# Ring/New page

Not to be confused with a ring of sets

## Contents

## Definition

Let [ilmath]R[/ilmath] be a *non-empty* set, let there be two binary operations (a kind of map where rather than [ilmath]f(a,b)[/ilmath] we write [ilmath]afb[/ilmath]):

- [ilmath]\oplus:R\times R\rightarrow R[/ilmath] - called "addition", [ilmath]\oplus:(a,b)\mapsto a\oplus b[/ilmath]
- [ilmath]\odot:R\times R\rightarrow R[/ilmath] - called "multiplication", [ilmath]\odot:(a,b)\mapsto a\odot b[/ilmath]

and let there be elements [ilmath]0_R\in R[/ilmath] and [ilmath]1_R\in R[/ilmath] (not necessarily distinct)^{[Note 1]} such that we have the following 7 properties^{[1]}:

TODO: This would be *much* nicer as a table....

- [ilmath](R,\oplus,0_R)[/ilmath] is an abelian group
- Group definition:
- [ilmath]\forall a,b,c\in R[(a\oplus b)\oplus c=a\oplus(b\oplus c)][/ilmath] - associativity
- [ilmath]\exists e\in R\ \forall a\in R[e\oplus a=a\oplus e=a][/ilmath] - existence of identity, on the group page we show it is unique
^{[Note 2]}, we denote it by [ilmath]0_R[/ilmath], so: [ilmath]\forall a\in R[a\oplus 0_R=0_R\oplus a=a][/ilmath] - [ilmath]\forall a\in R\ \exists b\in R[a\oplus b=b\oplus a=0_R][/ilmath] - existence of inverse, on the group page we show it is unique
^{[Note 3]}. Denoted by [ilmath]-a[/ilmath] as we're using additive notation^{[Note 4]}

- Being an Abelian group adds an additional property:
- [ilmath]\forall a,b\in R[a\oplus b=b\oplus a][/ilmath] - commutivity

- Group definition:
- [ilmath](R,\odot)[/ilmath] is a semigroup
- Semigroup definition:
- [ilmath]\forall a,b,c\in R[(a\odot b)\odot c=a\odot(b\odot c)][/ilmath]

- Semigroup definition:
- There is distributivity in play in.
- [ilmath]\odot[/ilmath] distributes across [ilmath]\oplus[/ilmath]
**Caution:**I think... it might be the other way around...**the following 2 rules are certainly correct however:**- [ilmath]\forall a,b,c\in R[a\odot(b\oplus c)=(a\odot b)\oplus(a\odot c)][/ilmath] and
- [ilmath]\forall a,b,c\in R[(a+b)c=ac+bc][/ilmath]

- [ilmath]\odot[/ilmath] distributes across [ilmath]\oplus[/ilmath]

Then [ilmath](R,\oplus:R\times R\rightarrow R,\odot:R\times R\rightarrow R,0_R)[/ilmath] is a ring, but as mathematicians are lazy we just write [ilmath](R,\oplus,\odot,0_R)[/ilmath], [ilmath](R,\oplus,\odot)[/ilmath] or even just "Let [ilmath]R[/ilmath] be a ring".

TODO: Be more formal about distributivity, I've checked my books, no one specified, they just say "it is distributive: "

### Further properties of elementary rings

There are 2 more additional properties we can apply to define rings:

- [ilmath]\exists e_\odot\ \forall a\in R[a\odot e_\odot=e_\odot\odot a=a][/ilmath] - a
*multiplicative identity*, this element if it exists is unique and denoted [ilmath]1_R[/ilmath] or just [ilmath]1[/ilmath] - [ilmath]\forall a,b\in R[a\odot b=b\odot a][/ilmath] - commutative with respect to [ilmath]\odot[/ilmath]

Giving us the following 4 types of elementary rings^{[Note 5]}:

- Ring - properties 1-7
- Ring with unity (AKA: u-ring, ring with identity) - properties 1-8
- Commutative ring (AKA: c-ring) - properties 1-7 and 9
- Commutative ring with unity (AKA: cu-ring or q-ring - properties 1-9

### Caveats

Some authors define a ring to be what we would call a *ring with unity* (which we shall call a u-ring throughout the site). Especially if the book covers the topics of rings and modules. We defined "commutative ring" and "ring with unity" above.

## See next

- Types of ring
- Ring morphism
- Ring homomorphism
- Kernel of a ring homomorphism - see also: kernel
- Image of a ring homomorphism - see also: image

- Ring isomorphism

- Ring homomorphism
- Unit of a ring
- Division ring
- Ring ideal
- Quotient ring
- Fundamental ring homomorphism theorem
- Ring isomorphism theorems
- Module

## Notes

- ↑ So we could have [ilmath]0_R=1_R[/ilmath] or we could have [ilmath]0_R\ne 1_R[/ilmath]
- ↑ there is only one inverse
- ↑ there is only one inverse for an element
- ↑ For multiplicative notation we'd use [ilmath]a^{-1} [/ilmath]
- ↑ field, integral domain are also all rings, there's like 6 kinds. We call "Elementary ring" just the ones listed