Wheel Theory
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A wheel is a type of
algebra Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...
(in the sense of
universal algebra Universal algebra (sometimes called general algebra) is the field of mathematics that studies algebraic structures in general, not specific types of algebraic structures. For instance, rather than considering groups or rings as the object of stud ...
) where division is always defined. In particular,
division by zero In mathematics, division by zero, division (mathematics), division where the divisor (denominator) is 0, zero, is a unique and problematic special case. Using fraction notation, the general example can be written as \tfrac a0, where a is the di ...
is meaningful. The
real number In mathematics, a real number is a number that can be used to measure a continuous one- dimensional quantity such as a duration or temperature. Here, ''continuous'' means that pairs of values can have arbitrarily small differences. Every re ...
s can be extended to a wheel, as can any
commutative ring In mathematics, a commutative ring is a Ring (mathematics), ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring prope ...
. The term ''wheel'' is inspired by the
topological Topology (from the Greek words , and ) is the branch of mathematics concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, wit ...
picture \odot of the
real projective line In geometry, a real projective line is a projective line over the real numbers. It is an extension of the usual concept of a line that has been historically introduced to solve a problem set by visual perspective: two parallel lines do not int ...
together with an extra point ⊥ (
bottom element In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined dually, that is, it is an ele ...
) such that \bot = 0/0. A wheel can be regarded as the equivalent of a
commutative ring In mathematics, a commutative ring is a Ring (mathematics), ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring prope ...
(and
semiring In abstract algebra, a semiring is an algebraic structure. Semirings are a generalization of rings, dropping the requirement that each element must have an additive inverse. At the same time, semirings are a generalization of bounded distribu ...
) where addition and multiplication are not a
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...
but respectively a
commutative monoid In abstract algebra, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being . Monoids are semigroups with identity ...
and a
commutative monoid In abstract algebra, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being . Monoids are semigroups with identity ...
with
involution Involution may refer to: Mathematics * Involution (mathematics), a function that is its own inverse * Involution algebra, a *-algebra: a type of algebraic structure * Involute, a construction in the differential geometry of curves * Exponentiati ...
.


Definition

A wheel is an
algebraic structure In mathematics, an algebraic structure or algebraic system consists of a nonempty set ''A'' (called the underlying set, carrier set or domain), a collection of operations on ''A'' (typically binary operations such as addition and multiplicatio ...
(W, 0, 1, +, \cdot, /), in which * W is a set, * 0 and 1 are elements of that set, * + and \cdot are
binary operation In mathematics, a binary operation or dyadic operation is a rule for combining two elements (called operands) to produce another element. More formally, a binary operation is an operation of arity two. More specifically, a binary operation ...
s, * / is a
unary operation In mathematics, a unary operation is an operation with only one operand, i.e. a single input. This is in contrast to ''binary operations'', which use two operands. An example is any function , where is a set; the function is a unary operation ...
, and satisfying the following properties: * + and \cdot are each
commutative In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Perhaps most familiar as a pr ...
and
associative In mathematics, the associative property is a property of some binary operations that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement for express ...
, and have \,0 and 1 as their respective identities. * / is an
involution Involution may refer to: Mathematics * Involution (mathematics), a function that is its own inverse * Involution algebra, a *-algebra: a type of algebraic structure * Involute, a construction in the differential geometry of curves * Exponentiati ...
, for example //x = x * / is multiplicative, for example /(xy) = /x/y * (x + y)z + 0z = xz + yz * (x + yz)/y = x/y + z + 0y * 0\cdot 0 = 0 * (x+0y)z = xz + 0y * /(x+0y) = /x + 0y * 0/0 + x = 0/0


Algebra of wheels

Wheels replace the usual division as a binary operation with multiplication, with a unary operation applied to one argument /x similar (but not identical) to the
multiplicative inverse In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when Multiplication, multiplied by ''x'' yields the multiplicative identity, 1. The multiplicative inverse of a ra ...
x^, such that a/b becomes shorthand for a \cdot /b = /b \cdot a, but neither a \cdot b^ nor b^ \cdot a in general, and modifies the rules of
algebra Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...
such that * 0x \neq 0 in the general case * x/x \neq 1 in the general case, as /x is not the same as the
multiplicative inverse In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when Multiplication, multiplied by ''x'' yields the multiplicative identity, 1. The multiplicative inverse of a ra ...
of x. Other identities that may be derived are * 0x + 0y = 0xy * x/x = 1 + 0x/x * x-x = 0x^2 where the negation -x is defined by -x = ax and x - y = x + (-y) if there is an element a such that 1 + a = 0 (thus in the general case x - x \neq 0). However, for values of x satisfying 0x = 0 and 0/x = 0, we get the usual * x/x = 1 * x-x = 0 If negation can be defined as above then the
subset In mathematics, a Set (mathematics), set ''A'' is a subset of a set ''B'' if all Element (mathematics), elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they a ...
\ is a
commutative ring In mathematics, a commutative ring is a Ring (mathematics), ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring prope ...
, and every commutative ring is such a subset of a wheel. If x is an
invertible element In mathematics, the concept of an inverse element generalises the concepts of opposite () and reciprocal () of numbers. Given an operation denoted here , and an identity element denoted , if , one says that is a left inverse of , and that ...
of the commutative ring then x^ = /x. Thus, whenever x^ makes sense, it is equal to /x, but the latter is always defined, even when x=0.


