Associated Topics || Dr. Math Home || Search Dr. Math

Connected Sets in Topology

```
Date: 04/22/98 at 11:13:49
From: DeAndrea Flowers
Subject: Topology

I know that this is supposed to be for high school level math, but I
am really stuck with this homework assignment. Can you please help me?

The question states:

Let X be a topological space with x an element of X. Show that
{x} is connected.

I really have no idea of how to go about showing this.

Another problem that I am having trouble solving is:

Give an example to show that the union of two connected sets is
not necessarily connected.

The only thing that I know is the definition of connected:

A topological space X is said to be disconnected if there are
disjoint nonempty open subsets U and V of X for which
X=(U and V). If it is not disconnected then it is connected.

Can you please try to give me some advice on how to solve these
problems, or maybe suggest another Web site that might be able to
help? Thank you.
```

```
Date: 04/22/98 at 14:19:23
From: Doctor Tom
Subject: Re: Topology

Hi DeAndrea,

If you're a beginner in topology (which I guess you are), the best way
to think about these problems is in terms of some specific examples.

I have found that for the elementary stuff, the best specific example
is just the usual two-dimensional plane or the one-dimensional
Euclidean line, or subsets of them, with the usual definition of open
sets in Euclidean space.

"Connected" means pretty much what you think it should. If I look at
two circles of the plane that don't intersect, they are disconnected,
in the sense that there's a gap between them - you can't get from
one to the other and you can split the space up into the two circles,
call them U and V, and you will see that this division makes up the
whole space, so it's disconnected.

But look at one circle. You think of it as connected, right?  Well,
try to imagine dividing it into two open sets. You can hack off an
open chunk, but you'll always find that what's left isn't open - it
includes the boundary, so the other part is not open.

If this is confusing, just think of one dimension. If I have two open
intervals: (0,1) and (2,3), they're clearly disconnected, they're both
open, and will satisfy the U and V of the definition of disconnected
above. But try to hack (0,1) into two open sets. If you take one piece
as (0, 1/2), the other piece will be [1/2,1). Note that the number 1/2
is in the second chunk, so it's not open. This will happen no matter
how you try to divide it.

Your original question is easy - since there's only one element in
{x}, you CAN'T divide it into two non-empty sets - one will have one
element and the other zero, so one of the sets will have to be empty.
Therefore, it can't be disconnected, so it must be connected.

-Doctor Tom,  The Math Forum
Check out our web site! http://mathforum.org/dr.math/
```
Associated Topics:
College Euclidean Geometry
High School Euclidean/Plane Geometry
High School Sets

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search