Sup and inf of a set of limit points

In summary, the conversation is about proving that the supremum and infimum of a set of limit points of a sequence are themselves limit points. The approach suggested is to use the definition of a limit point and carefully write out the proof with appropriate inequalities. The original set of points is denoted as S and the sequence is of nonnegative real numbers.
  • #1
Yankees24
3
0

Homework Statement



I have to prove that the supremum and infimum of a set of limit points of a a sequence {an} are themselves limit points.


Homework Equations





The Attempt at a Solution



I have been messing around with definitions but have not made any progress. Please help. Thank you
 
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  • #2
The general idea is, there are limit points close to the sup of the limit points. And there are points in the given set that are close to those limit points. So there are points in the set close to the sup of the limit points. You just have to write it carefully with appropriate inequalities.
 
  • #3
Great! Could you possibly give me an idea of where to begin with the careful proof? This is usually where I struggle. Thank you!
 
  • #4
Yankees24 said:
Great! Could you possibly give me an idea of where to begin with the careful proof? This is usually where I struggle. Thank you!

If you call original set of points ##S## and the sup of the limit points ##s## and you want to show ##s## is a limit point of ##S## you would start with the definition for ##s## to be a limit point of ##S##. That is what you have to prove. And you have already neglected to mention what ##S## is a set of e.g., the real numbers.
 
  • #5
Ok thanks I will see how it goes. And yes I meant to say a sequence of nonnegative real numbers.
 

FAQ: Sup and inf of a set of limit points

What is the definition of the sup and inf of a set of limit points?

The supremum (sup) of a set of limit points is the smallest number that is greater than or equal to all of the limit points in the set. The infimum (inf) is the largest number that is less than or equal to all of the limit points in the set.

How do you find the sup and inf of a set of limit points?

To find the sup, you need to first identify all of the limit points in the set. Then, you can either use a visual representation, such as a graph, or use algebraic methods to determine the smallest number that is greater than or equal to all of the limit points. Similarly, to find the inf, you would use the same process to determine the largest number that is less than or equal to all of the limit points.

Can a set of limit points have multiple sup and inf values?

No, a set of limit points can only have one sup and one inf value. This is because the sup and inf are unique and represent the smallest and largest possible values in the set.

What is the relationship between the sup and inf of a set of limit points?

The sup and inf of a set of limit points are always related in the following way: sup ≥ inf. In other words, the sup is always greater than or equal to the inf.

How are the sup and inf of a set of limit points used in calculus?

The sup and inf of a set of limit points are important concepts in calculus, particularly in the study of limits and continuity. They help to define the behavior of a function at a particular point and can be used to prove the existence or non-existence of a limit. Additionally, the sup and inf are used in the Intermediate Value Theorem, which states that if a function is continuous on a closed interval, then it must take on every value between its sup and inf at least once within that interval.

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