Table of Contents

## How do you prove the intersection of two open sets is open?

The intersection of two open intervals is an open interval (possibly empty), so (1) is a union of open intervals and as such is an open set. This shows that the intersection of two open sets is open, and the result for any finite collection of open sets is easily established by induction.

## How do you prove a set is an open set?

A set is open if and only if it is equal to the union of a collection of open balls. Proof. According to Theorem 4.3(2) the union of any collection of open balls is open. On the other hand, if A is open then for every point x ∈ A there exists a ball B(x) about x lying in A.

**How do you prove a set in R 2 is closed?**

In R2 a set is closed if it contains all of its limit points. So, you can prove C is closed by considering a sequence in C and show that if it converges then the limit is in C. More generally, if f:R2→R is a continuous function then the set {(x,y)∈R2∣f(x,y)=c}, for any constant c∈R, is closed.

**How do you prove the union of open sets are open?**

There is a simpler proof: Take x∈A1∪A2. Then x∈A1 or x∈A2. If x∈A1, as A1 is open, there exists an r>0 such that B(x,r)⊂A1⊂A1∪A2 and B(x,r) is an open set.

### How do I prove my AB is closed?

Yes, if A is open and B is closed, then B∖A is closed. To prove it, just note that X∖A is closed (where X is the whole space), and B∖A=B∩(X∖A), so B∖A is the intersection of two closed sets and is therefore closed.

### How do you prove a closed?

To prove that a set is closed, one can use one of the following: — Prove that its complement is open. — Prove that it can be written as the union of a finite family of closed sets or as the intersection of a family of closed sets. — Prove that it is equal to its closure.

**How can a set be both open and closed?**

If a set has no boundary points, it is both open and closed. Since there aren’t any boundary points, therefore it doesn’t contain any of its boundary points, so it’s open. Since there aren’t any boundary points, it is vacuously true that it does contain all its boundary points, so it’s closed.

**How do you prove a set is open in a metric space?**

We take as the definition of an open set in a metric space to mean that for every point in the set, there exists an open ball containing the point that is completely contained in the set. If a set is a union of open balls then every point is in some ball which is contained in the set, so the set is open.

## Can a set be not open nor closed?

Intuitively, an open set is a set that does not include its “boundary.” Note that not every set is either open or closed, in fact generally most subsets are neither. The set [0,1)⊂R is neither open nor closed.

## When is a set of real numbers open?

Recall that a set of real numbers is open if and only if it is a countable disjoint union of open intervals. Also recall that: 1. a countable union of open sets is open, and 2. a countable intersection of closed sets is closed. These two properties are the main motivation for studying the following.

**What is an open set?**

Relevant For… Open sets are the fundamental building blocks of topology. In the familiar setting of a metric space, the open sets have a natural description, which can be thought of as a generalization of an open interval on the real number line.

**Can closed sets be complements of open sets?**

As always with characterizations, this characterization is an alternative de nition of a closed set. In fact, many people actually use this as the de nition of a closed set, and then the de nition we’re using, given above, becomes a theorem that provides a characterization of closed sets as complements of open sets.

### What is the de nition of a closed set?

In fact, many people actually use this as the de nition of a closed set, and then the de nition we’re using, given above, becomes a theorem that provides a characterization of closed sets as complements of open sets. Theorem: A set is closed if and only if it contains all its limit points.