# Wheel theory

A wheel is a type of algebra (in the sense of universal algebra) where division is always defined. In particular, division by zero is meaningful. The real numbers can be extended to a wheel, as can any commutative ring.
The term wheel is inspired by the topological picture $\odot$ of the real projective line together with an extra point ⊥ (bottom element) such that $\bot =0/0$.
A wheel can be regarded as the equivalent of a commutative ring (and semiring) where addition and multiplication are not a group but respectively a commutative monoid and a commutative monoid with involution.

## Definition

A wheel is an algebraic structure $(W,0,1,+,\cdot ,/)$, in which
* $W$ is a set,
* $0$ and $1$ are elements of that set,
* $+$ and $\cdot$ are binary operations,
* $/$ is a unary operation,
and satisfying the following properties:
* $+$ and $\cdot$ are each commutative and associative, and have $\,0$ and $1$ as their respective identities.
* $/$ is an involution, 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 $x^{-1}$, such that $a/b$ becomes shorthand for $a\cdot /b=/b\cdot a$, but neither $a\cdot b^{-1}$ nor $b^{-1}\cdot a$ in general, and modifies the rules of algebra 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 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 $\{x\mid 0x=0\}$ is a commutative ring, and every commutative ring is such a subset of a wheel. If $x$ is an invertible element of the commutative ring then $x^{-1}=/x$. Thus, whenever $x^{-1}$ 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 of $A$. Define the congruence relation $\sim _{S}$ 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 $A\times A~/{\sim _{S}}$ (and denoting the equivalence class containing $(x_{1},x_{2})$ as $[x_{1},x_{2}]$) with the operations
    $0=[0_{A},1_{A}]$     (additive identity)
    $1=[1_{A},1_{A}]$     (multiplicative identity)
    $/[x_{1},x_{2}]=[x_{2},x_{1}]$     (reciprocal operation)
    $[x_{1},x_{2}]+[y_{1},y_{2}]=[x_{1}y_{2}+x_{2}y_{1},x_{2}y_{2}]$     (addition operation)
    $[x_{1},x_{2}]\cdot [y_{1},y_{2}]=[x_{1}y_{1},x_{2}y_{2}]$     (multiplication operation)
In general, this structure is not a ring unless it is trivial, as $0x\neq 0$ in the usual sense - here with $x=[0,0]$ we get $0x=[0,0]$, although that implies that $\sim _{S}$ is an improper relation on our wheel $W$.
This follows from the fact that $[0,0]=[0,1]\implies 0\in S$, 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 extended to a wheel by adjoining a bottom element 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 numbers, the corresponding projective "line" is geometrically a circle, and then the extra point $0/0$ gives the shape that is the source of the term "wheel". Or starting with the complex numbers instead, the corresponding projective "line" is a sphere (the Riemann sphere), and then the extra point gives a 3-dimensional version of a wheel.