Proving the limsup=lim of a sequence

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In summary, the problem asks to show that if the limit of a sequence bn is equal to a number b, then the limit superior of bn is also equal to b. This can be proven by showing that for all n sufficiently large, the absolute value of bn is less than or equal to b plus some small value epsilon. This implies that the supremum of the set of all subsequential limits is also equal to b, and therefore the limit superior is equal to b.
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gottfried
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Homework Statement


Show that the if lim bn = b exists that limsup bn=b.

The Attempt at a Solution



Let limsup = L and lim = b

We know for all n sufficiently large
|bn-b|<ε
|bn| < b+ε

Therefore L ≤ b+ε and
|bn| < L ≤ b+ε

I'm trying to get |bn-L|<ε or |L-b|<ε both of which I believe imply that b=L.
The problem is I can't get my absolute value signs to be correct.
 
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  • #2
I'm not sure why you would do that. The "limsup" of a sequence is defined as the supremum of the set of all subsequential limits. If the sequence itself converges, then every subsequence converges to that limit. That is the "set of all subsequential limits" contains ony a single number.
 

Related to Proving the limsup=lim of a sequence

1. What is the definition of limsup and lim of a sequence?

The limsup (limit superior) of a sequence is the largest limit point of the sequence, while the lim (limit) is the value that the sequence approaches as the number of terms in the sequence approaches infinity.

2. How do you prove that the limsup of a sequence is equal to the lim of the sequence?

To prove that the limsup of a sequence is equal to the lim of the sequence, you need to show that the limsup is both an upper bound and a limit point of the sequence. This can be done by using the definition of the limsup and lim, along with the properties of limit points and upper bounds.

3. Can the limsup and lim of a sequence be different?

Yes, the limsup and lim of a sequence can be different. This can happen when the sequence has multiple limit points or when the sequence does not converge to a single value.

4. What is the significance of proving the limsup=lim of a sequence?

Proving that the limsup of a sequence is equal to the lim of the sequence helps to establish the convergence of the sequence. It also allows us to evaluate the behavior of the sequence as the number of terms increases, providing insights into the long-term behavior of the sequence.

5. Are there any common techniques for proving the limsup=lim of a sequence?

Yes, some common techniques for proving the limsup of a sequence is equal to the lim of the sequence include using the definition of limsup and lim, using the properties of limit points and upper bounds, and using the squeeze theorem.

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