# Talk:Dual number

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I added a division section, showing how dual numbers can be divided. I'll add other calculation sections later on. I want to add this exponentiation stuff to the page, but I can't find an independent source for it. Anyone know of one?

Exponentiation

${(a+b\varepsilon)^{c+d\varepsilon}}$
$= {a^{c+d\varepsilon}(1+{b\varepsilon \over a^{c+d\varepsilon}})^{c+d\varepsilon}} = {a^c a^{d\varepsilon}(1+{b\varepsilon \over a^c a^{d\varepsilon}})^{c+d\varepsilon}}$
$= {a^c(1 + \varepsilon ln(a)d)(1+{b\varepsilon \over a^c})^{c+d\varepsilon}} = {a^c(1 + \varepsilon ln(a)d)(1+{b\varepsilon \over a^c(1 + \varepsilon ln(a)d)})^{c+d\varepsilon}}$
$= {a^c(1 + \varepsilon ln(a)d)(1+{b\varepsilon \over a^c})^{c+d\varepsilon}}$
$= {a^c(1 + \varepsilon ln(a)d)({e^{b\varepsilon \over a^c}})^{c+d\varepsilon}} = {a^c(1 + \varepsilon ln(a)d)e^{{b\varepsilon \over a^c}(c+d\varepsilon)}} = {a^c(1 + \varepsilon ln(a)d)(e^{{b\varepsilon c \over a^c}}e^{b\varepsilon d\varepsilon \over a^c})}$
$= {a^c(1 + \varepsilon ln(a)d)((1 + {bc\varepsilon \over a^c})e^0)}$
$= {(1 + \varepsilon ln(a)d)(a^c + bc\varepsilon)} = {a^c + bc\varepsilon + \varepsilon ln(a)d a^c + \varepsilon ln(a)d bc\varepsilon}$
$= {a^c + \varepsilon(bc + ln(a)da^c)}$

Which is definitely a dual number when a is greater than 0.

There's not really a need to explicitly write out the expressions for operations such as exponentiation. For any binary function on the reals, f, the natural extension to dual numbers is given by f(a+be,c+de) = f(a,c)+(b f1(a,c)+d f2(a,c))e, where f1 and f2 are the two partial derivatives of f with respect to its arguments. Sigfpe 01:02, 14 December 2006 (UTC)

I would like to know which Slavic languages in addition to Slovene use dual number.

I believe Slovenian and Sorbian are the two and only Slavic languages using the dual number. BT 18:07 23 Jun 2003 (UTC)

## References

I have removed the reference to Clifford since the assertion given was not true when I checked the text of his article. The dual numbers are well-recognized by this name, but the origin of this convention has not been provided. There is a 1906 reference to Joseph Grunbaum given at Inversive ring geometry#Historical notes.Rgdboer 23:22, 17 August 2006 (UTC)

## Category "Supernumber"?

I just saw the category entry "Supernumber", which is a term I've just now seen for the first time - can someone familiar with this concept elaborate or give reference(s)? I've recently rewritten and expanded the hypercomplex number article, which should include all algebraic systems with dimensionality that are commonly refered to as "numbers". If there's a distinct "supernumber" program that's not captured there, it should be added. Thanks, Jens Koeplinger 12:30, 25 August 2006 (UTC)

## Images

It would be nice to have a picture or two to illustrate dual numbers (assuming that's even possible). As it's written now, I can't quite grasp what a dual number is from the first couple of paragraphs. (Actually, I can't grasp the concept at all, but I might just be having a slow day today.) — Loadmaster 21:27, 17 April 2007 (UTC)

Yes, graphics would be helpful. Its really the cartesian plane with the multiplicative embellishment. A figure might accent the "circle" x = -/+ 1, the slope y/x , and the product by examples. Problems enter when pushing to close to C with its Euclidean fixity. For instance, z z* = xx for dual numbers, so the "modulus" fails to separate points on lines x = const. Also, the product on the "circle" calls for adding slopes, a dangerous suggestion for beginners in analytic geometry to see. *** Graphics would also be a help at split-complex number. Rgdboer 22:13, 29 April 2007 (UTC)

ε is kind of like a tiny real number. Having (2+3ε)(4-5ε) = 8+2ε is like having 2.0003×3.9995=8.000199985, but you only care about precision to the 0.0001 place, so you round it to 8.0002. DanBishop (talk) 07:19, 23 May 2009 (UTC)

