Which sets are open, closed, or neither?

In summary: A set is closed if it is defined entirely in terms of "<" or ">", open if it is defined entirely in terms of "<=" or ">=", neither if it has both. The set {x| 0<= x< 1} is closed because it is defined entirely in terms of "<=" or ">=".
  • #1
calvino
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Determine with justification which sets are open, closed, or neither

i) {(x,y,z): x^2+ y^2 + z^2 +(xyz)^2 >= -1}

ii) {(x,y,z): x^2 + y^2 +z^2 >= 1}

iii) {(x,y,z): x^2- y- z >1}

iv) {(x,y): 3>= x^2- xy + y^2 >1

v) {(x,y): x^2 - y^2 >=0 }


So, my first insinct is to go about it using the definition of open sets. So I try and find a neighbourhood around a point in the set that is not completely contained in the set. What confuses me is that that method is not very definite. What if I can't find that neighbourhood?

Any help on how I should go about starting this question off?
 
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  • #2
I don't understand what you mean by "that method is not very definite". The definition is very definite!

However, you might find it easier to use a property that some text use as the definition: a set is open if and only if it contains none of its boundary points. "boundary" points may be difficult to define in general but with sets like you are given they are easy.
 
  • #3
thanks.

now I'm just curious about what neither open nor closed means. Do you think you could explain to me a bit about that (or give an example?)?
 
  • #4
A set is open if it contains none of its boundary points. A set is closed if it contains all of its boundary points. A set is neither open nor closed if it contains some but not all of its boundary points.

The set {x| 0<= x< 1} has "boundary" {0, 1}. It contains one of those but not the other and so is neither open nor closed.

For simple intervals like these, a set is open if it is defined entirely in terms of "<" or ">", closed if it is defined entirely in terms of "<=" or ">=", neither if it has both.

That is, however, for "simple intervals". Is the "set of all rational numbers between 0 and 1 (but not including 0 and 1)" open or closed (or neither)?
 
  • #5
So in this case, by simply looking at the boundary points of the set, I come up with the following answers, in order.

closed, closed, open, neither, closed

Is this right, or do I have to consider manipulating the functions which make the set?
 

FAQ: Which sets are open, closed, or neither?

What is the definition of an open set?

An open set is a set in which all of its points are interior points, meaning that every point in the set has a neighborhood contained entirely within the set.

How do you determine if a set is closed?

A set is closed if it contains all of its limit points. This means that for every convergent sequence of points in the set, the limit of that sequence must also be in the set.

Can a set be both open and closed?

Yes, a set can be both open and closed. This type of set is known as a clopen set. An example of a clopen set is the set of all real numbers between 0 and 1, including 0 and 1.

How can you tell if a set is neither open nor closed?

A set is neither open nor closed if it contains some of its limit points but not all of them. In other words, it is not open because it does not contain all of its interior points, and it is not closed because it does not contain all of its limit points.

Are there any other types of sets besides open, closed, and neither?

Yes, there are other types of sets, such as compact sets, connected sets, and dense sets. These types of sets have specific mathematical definitions and properties that differentiate them from open, closed, and neither sets.

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