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Wheel theory : ウィキペディア英語版
Wheel theory
Wheels are a type of 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.
Also the Riemann sphere can be extended to a wheel by adjoining an element 0/0. The Riemann sphere is an extension of the complex plane by an element \infty, where z/0=\infty for any complex z\neq 0. However, 0/0 is still undefined on the Riemann sphere, but defined in wheels.
== The algebra of wheels ==

Wheels discard the usual notion of division being a binary operator, replacing it with multiplication by a unary operator /x similar (but not identical) to the multiplicative inverse x^, such that a/b becomes shorthand for a \cdot /b = /b \cdot a, and modifies the rules of algebra such that
* 0x \neq 0\ in the general case.
* x - x \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.
Precisely, a wheel is an algebraic structure with operations binary addition +, multiplication \cdot, constants 0, 1 and unary /, satisfying:
* Addition and multiplication are commutative and associative, with 0 and 1 as their respective identities.
* /(xy) = /x/y\ and //x = x\
* xz + yz = (x + y)z + 0z\
* (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\
If there is an element a with 1 + a = 0, then we may define negation by -x = ax and x - y = x + (-y).
Other identities that may be derived are
* 0x + 0y = 0xy\
* x-x = 0x^2\
* x/x = 1 + 0x/x\
However, if 0x = 0 and 0/x = 0 we get the usual
* x-x = 0\
* x/x = 1\
The subset \ is always a commutative ring if negation can be defined as above, and every commutative ring is such a subset of a wheel. If x is an invertible element 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.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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