The epsilon (ε) in this article has nothing to do with "a tiny real number" such as found in the article epsilon-delta argument. Instead this epsilon represents a unit vector perpendicular to the real line at the origin. Therefore it generates a plane out of the line and ε, points in the plane being represented by dual numbers x + y ε. An old phrase for this generation of a new direction from the real line is "an imaginary number". Just as the imaginary number i generates the ordinary complex plane with its algebra based on ii = − 1, so in this article the "imaginary number" ε generates the dual number plane with its algebra based on ε2 = 0. So there is nothing tiny about ε except its square, which is less than tiny.Rgdboer (talk) 21:07, 24 May 2009 (UTC)
"a unit vector...": this should be "the unit distance along a line orthogonal to the complex number plane" (or to the quaternion skew field, etc.). ε is not an imaginary number, it is an abstract number. It might be easier to think of , the product of real r and dual ε, as a collapsing sphere with radius proportional to r. Then x+ε is a collapsing unit sphere surrounding x. — Preceding unsigned comment added by 2601:C:A780:961:2C3:51FF:FE74:7AF (talk) 21:30, 15 February 2014 (UTC)

## Division

It seems like division isn't explained fully. In particular, it seems like there's a special case for when both real parts are zero:

$\frac{0+b\epsilon}{0+d\epsilon} = \frac{b}{d}$.

Is that right? —Ben FrantzDale 03:37, 5 May 2007 (UTC)

No, if w and z are dual numbers, the quotient w /z means w z −1 , and the inverse of z exists only when its real part is non-zero.Rgdboer 22:09, 7 May 2007 (UTC)

I just corrected the section because id contained a clear mistake: Division is not defined if the equation

$ax=b$

has no solution x but also if it has no unique solution. Otherwise, zero could be divided by itself because 0·x = b "has a solution" (rather infinitely many, in fact the solution space is entire given algebraic structure) if b=0.--Slow Phil (talk) 14:01, 10 January 2011 (UTC)

I don't agree with your change or with your explanation. Take two dual numbers a and b with:
$a := 0 + rd,$
$b := 0 + sd,$
$r, s \in \mathbb{R}, r \ne 0,$
$dd = 0.$
Then for
$ax=b$
there exists a unique solution:
$x=s/r$.
This is the essence of what you just changed. No? Thanks, Jens Koeplinger (talk) 22:12, 14 January 2011 (UTC)
Nevermind, I see what you mean. Thanks, Koeplinger (talk) 15:54, 15 January 2011 (UTC)

## Silly Question

So, being that both of them can be expressed as 2x2 matrices, can one do math on both complex numbers and dual numbers together? Maybe even split-complex numbers...

I know they are different number systems altogether pretty much, but was just wondering if the math would work out or if there was any way to combine complex/dual numbers or how they would work with eachother.

If I'm right that the 2x2 matrices are not compatible with eachother, then would a 2x2 matrix with each of the four numbers being a 2x2 matrix themselves work? If so, would it be a matrix representing a complex number with four dual numbers inside or vice versa? 71.120.201.39 19:56, 11 May 2007 (UTC)

Given the nature of matrix multiplication (that you can apply the same algorithm to blocks of the matrix), this should work. That is,
$\begin{bmatrix} a+b i & c+d i \\ 0 & a+ bi\end{bmatrix}$
should be equivalent to
$\begin{bmatrix} a & -b & c & -d \\ b & a & d & c \\ 0 & 0 & a & -b \\ 0 & 0 & b & a \end{bmatrix}$.
So yes. —Ben FrantzDale 20:35, 11 May 2007 (UTC)
Refer to Real matrices (2 x 2) for the breakdown of the general 2 x 2 real matrix into one of the three types of complex number: ordinary, split, or dual.Rgdboer 23:07, 12 May 2007 (UTC)

## Diagonal Matrix Elements

Should the diagonals of the matrix representation be (a / sqrt(2)) rather than a? The way it's written we would have (a + 0e)^2 == 2 * a^2 for pure real numbers.

## Moebius transformations and parabolic rotations

I would propose to add to Geometric section (after "multiplication" rotations were described as shears) the following passage:

Less trivial and more "parabolic" rotations of dual numbers can be obtained by a usage of Moebius transformation, see arXiv:0707.4024

By the way, the cited paper contains some pictures related to dual numbers, let me know if you would like to use them in the Wiki-article. V.V.Kisil 20:44, 20 August 2007 (UTC)

The extra parabolic rotations are developed from the concept of "Galilean angle" in Isaak Yaglom's book you cite in the bibliography of "Inventing a wheel, the parabolic one". I agree that the article would be improved by expanding on this idea. However, the Galilean invariance article has a more physical bent, and these rotations are likened to parabolic particle trajectories in Yaglom's text. The idea is at home in both places, I'd just been thinking of the other first. Your "Inventing a wheel" article is clear enough (at the outset) for beginning students, a real credit to your composition. Rgdboer 23:17, 22 August 2007 (UTC)

## Differentiation section

There are some primes missing. This is because the source markup contains primes quite with primes. I don't know how to escape primes within primes so maybe someone else could fix it. Sigfpe (talk) 15:51, 25 July 2008 (UTC)

## Algebraic Geometry

In algebraic geometry dual numbers over fields other than the reals are useful to define an infinitesimal deformation (essentially a flat family over the ring of dual numbers over the field of interest). 69.234.22.86 (talk) 02:49, 10 August 2008 (UTC)

