Show sequence is bounded in an interval

In summary: Therefore, |a_n| \leq \max\{|c|,|d|\}.In summary, a sequence {a_n} is bounded if and only if there is an interval [c, d] such that {a_n} is a sequence in [c,d]. This means that there is a number M, which can be defined as the maximum of |c| and |d|, such that |a_n| is always less than or equal to M.
  • #1
darkestar
12
0

Homework Statement



Show that a sequence {a_n} is bounded if and only if there is an interval [c, d] such that {a_n} is a sequence in [c,d].

Homework Equations



A sequence {a_n} is bounded provided that there is a number M such that |a_n| <= M.

The Attempt at a Solution



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  • #2
The first part looks fine. I'm having difficulty reading the second part, but it looks like you assume the c=-d, which isn't necessarily a valid assumption.
 
  • #3
For the second part I let c = -d because I was trying to end with a result such as the sequence being in the interval [-M, M]. If I leave it as the interval [c, d] then I'm not sure how to end with the conclusion that every element of the sequence is less than a number M.
 
  • #4
Consider max{|c|,|d|}.
 
  • #5
So I can define M to be max{ |c|, |d| }.

Therefore, |a_n| <= M.

Would this be all that I need to write for the proof in that direction?
 
  • #6
It might be nice if you write things out a little more explicitly, but yes, it's all that you need.
 
  • #7
I'm not really sure how I should be more explicit for the proof.
 
  • #8
[tex]-\max\{|c|,|d|\} \leq c \leq a_n \leq d \leq \max\{|c|,|d|\}[/tex]
 

FAQ: Show sequence is bounded in an interval

What does it mean for a sequence to be bounded in an interval?

When a sequence is bounded in an interval, it means that all the terms in the sequence fall within a specific range or interval of numbers. This interval can be defined by a lower and upper bound.

How do you check if a sequence is bounded in an interval?

To check if a sequence is bounded in an interval, you can use the limit test. If the limit of the sequence exists and falls within the given interval, then the sequence is bounded. Another way is to find the maximum and minimum values of the sequence and see if they fall within the given interval.

What happens if a sequence is not bounded in an interval?

If a sequence is not bounded in an interval, it means that the terms of the sequence are either increasing or decreasing without limit. This could also mean that the sequence is oscillating between two values without converging to a specific value.

Can a sequence be bounded in multiple intervals?

Yes, a sequence can be bounded in multiple intervals. This means that the terms of the sequence fall within different ranges or intervals. For example, a sequence could be bounded in the intervals [0, 1] and [3, 5], meaning all the terms fall between 0 and 1, as well as between 3 and 5.

Is a bounded sequence always convergent?

No, a bounded sequence is not always convergent. A sequence can be bounded but not convergent if it has multiple limit points or if it is oscillating between two values without converging to a specific value. However, a bounded sequence must be convergent if it is also monotonic (either increasing or decreasing).

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