Proof of limsup(anbn)<=limsup(an)limsup(bn) and Example

In summary, the conversation discusses the proof that if (an) and (bn) are bounded positive sequences, then limsup(anbn) is less than or equal to the product of limsup(an) and limsup(bn). An example is also given to show that there is not always equality. The conversation also includes a reminder to attempt the problem before seeking help.
  • #1
stukbv
118
0

Homework Statement


if (an) and (bn) are bounded positive sequences prove that
limsup(anbn)<= limsup(an)limsup(bn)
and give an example to show there is not equality in general
 
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  • #2
If you do not show at least some attempt to do this problem yourself, this thread will be deleted.
 
  • #3
all i know is, if we let l = limsupan and m = limsupbn ,
then we can say that there exists an n such that
l-e < an < l+e
m-e<bn < l + e

so |anbn| < (l+e)(m+e)
...
 
  • #4
Hi stukbv! :smile:

Can you first show that

[tex]\sup{(a_nb_n)}\leq \sup{a_n}\sup{b_n}[/tex]
 

FAQ: Proof of limsup(anbn)<=limsup(an)limsup(bn) and Example

What is the proof for the inequality limsup(anbn)<=limsup(an)limsup(bn)?

The proof for this inequality involves using the definition of limsup to show that limsup(anbn) is less than or equal to limsup(an) multiplied by limsup(bn). This can be done by considering the subsequences of anbn and anbn - anbn, and using the properties of limsup.

How does the example illustrate the inequality limsup(anbn)<=limsup(an)limsup(bn)?

The example typically used to illustrate this inequality is the sequence an = bn = (-1)^n. In this case, limsup(anbn) = 1, while limsup(an) and limsup(bn) are both equal to 1/2. This shows that the inequality holds, as 1 is less than or equal to 1/2 multiplied by 1/2.

Is the inequality limsup(anbn)<=limsup(an)limsup(bn) always true?

Yes, the inequality limsup(anbn)<=limsup(an)limsup(bn) is always true for any two sequences {an} and {bn}. This can be proved using the basic properties of limsup and the definition of the limit of a sequence.

Can this inequality be extended to more than two sequences?

Yes, this inequality can be extended to any finite number of sequences. In other words, if we have n sequences {an1}, {an2}, ..., {ann}, then limsup(an1an2...ann) is less than or equal to limsup(an1) multiplied by limsup(an2) multiplied by ... multiplied by limsup(ann).

What is the significance of the inequality limsup(anbn)<=limsup(an)limsup(bn)?

This inequality is significant because it allows us to compare the growth rates of two sequences. It tells us that the limsup of the product of two sequences is always less than or equal to the product of their individual limsups, which can provide valuable information in various mathematical and scientific contexts.

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