### Finite Set Operations

The operations and apply to two sets; we can however use them to combine more than one set. If we have a finite collection of sets , we write

(1)

**Activity. **Give a better definition of and . *(Hint.* What did we say about definitions involving ?)

**De Morgan's Laws. **For any finite collection of sets ,

**Exercise. **Prove this version of de Morgan's Laws.

### Transfinite Operations

There is no reason to restrict our attention to {\em finite} collections of sets. For example, we might consider

the collection of intervals of the form , where is allowed to be an natural number,

or perhaps

the set of all disks centered at the origin

each of which consists of an infinite number of sets. We usually use set-builder notation to write something like

(2)

or

(3)

to emphasize that what we are dealing with is a(n infinite) *set of sets*. (Here we have written for the disk of radius centered at the origin.) We call the set to the right of the pipe the *index set*: in these collections the index sets are and the interval , respectively.

We can define unions and intersections of such infinite collections of sets by recalling that acts like a souped-up version of and acts like a souped-up version of .

Given a collection of sets , we set

**Definition. **

**E.g. **

**Proof. **Let . Then there is some with , so . Thus .

Now let . Then there is some with . But , so .

**E.g. **

**Proof. **Certainly the origin is in each disk . We will show it is the only such point.

Consider any point in the plane. Let be the distance between and the origin. Then if we select , we see that . Thus cannot be in every .

**de Morgan's Laws. **For any collection , we have

**Exercise. **Prove this version of de Morgan's Laws.

**Exercise. **What is each of the following sets?

**Exercise **Express each set below as either a generalized union or a generalized intersection, and also express it in set-builder notation.