Calculating sup, inf, limsup, liminf, and limit points of a sequence

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In summary, the conversation discusses the computation of the sup, inf, limsup, liminf, and limit points of a given sequence. The speaker writes down the first few terms to observe the behavior of the sequence and finds that the limit points are 1 and -1. They determine that limsup = 1 and liminf = -1, but are unsure about the inf value. The speaker is unsure if they used the correct method and asks if their values are correct. They are confirmed that their sup is fine, but their inf is incorrect and should be -1. The conversation ends with the speaker expressing gratitude for the clarification.
  • #1
*melinda*
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Question:
(I've got a few like this, so I'd like to know if I am doing them correctly.)

Compute the sup, inf, limsup, liminf, and all the limit points of the following sequence [itex]x_1, x_2,...[/itex] where

[tex]x_n = 1/n + (-1)^n[/tex]

What I did was write down the first few terms to get an idea of the behavior of the sequence.
From this I found that the limit points were 1 and -1
limsup = 1
liminf = -1
sup = 1.5 (I think)
inf = 0 (I'm not sure)

I'm not so sure about the inf because I don't think inf = liminf is allowed, but in this case the lower endpoint and the liminf both equal -1, I think...

Did I determined these values by the correct method, or is there some formal way of which I am unaware to 'compute' these things?

Also, are my values correct?

thanks-a-bunch :smile:
 
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  • #2
write out the first few terms of the sequence and watch the trend of the terms. Your inf is wrong. Your sup is fine.
 
  • #3
So that would mean that my inf = -1, just like I thought it should be. :rolleyes:

Thanks a lot!
 

FAQ: Calculating sup, inf, limsup, liminf, and limit points of a sequence

What is the difference between sup and inf?

The sup (supremum) of a set is the least upper bound, meaning it is the smallest number that is greater than or equal to all the numbers in the set. The inf (infimum) is the greatest lower bound, meaning it is the largest number that is less than or equal to all the numbers in the set.

How do you calculate the sup and inf of a set?

To calculate the sup of a set, you need to find the largest number in the set. To calculate the inf, you need to find the smallest number in the set. Both can be found by arranging the numbers in the set in ascending or descending order and then selecting the first or last number respectively.

Can the sup or inf be a part of the set?

Yes, the sup and inf can be a part of the set. For example, if the set is {1, 2, 3}, then both the sup and inf are included in the set as 3 is the largest number and 1 is the smallest number in the set.

What is the significance of computing sup and inf?

Computing sup and inf is useful in analyzing and understanding the behavior of a set of numbers. It can be used to determine the range of values in a set, the existence of maximum or minimum values, and can help in solving optimization problems in various fields such as economics, engineering, and statistics.

Is the sup or inf always unique for a set?

No, the sup and inf may not always be unique for a set. If there are multiple numbers that are equal to the sup or inf, then there is no unique value. For example, in the set {1, 2, 3, 3}, both 3 and 2 can be considered as the sup and inf.

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