Real Analysis: Proving Lim(yn)=0 from Lim(xn)=Infinity & Lim(xnyn)=L

In summary, we are trying to prove that if (xn) and (yn) are sequences of real numbers such that lim(xn)= infinity and lim(xnyn)=L, then Lim(yn)=0. A hint for the proof is to consider the limit of 1/xn as n approaches infinity.
  • #1
rohitmishra
7
0
Question : Let (xn) and (yn) be sequences of real numbers such that lim(xn)= infinity and lim(xnyn)=L for some real number L.

Prove Lim(yn)=0.


I've been trying to solve this question for a long time now. I've no success yet. Can anyone guide me as to how i can approach it.
 
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  • #2
welcome to pf!

hi rohitmishra! welcome to pf! :smile:

(have an infinity: ∞ and try using the X2 icon just above the Reply box :wink:)

hint: can you prove what limn->∞ 1/xn is? :smile:
 
  • #3
You know |x_n*y_n - L| < epsilon. Why not see what you can conclude if you let epsilon = |L/2|?
 
  • #4
I have lim(xnyn) =L.

How can I start of a prove assuming two sequences That is x(n) = 2x(n) which wud tend to infinity and Lim (yn) = 1/x(n) = 0.

Can you tell me what the Hypothesis wud be ?
 
  • #5
limn->∞ 1/xn = 0
 
  • #6
ok so you know lim 1/xn and you know lim xnyn, so … ? :smile:
 
  • #7
Can you help me how i can construct the proof.. ? I understand what you meant. But, How can I assume my yn is 1/xn ?
 
  • #8
rohitmishra said:
I understand what you meant. But, How can I assume my yn is 1/xn ?

no you don't :redface:, because it isn't

try again :smile:
 

1. How do you prove that Lim(yn) equals 0 given Lim(xn) equals Infinity and Lim(xnyn) equals L?

To prove this statement, we can use the Squeeze Theorem. First, we know that Lim(xn) equals Infinity, meaning that the sequence xn grows without bound. This also implies that Lim(|xn|) equals Infinity. Then, using the given information, we can rewrite Lim(xnyn) as Lim(xn) * Lim(yn) = Infinity * Lim(yn) = Infinity. Since both xn and |xn| grow without bound, we can use the Squeeze Theorem to show that Lim(yn) must equal 0.

2. What is the role of the Squeeze Theorem in proving this statement?

The Squeeze Theorem is a useful tool in proving limits for sequences. It states that if two sequences, xn and yn, approach the same limit, and there exists a third sequence, zn, where xn ≤ zn ≤ yn for all n, then the limit of zn must also be equal to the limit of xn and yn. In this case, we use the Squeeze Theorem to show that Lim(yn) must equal 0 since it is sandwiched between two sequences that grow without bound.

3. Can you provide an example of a sequence where Lim(yn) equals 0 given Lim(xn) equals Infinity and Lim(xnyn) equals L?

One example could be xn = n and yn = 1/n. In this case, Lim(xn) = Infinity, Lim(xnyn) = Lim(n/n) = 1, and Lim(yn) = Lim(1/n) = 0.

4. How does proving this statement relate to the concept of limits in Real Analysis?

In Real Analysis, limits are used to describe the behavior of a function as the input values approach a certain value or infinity. In this case, we are using limits to prove that given certain conditions, the limit of a sequence must equal a specific value, in this case, 0. This is an important concept in Real Analysis as it allows us to make conclusions about the behavior of a function without necessarily needing to know its exact value at a certain point.

5. Are there any other methods for proving this statement aside from using the Squeeze Theorem?

Yes, there are other methods that can be used to prove this statement, such as the ε-δ definition of limits or the Cauchy Criterion for limits. However, the Squeeze Theorem is often the most straightforward and efficient method for proving this type of statement. It is always important to choose the method that best suits the specific problem at hand.

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