## "Polar" form

Complex numbers have a polar form that simplifies multiplication and division: (r cis θ) × (s cis φ) = rs cis (θ + φ). Analogously, If we define P(a, β) = a + aβε, then P(a, β) × P(c, δ) = (a + aβε)(c + cδε) = ac+ac(β+δ)ε = P(ac, β+δ). Is there a standard notation for this? DanBishop (talk) 06:50, 23 May 2009 (UTC)

The cis notation, not uncommon but by no means standard, expresses the exponential function as in Euler's formula:
cis(θ) =$\ \exp (i \theta) = \cos \theta + i \sin \theta$.
In dual numbers one also has
$\exp (\theta \epsilon)\ \exp (\phi \epsilon) = \exp ((\theta + \phi) \epsilon)$.
Extending the cis notation into other number rings beyond the ordinary complex plane is not advisable; the study of transformation groups and their "infinitesimal algebras" by means of Lie theory refers to the exponential function at every juncture.Rgdboer (talk) 20:58, 24 May 2009 (UTC)

## Algebraic Properties section

Slow Phil - you've done an edit that concerns me. Let me go line by line (rather than rudely reverting it).

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 x2,
R[x]/x2.
The image of x in the quotient is the "imaginary" unit j.

The article here is about the dual numbers, I don't see why a statement about the split-complex numbers should be here. If it would relate to dual numbers then you would also have to clarify that $x^2 := 0$ is your ideal; because the split-complex, complex, and dual numbers all can be understood as a quotient ring $R [ x ] / x^2$. However, the next section "Generalization" already states that, so I see no need to duplicate this information.

With this description, it is clear that the dual numbers form a commutative ring with characteristic 0.

I don't like wording "... it is clear that ...". To me, it is already clear from the lead-in that the algebra is commutative, that it is a ring, and that its characteristic is zero. I see that the article in the current form doesn't state the system's characteristic yet, so it would seem sufficient to add a simple note somewhere, "... with characteristic 0". The reader then can follow the link to find out more about what that means. This would be informative, and would not make a judgment whether this property should or should not be clear to the reader.

Moreover if we define scalar multiplication in the obvious manner, the dual numbers actually form a commutative and associative algebra over the reals of dimension two.

Since the dual numbers are defined on a vector space over the reals, this includes scalar multiplication already. Again, I don't like to judge whether something should be "obvious" to the reader. Instead of adding this paragraph, I suggest adding (and referencing) vectors space, for example in the lead in: Change "The collection of dual numbers forms a particular two-dimensional commutative unital associative algebra over the real numbers." into "The dual numbers are a two dimensional vector space over the reals, equipped with a commutative, associative, and unital vector multiplication." Something alone these lines. Maybe the entire lead-in needs to be straightened out.

The algebra is not a division algebra or field since the imaginary elements are not invertible. In fact, all of the nonzero imaginary elements are zero divisors (also see the section "Division").

My suggestion here would be to maybe add to the lead-in: "The dual numbers are not a division algebra, since multiplication is generally not invertible" or similar. Then, the existing section "Division" in the article should probably be moved up, either before or after "Geometry". Personally I would put it before "Geometry", but others may feel differently. On a sidenote, I suggest against using emphasis (here: "not").

Overall, it looks as if the entire article could use a bit of smoothing out; but not by adding another section with mostly duplicate information. Thanks, Jens Koeplinger (talk) 19:46, 29 January 2011 (UTC)

PS: I've just read over the German version of this article, and see where your edit is coming from. Thank you for working on merging material across the languages! Koeplinger (talk) 20:01, 29 January 2011 (UTC)
In addition to Koeplinger's observations, the indeterminate of a polynomial ring is usually denoted with a capital X or Y. Fixed this and some other slips.Rgdboer (talk) 00:50, 30 January 2011 (UTC)

## "Parabolic numbers"?

I note that another name for dual numbers, parabolic numbers, has been added. I fail to readily find a mention thereof with a Google search. Is this name notable enough to mention? — Quondum 05:03, 5 November 2012 (UTC)

Perhaps, if we can find a source. Tkuvho (talk) 16:35, 3 December 2012 (UTC)

## Exponentiation

The following lines have been removed as they require a demonstration or reference:

Exponentiation of dual numbers follows the general rule:
$(a+b\varepsilon)^{c+d\varepsilon}=a^c+\varepsilon(b (c a^{c-1})+d (a^c \ln a))$

The subject can be discussed in this Talk space.Rgdboer (talk) 03:12, 23 December 2013 (UTC)

I haven't gone through the detail, but since the exponential mapping has such a simple form, the inverse mapping (the logarithm) seems straightforward to define, and presumably from this exponentiation (only for a > 0). So a demonstration should be easy, if it is considered worthwhile. However, I don't feel a burning need for this to be in the article. —Quondum 04:35, 23 December 2013 (UTC)