# Is there a third dimension of numbers?

Is there a third dimension of numbers like real numbers, imaginary numbers, [blank] numbers?

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– Aryabhata Apr 10 '11 at 16:30
Wow, that is annoying. Are three-dimensional numbers even possible/necessary? – awesomeguy Apr 10 '11 at 16:42

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Quaternions are four-dimensional, but they are used in computer graphics to deal with 3-D situations. – André Nicolas Apr 10 '11 at 16:54

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A good hint that there is no three dimensional numbers is the fact that $(a_1^3 + b_1^3 + c_1^3)(a_1^3 + b_2^3 + c_2^3)$ doesn't always equal the sum of three cube numbers (the axiom that product of two three cubes is another three cube). The quaternions barely satisfy the condition for four dimensional except for commutativity. – Mark Apr 10 '11 at 20:27

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EDIT for above comment: I accidentally wrote cubes for some reason. I meant the product of three squares $(a_1^2+b_1^2+c_1^2)(a_2^2+b_2^2+c_2^2)$ doesn't always equal the sum of three square numbers. It's well known in any elementary number theory book that Euler proved its true for product of four square hinting that four dimensional numbers exist. – Mark Apr 10 '11 at 22:36

## 7 Answers

Alas, there are no algebraically coherent "triplexes". The next step in the construction as has been said already are "quaternions" with 4 dimensions.

Many young aspiring mathematicians have tried to find them since Hamilton in the 19th century. This impossibility links geometric dimensionality, fundamental properties of polynomial equations, algebraic systems and many other aspects of mathematics. It is really worth studying.

A quite recent book by modern mathematicians which details all this for advanced college undergraduates is Numbers by Ebbinghaus, Hermes, Hirzebruch, Koecher, Mainzer, Neukirch, Prestel, Remmert, and Ewing.

However, the set of quaternions with zero real part is an interesting system of dimension 3 with very interesting properties, linked to the composition of rotations in space.

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You may also find of interest some more general results besides the mentioned Frobenius Theorem. Weierstrass (1884) and Dedekind (1885) showed that every finite dimensional commutative extension ring of $\mathbb R$ without nilpotents ($\rm\:x^n = 0 \ \Rightarrow\ x = 0\:$) is isomorphic as a ring to a direct sum of copies of $\rm\:\mathbb R\:$ and $\rm\:\mathbb C\:.\:$ Wedderburn and Artin proved a generalization that every finite-dimensional associative algebra without nilpotent elements over a field $\rm\:F\:$ is a finite direct sum of fields. Such structure theoretic results greatly simplify classifying such rings when they arise in the wild. For example, I applied a special case of these results last week to prove that a finite ring is a field if its units $\cup\ \{0\}$ comprise a field of characteristic $\ne 2\:.\:$ For another example, a sci.math reader once proposed an extension of the real numbers with multiple "signs". This turns out to be a very simple case of the above results. Below is my 2009.6.16 sci.math post on this topic.

The results in Eitzen's paper Understanding PolySign Numbers the Standard Way, characterizing Tim Golden's so-called PolySign numbers as ring direct sums of $\mathbb R$ and $\mathbb C$, have been known for over a century and a half. Namely that $\rm\:P_n =\: \mathbb R[x]/(1+x+x^2+\:\cdots\: + x^{n-1})\$ is isomorphic to a certain ring direct sum of $\:\mathbb R$ and $\:\mathbb C\:,\:$ is just a special case of more general results due to Weierstrass and Dedekind in the 1860s. These classic results are so well-known that you will find them mentioned even in many elementary textbooks on number systems and their generalizations. For example, in Numbers by Ebbinghaus et.al. p.120:

Weierstrass (1884) and Dedekind (1885) showed that every finite dimensional commutative ring extension of R with unit element but without nilpotent elements, is isomorphic to a ring direct sum of copies of R and C.

Ditto for historical expositions, e.g. Bourbaki's Elements of the History of Mathematics, p. 119:

By 1861, Weierstrass, making precise a remark of Gauss, had, in his lectures, characterized commutative algebras without nilpotent elements over R or C as direct products of fields (isomorphic to R or C); Dedekind had on his side reached the same conclusions around 1870, in connection with his "hypercomplex" conception of the theory of commutative fields, their proofs were published in 1884-85 [1,2]. [...] These methods rely above all on the consideration of the characteristic polynomial of an element of the algebra relative to its regular representation (a polynomial already met in the work of Weierstrass and Dedekind quoted earlier) and on the decomposition of the polynomial into irreducible factors.

