Proving Finite Supremum of Independent Random Variables

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In summary: Thank you.In summary, the conversation discusses the relationship between independent random variables and their convergence. The key result used is the Borel-Cantelli lemma, which states that if the series of probabilities of a random variable exceeding a certain value is convergent, then the probability of the random variable exceeding that value is 0. The converse of this lemma can be applied to show that if the series of probabilities is divergent for all values, then the random variable can converge to infinity. There is some confusion in the conversation about the notation and terminology used.
  • #1
bennyzadir
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Let be $X_1, X_2, \dots $ independent random variables. My question is how can we show that $sup_n X_n <\infty$ almost surely $ \iff \sum_{n=1}^{\infty} \mathbb{P}(X_n>A)<\infty $ for some positive finite A number.

Thank you very much for your help in advance!
 
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  • #2
Of course, setting $\displaystyle \text{sup}_{n} X_{n}=B$, for A>B the 'infinite sum' vanishes. Do You intend to get Your question with the hypothesis A<B?...

Kind regards

$\chi$ $\sigma$
 
  • #3
chisigma said:
Of course, setting $\displaystyle \text{sup}_{n} X_{n}=B$, for A>B the 'infinite sum' vanishes.
\(\sup_n X_n\) is not a number but a function from the sample space to reals (plus infinity)...
 
  • #4
Hint: use Borel-Cantelli lemma.
 
  • #5
Evgeny.Makarov said:
\(\sup_n X_n\) is not a number but a function from the sample space to reals (plus infinity)...

I'm afraid that the question has been wrongly expressed and in particular there is confusion between the random variables $X_{n}$ and the probabilities $P_{n}= P\{X_{n}>A\}$...

Kind regards

$\chi$ $\sigma$
 
  • #6
I don't see any confusion in the question. The sequence $X$ is a function of two arguments, i.e., $X:\mathbb{N}\times\Omega\to\mathbb{R}$. We denote $X(n,\cdot)$ by $X_n$. Then $\sup_nX_n$ is a function $\Omega\ni\omega\mapsto\sup_nX_n(\omega)$. In particular, $X_n$ is not a number; otherwise, $\sup_nX_n$ would also be a number and $\sup_nX_n<\infty$ would be either true or false. As it is, $\sup_nX_n<\infty$ has its own Boolean value for each $\omega\in\Omega$. It is an event (i.e., a subset iof $\Omega$) that holds almost surely, i.e., the probability measure of this event is 1.
 
  • #7
girdav said:
Hint: use Borel-Cantelli lemma.

Thank you for everybody who replied to my post, special thanks to girdav for the hint and to Evgeny.Makarov for 'protecting' my question.
Anyway, I am afriad I still can't solve the problem. To tell the truth I don't really see how the lemma could be used in this case. Which form of the lemma do you mean and how does it give the result?

I would be really grateful if you could help me!
 
  • #8
I meant this result: http://en.wikipedia.org/wiki/Borel–Cantelli_lemma .

If we assume that for some $A>0$ the series $\sum_{n\geq 0}P(X_n\geq A)$ is convergent then by Borel-Cantelli lemma $P(\limsup_n X_n\geq A)=0$ . Now assume that for all $A$ we have that $\sum_{n\geq 0}P(X_n\geq A)$ is divergent, and apply a converse of Borel-Cantelli lemma, which works for independent random variables.
 
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  • #9
girdav said:
I meant this result:.

If we assume that for some $A>0$ the series $\sum_{n\geq 0}P(X_n\geq A)$ is convergent then by Borel-Cantelli lemma $P(\limsup_n X_n\geq A)=0$ . Now assume that for all $A$ we have that $\sum_{n\geq 0}P(X_n\geq A)$ is divergent, and apply a converse of Borel-Cantelli lemma, which works for independent random variables.

Thank you so much for your quick and clear answer. I really appreciate it!
 
  • #10
chisigma said:
I'm afraid that the question has been wrongly expressed and in particular there is confusion between the random variables $X_{n}$ and the probabilities $P_{n}= P\{X_{n}>A\}$...

Kind regards

$\chi$ $\sigma$
I'm afraid your wording is as wrong as ever. You're always putting some weird notations and it's quite obvious that you're not used to using common things in probability. Hence the probability that you understood wrongly is superior to the probability of the question being wrongly worded.
 
  • #11
Moo said:
I'm afraid your wording is as wrong as ever...

I'm afraid... following Dante Alighieri's sentence reported in the signature... that I'm no time to waste with monkeys ...

Kind regards

$\chi$ $\sigma$
 
  • #12
chisigma said:
I'm afraid... following Dante Alighieri's sentence reported in the signature... that I'm no time to waste with monkeys ...

Kind regards

$\chi$ $\sigma$
A monkey that can probably speak a better English than yours, but that wouldn't bother looking for the translation of an Italian sentence no one cares about. And I did mean wording, not working.

Sincerely yours,

Monkey cow.
 
  • #13
It appears that the original question has been answered so all other comments can be taken care of through private messages. If anyone ever has a complaint or comment they are free to send me a message or handle it personally with other users.
 

FAQ: Proving Finite Supremum of Independent Random Variables

What is the concept of "probability almost surely"?

"Probability almost surely" is a concept in probability theory that refers to the likelihood of an event occurring. It means that the event is almost certain to happen, with a probability of 1.

How is "probability almost surely" different from "probability one"?

While both terms refer to an event having a probability of 1, "probability almost surely" is used in situations where there may be a small chance of the event not occurring due to certain technicalities. "Probability one" is used when the event is guaranteed to occur regardless of any technicalities.

Can an event have a probability of "almost surely" but not "one"?

Technically, no. If an event has a probability of "almost surely," it means that the event will occur with a probability of 1, but there may be some exceptional cases that could prevent it from happening. However, if an event has a probability of 1, it will always happen without any exceptions.

In what situations is the concept of "probability almost surely" used?

"Probability almost surely" is often used in situations where there may be some uncertainty or ambiguity in the event, but it is still highly likely to occur. It is also used in discussions of infinite sequences and events with a large number of possible outcomes.

How is "probability almost surely" related to the concept of "almost never"?

The concepts of "probability almost surely" and "almost never" are opposite sides of the same coin. While "probability almost surely" refers to an event being almost certain to occur, "almost never" refers to an event being almost certain not to occur. Both concepts have a probability of 1, but they are used in different contexts.

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