# Ring of sets

A Ring of sets is also known as a **Boolean ring**

Note that every Algebra of sets is also a ring, and that an Algebra of sets is sometimes called a **Boolean algebra**

## Definition

A Ring of sets is a non-empty class [ilmath]R[/ilmath]^{[1]} of sets such that:

- [math]\forall A\in R\forall B\in R[A\cup B\in R][/math]
- [math]\forall A\in R\forall B\in R[A-B\in R][/math]

## A ring that exists

Take a set [ilmath]X[/ilmath], the power set of [ilmath]X[/ilmath], [ilmath]\mathcal{P}(X)[/ilmath] is a ring (further still, an algebra) - the proof of this is trivial.

This ring is important because it means we may talk of a "ring generated by"

## First theorems

The empty set belongs to every ring

Take any [math]A\in R[/math] then [math]A-A\in R[/math] but [math]A-A=\emptyset[/math] so [math]\emptyset\in R[/math]

Given any two rings, [ilmath]R_1[/ilmath] and [ilmath]R_2[/ilmath], the intersection of the rings, [ilmath]R_1\cap R_2[/ilmath] is a ring

We know [math]\emptyset\in R[/math], this means we know at least [math]\{\emptyset\}\subseteq R_1\cap R_2[/math] - it is non empty.

Take any [math]A,B\in R_1\cap R_2[/math] (which may be the empty set, as shown above)

Then:

- [math]A,B\in R_1[/math]
- [math]A,B\in R_2[/math]

This means:

- [math]A\cup B\in R_1[/math] as [ilmath]R_1[/ilmath] is a ring
- [math]A-B\in R_1[/math] as [ilmath]R_1[/ilmath] is a ring
- [math]A\cup B\in R_2[/math] as [ilmath]R_2[/ilmath] is a ring
- [math]A-B\in R_2[/math] as [ilmath]R_2[/ilmath] is a ring

But then:

- As [math]A\cup B\in R_1[/math] and [math]A\cup B\in R_2[/math] we have [math]A\cup B\in R_1\cap R_2[/math]
- As [math]A- B\in R_1[/math] and [math]A- B\in R_2[/math] we have [math]A- B\in R_1\cap R_2[/math]

Thus [math]R_1\cap R_2[/math] is a ring.

## References

- ↑ Page 19 -Measure Theory - Paul R. Halmos