Nowadays these fundamental results are merely special cases of more general structure theories for algebras that are part of any first course on algebras (but not always met in a first course on abstract algebra). A web search turns up more on the subsequent history, e.g. excerpted from

Y. M. Ryabukhin, Algebras without nilpotent elements, I,
Algebra i Logika, Vol. 8, No. 2, pp. 181-214, March-April, 1969

Algebras without nilpotent elements have been studied long ago. So, Weierstrass characterized in his lectures in 1861 finite-dimensional associative-commutative algebras without nilpotent elements over the field of real or complex numbers as finite direct sums of fields. To be exact, some nonessential restrictions have there been imposed. In 1870 Dedekind removed those nonessential restrictions. The following theorem of Weierstrass-Dedekind is now considered as a classical one: every finite-dimensional associative-commutative algebra without nilpotent elements over a field F is a finite direct sum of fields. The results of Weierstrass and Dedekind (for the case when F is the field of complex or real numbers) have been published in [1,2]. The results of works of Molien, Cartan, Wedderburn and Artin [3-6] imply that Dedekind's theorem holds for any field F. Moreover, the following theorem of Wedderburn-Artin holds: every finite-dimensional associative algebra without nilpotent elements over a field F is a finite direct sum of fields." [...]

1. K. Weierstrass, "Zur Theorie der aus n Haupteinheiten gebildeten complexen Grossen," Gott. Nachr. (1884).
2. R. Dedekind, "Zur Theorie der aus n Haupteinheiten gebildeten complexen Grossen," Gott. Nachr. (1885).
3. F. Molien, "Ueber Systeme hoherer complexer Zahlen," Math. Ann., XLI, 83-156 (1893).
4. E. Cartan, "Les groupes bilineaires et les systemes de nombres complexes," Ann. Fac. Sci., Toulouse (1898).
5. J. Wedderburn, "On hypercomplex numbers," Proc. London Math. Soc. (2), VI, 349-352 (1908).
6. E. Artin, "Zur Theorie der hyperkomplexen Zahlen," Abh. Math. Sere. Univ. Hamburg, 5, 251-260 (1927).

and excerpted from its sequel

Y.M. Ryabukhin, Algebras without nilpotent elements, II,
Algebra i Logika, Vol. 8, No. 2, pp. 215-240, March-April, 1969

In [1] we proved structural theorems on the decomposition of algebras without nilpotent elements into direct sums of division algebras; certain chain conditions were imposed on these algebras.

Yet it is possible to prove structural theorems also without imposing any chain conditions. In this case the direct sums are replaced by subdirect sums and instead of division algebras we shall consider algebras without zero divisors.

The first structural theorem of this kind is apparently the classical theorem of Krull [2]:

Any associative-commutative ring without nilpotent elements can be represented by a subdirect sum of rings without zero divisors. Krull's theorem was subsequently extended to the case of any associative ring. This was done by various authors and in various directions. In [3], Thierrin came very close to a final generalization of Krull's theorem to the associative, but not commutative case. The final result was obtained in [4]:

Any associative ring without nilpotent elements can be represented by a subdirect sum of rings without zero divisors. At the Ninth All-Union Conference on General Algebra (held at Gomel'), I. V. L'vov reported an even stronger result:

Any alternative ring without nilpotent elements can be represented by a subdirect sum of rings without zero divisors.

It could be assumed that the theorem on decomposition into a subdirect sum of algebras without zero divisors holds for any ring. Yet this assumption is erroneous (see [1]), since there exists a finite-dimensional simple, special Jordan algebra without nilpotent elements that has zero divisors and cannot therefore be decomposed into a subdirect sum of algebras (or rings) without zero divisors.

There naturally arises the following question: what conditions must a ring without nilpotent elements satisfy to permit its representation by a subdirect sum of rings without zero divisors?

In this paper we answer this question:

An algebra R over an associative-commutative ring F with unity can be represented by a subdirect sum of rings without zero divisors, iff it is a conditionally associative algebra without nilpotent elements.