Examples


Wheel of fractions

Let A be a commutative ring, and let S be a multiplicative
submonoid In abstract algebra, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being . Monoids are semigroups with identity ...
of A. Define the
congruence relation In abstract algebra, a congruence relation (or simply congruence) is an equivalence relation on an algebraic structure (such as a group (mathematics), group, ring (mathematics), ring, or vector space) that is compatible with the structure in the ...
\sim_S on A \times A via : (x_1,x_2)\sim_S(y_1,y_2) means that there exist s_x,s_y \in S such that (s_x x_1,s_x x_2) = (s_y y_1,s_y y_2). Define the ''wheel of fractions'' of A with respect to S as the quotient A \times A~/ (and denoting the
equivalence class In mathematics, when the elements of some set S have a notion of equivalence (formalized as an equivalence relation), then one may naturally split the set S into equivalence classes. These equivalence classes are constructed so that elements ...
containing (x_1,x_2) as _1,x_2/math>) with the operations : 0 = _A,1_A/math> (additive identity) : 1 = _A,1_A/math> (multiplicative identity) : / _1,x_2= _2,x_1/math> (reciprocal operation) : _1,x_2+ _1,y_2= _1y_2 + x_2 y_1,x_2 y_2/math> (addition operation) : _1,x_2\cdot _1,y_2= _1 y_1,x_2 y_2/math> (multiplication operation) In general, this structure is not a ring unless it is trivial, as 0x\ne0 in the usual sense – here with x= ,0/math> we get 0x= ,0/math>, although that implies that \sim_S is an improper relation on our wheel W. This follows from the fact that ,0 ,1implies 0\in S, which is also not true in general.


Projective line and Riemann sphere

The special case of the above starting with a field produces a
projective line In projective geometry and mathematics more generally, a projective line is, roughly speaking, the extension of a usual line by a point called a '' point at infinity''. The statement and the proof of many theorems of geometry are simplified by the ...
extended to a wheel by adjoining a
bottom element In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined dually, that is, it is an ele ...
noted ⊥, where 0/0=\bot. The projective line is itself an extension of the original field by an element \infty, where z/0=\infty for any element z\neq 0 in the field. However, 0/0 is still undefined on the projective line, but is defined in its extension to a wheel. Starting with the
real number In mathematics, a real number is a number that can be used to measure a continuous one- dimensional quantity such as a duration or temperature. Here, ''continuous'' means that pairs of values can have arbitrarily small differences. Every re ...
s, the corresponding projective "line" is geometrically a
circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...
, and then the extra point 0/0 gives the shape that is the source of the term "wheel". Or starting with the
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
s instead, the corresponding projective "line" is a sphere (the
Riemann sphere In mathematics, the Riemann sphere, named after Bernhard Riemann, is a Mathematical model, model of the extended complex plane (also called the closed complex plane): the complex plane plus one point at infinity. This extended plane represents ...
), and then the extra point gives a 3-dimensional version of a wheel.


See also

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Citations


References

* (a draft) * * (also available onlin
here
. * * {{cite journal , last1=Bergstra , first1=Jan A. , last2=Ponse , first2=Alban , title=Division by Zero in Common Meadows , journal=Software, Services, and Systems: Essays Dedicated to Martin Wirsing on the Occasion of His Retirement from the Chair of Programming and Software Engineering , series=Lecture Notes in Computer Science , date=2015 , volume=8950 , pages=46–61 , doi=10.1007/978-3-319-15545-6_6 , url=https://link.springer.com/chapter/10.1007/978-3-319-15545-6_6 , publisher=Springer International Publishing , isbn=978-3-319-15544-9 , s2cid=34509835 , language=en, arxiv=1406.6878 Fields of abstract algebra