Dual number

In algebra, the dual numbers are a hypercomplex number system first introduced in the 19th century. They are expressions of the form , where and are real numbers, and is a symbol taken to satisfy formula_1.
Dual numbers can be added componentwise, and multiplied by the formula
which follows from the property and the fact that multiplication is a bilinear operation.
The dual numbers form a commutative algebra of dimension two over the reals, and also an Artinian local ring. They are one of the simplest examples of a ring that has nonzero nilpotent elements.
History.
Dual numbers were introduced in 1873 by William Clifford, and were used at the beginning of the twentieth century by the German mathematician Eduard Study, who used them to represent the dual angle which measures the relative position of two skew lines in space. Study defined a dual angle as , where is the angle between the directions of two lines in three-dimensional space and is a distance between them. The -dimensional generalization, the Grassmann number, was introduced by Hermann Grassmann in the late 19th century.
Definition in abstract algebra.
In abstract algebra, the algebra of dual numbers is often defined as the quotient of a polynomial ring over the real numbers formula_3 by the principal ideal generated by the square of the indeterminate, that is
Representation in algebras.
The dual number formula_5 can be represented by the matrix formula_6. This works because the matrix formula_7 squares to the zero matrix, similar to the dual number formula_8.
There are other ways to represent dual numbers as matrices. Let us consider just the case of formula_9 real matrices. Assuming that the dual number formula_10 is represented by the identity matrix, then formula_8 can be represented by any matrix of the form
where formula_13 except when formula_14
Differentiation.
One application of dual numbers is automatic differentiation. Consider the real dual numbers above. Given any real polynomial , it is straightforward to extend the domain of this polynomial from the reals to the dual numbers. Then we have this result:
where is the derivative of .
More generally, we can extend any (analytic) real function to the dual numbers by looking at its Taylor series:
since all terms of involving or greater are trivially 0 by the definition of .
By computing compositions of these functions over the dual numbers and examining the coefficient of in the result we find we have automatically computed the derivative of the composition.
A similar method works for polynomials of variables, using the exterior algebra of an -dimensional vector space.
Geometry.
The "unit circle" of dual numbers consists of those with since these satisfy where . However, note that
so the exponential map applied to the -axis covers only half the "circle".
Let . If and , then is the polar decomposition of the dual number , and the slope is its angular part. The concept of a "rotation" in the dual number plane is equivalent to a vertical shear mapping since .
In absolute space and time the Galilean transformation
that is
relates the resting coordinates system to a moving frame of reference of velocity . With dual numbers representing events along one space dimension and time, the same transformation is effected with multiplication by .
Cycles.
Given two dual numbers and , they determine the set of such that the difference in slopes ("Galilean angle") between the lines from to and is constant. This set is a cycle in the dual number plane; since the equation setting the difference in slopes of the lines to a constant is a quadratic equation in the real part of , a cycle is a parabola. The "cyclic rotation" of the dual number plane occurs as a motion of its projective line. According to Isaak Yaglom, the cycle is invariant under the composition of the shear
with the translation
Division.
Division of dual numbers is defined when the real part of the denominator is non-zero. The division process is analogous to complex division in that the denominator is multiplied by its conjugate in order to cancel the non-real parts.
Therefore, to divide an equation of the form
we multiply the top and bottom by the conjugate of the denominator:
which is defined when is non-zero.
If, on the other hand, is zero while is not, then the equation
This means that the non-real part of the "quotient" is arbitrary and division is therefore not defined for purely nonreal dual numbers. Indeed, they are (trivially) zero divisors and clearly form an ideal of the associative algebra (and thus ring) of the dual numbers.
Applications in mechanics.
Dual numbers find applications in mechanics, notably for kinematic synthesis. For example, the dual numbers make it possible to transform the input/output equations of a four-bar spherical linkage, which includes only rotoid joints, into a four-bar spatial mechanism (rotoid, rotoid, rotoid, cylindrical). The dualized angles are made of a primitive part, the angles, and a dual part, which has units of length. See screw theory for more.
Generalizations.
This construction can be carried out more generally: for a commutative ring one can define the dual numbers over as the quotient of the polynomial ring by the ideal : the image of then has square equal to zero and corresponds to the element from above.
Arbitrary module of elements of zero square.
There is a more general construction of the dual numbers. Given a commutative ring formula_25 and a module formula_26, there is a ring formula_27 called the ring of dual numbers which has the following structures:
It is the formula_25-module formula_29 with the multiplication defined by formula_30 for formula_31 and formula_32
The algebra of dual numbers is the special case where formula_33 and formula_34
Superspace.
Dual numbers find applications in physics, where they constitute one of the simplest non-trivial examples of a superspace. Equivalently, they are supernumbers with just one generator; supernumbers generalize the concept to distinct generators , each anti-commuting, possibly taking to infinity. Superspace generalizes supernumbers slightly, by allowing multiple commuting dimensions.
The motivation for introducing dual numbers into physics follows from the Pauli exclusion principle for fermions. The direction along is termed the "fermionic" direction, and the real component is termed the "bosonic" direction. The fermionic direction earns this name from the fact that fermions obey the Pauli exclusion principle: under the exchange of coordinates, the quantum mechanical wave function changes sign, and thus vanishes if two coordinates are brought together; this physical idea is captured by the algebraic relation .
Projective line.
The idea of a projective line over dual numbers was advanced by Grünwald and Corrado Segre.
Just as the Riemann sphere needs a north pole point at infinity to close up the complex projective line, so a line at infinity succeeds in closing up the plane of dual numbers to a cylinder.
Suppose is the ring of dual numbers and is the subset with . Then is the group of units of . Let . A relation is defined on B as follows: when there is a in such that and . This relation is in fact an equivalence relation. The points of the projective line over are equivalence classes in under this relation: . They are represented with projective coordinates .
Consider the embedding by . Then points , for , are in but are not the image of any point under the embedding. is mapped onto a cylinder by projection: Take a cylinder tangent to the double number plane on the line , . Now take the opposite line on the cylinder for the axis of a pencil of planes. The planes intersecting the dual number plane and cylinder provide a correspondence of points between these surfaces. The plane parallel to the dual number plane corresponds to points , in the projective line over dual numbers.