Let us recall that an algebra R is said to be conditionally associative, iff we have in R the conditional identity x(yz) = 0 iff (xy)z = 0.

We say a (not necessarily associative) algebra R does not have nilpotent elements, iff in R we have the conditional identity x^2 = 0 iff x = 0.

From this theorem we easily obtain the above-mentioned results of [2-4], as well as the result of L'vov (it suffices to take as the ring F the ring Z of integers). [...]

1. Yu. M. Ryabukhin, "Algebras without nilpotent elements,I," this issue, pp. 215-240.
2. W. Krull, "Subdirect representations of sums of integral domains," Math. Z., 52, 810-823 (1950).
3. J. Thierrin, "Completely simple ideals of a ring," Acad. Belg. Bull. C1. Sci., 5 N 43, 124-132 (1957}.
4. V. A. Andrunakievich and Yu. M. Ryabukhin, "Rings without nilpotent elements in completely simple ideals," DAN SSSR, 180, No. 1, 9 (1968).
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Bill, something seems to be wrong with the title of Dedekind's paper. Two blanks are missing. When you add these, the title becomes identical to Weierstrass' which also seems weird to me. – Rasmus Apr 10 '11 at 20:15

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@Ras No, it's correct. The papers do in fact have the same titles. – Gone Apr 10 '11 at 21:04
Interesting, was it sort of a collaboration between them? – Rasmus Apr 11 '11 at 7:15

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Wow. This must have taken hours! Sadly, I am limited to (+1)... – The Chaz 2.0 Jul 13 '11 at 2:51

Every finite-dimensional division algebra over $\mathbb{R}$ is one of $\mathbb{R}$, $\mathbb{C}$ or $\mathbb{H}$. This is what is called the Frobenius Theorem. You may refer to here for details.

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Every finite dimensional associative division algebra over $\mathbb R$ is one of the above. The octonions $\mathbb O$ is also a division algebra, but it is not associative. – Calle Apr 10 '11 at 17:29
@sis440: You mean Frobenius. More broadly, you might also like to lookup theorems by Dickson, Artin and Wederburn which gives a broader picture of the classification of algebra. – ogerard Apr 10 '11 at 17:35

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Oh, I forgot that division algebra need not be associaitive... You are right, $\mathbb{O}$ has to be added if associativity is omitted. – sos440 Apr 10 '11 at 17:35

You might look up quaternions.

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This answer is useful. – The Chaz 2.0 Jul 13 '11 at 2:52
At the time, I did not know how to insert links. – André Nicolas Jul 13 '11 at 3:26
I was hovering over the +1 and it said "This answer is useful". I agree(d), and clicked! ..and quoted the popup  :) – The Chaz 2.0 Jul 13 '11 at 3:39

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Will try hovering over one of your answers, and do what the gnome in the machine asks me to do. – André Nicolas Jul 13 '11 at 4:31

In addition to complex numbers and quaternions, you might want to look up Clifford Algebras which encapsulate both and extend to arbitrary dimension. Complex and quaternios are sub-algebras of the Clifford Algebras over $\mathbb{R}^2$ and $\mathbb{R}^3$ respectively.

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There are of course, examples of R3, in the form X, Y, Z, which behave as three separate real numbers, with an invarient when X -> Y -> Z -> X applies.

Such numbers turn up in the study of the heptagon and enneagon. The integer systems are discrete points forming a lattice in this space, the invariants cycle through the solutions to the heptagonal ($x^3-x^2+2x-1=0$) and enneagonal equations, which transforms eg {7} to {7/3} to {7/2}.

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This answer makes no sense. – Zev Chonoles♦ 2 days ago
I'm pretty sure that the process is similar to the process of replacing x+iY by x-iy, but through the solutions of 3d.  If you replace i² = -1 by j² = +1, you get a hyper-complex space, which transforms a+b sqrt(5) into a-b sqrt(5).  It can be done over any number of dimensions, if the values solve an integer polynomial of the same order. – wendy.krieger 2 days ago

If you think of the dimensions for numbers as going real numbers (1st dimension), fuzzy numbers (2nd dimension), then the 3rd dimension ends up fuzzy numbers of dimension two. For more details see A. Kaufmann and M. M. Gupta's Introduction to Fuzzy Arithmetic or George and Maria Bojadziev's Fuzzy Sets, Fuzzy Logic, Applications.

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