypernumber program developed by Charles Musès.