5
$\begingroup$

For the sequence $$S_n=((-1)^j\times j); \forall j \in \mathbb{N}$$

I am having hard time understanding what

Limit of the sequence does not exist but $$\limsup_{n\rightarrow\infty}(S_n)$$ exist and =$\infty$ and $$\liminf_{n\rightarrow\infty}(S_n)$$ exists and =$-\infty$ mean.

Added: My only concern here is in case of limit, whenever limit is equal to infinity, we say limit does not exist. But Why does Limit sup =infinity mean limit exists? Why is there such difference?

$\endgroup$

2 Answers 2

7
$\begingroup$

The $\limsup$ is the largest real number, or $\pm\infty$, which is a limit of a subsequence of $S_n$, whereas $\liminf$ is the smallest real number (or $\pm\infty$) which is a limit of a subsequence of $S_n$.

It follows that $\lim a_n$ exists (in the broad sense) if and only if $\liminf a_n=\limsup a_n$.

The existence of $\liminf,\limsup$ follows from the completeness of $\mathbb R$.


If $S_n$ is a sequence such that $\limsup S_n=\infty$ then there is a subsequence, $S_{n_k}$ which is strictly increasing. Therefore its limit is $\infty$.

To the edit:

One cannot treat $\infty$ as a real number. It's good when a sequence has a limit within the real numbers, but sometimes it is sufficient that it is convergent, i.e. satisfies some definition.

To say that the limit of a sequence is $\infty$ is to say that although the sequence does not tend to a real number, it behaves "nice enough". This is in contrast to sequences which have no limit at all and just jump around between numbers.

This is why we sometimes say that it converges in a broad sense when it has the limit $\pm\infty$. Sometimes, when context is clear enough, we may omit the "broad sense" part too.

$\endgroup$
5
  • $\begingroup$ I think you're confusing which one is the smallest and largest in your first paragraph. $\endgroup$
    – Arthur
    Commented Dec 25, 2012 at 21:49
  • $\begingroup$ @Arthur: Thanks. $\endgroup$
    – Asaf Karagila
    Commented Dec 25, 2012 at 21:50
  • $\begingroup$ I was unclear but the main part of my question was, in case of limits, when we calculate $\lim{f(n)}=\infty$, we say limit does not exist. Is not lim=$\infty$ same as limit not existing? Here, even though $\lim=\infty$ we said limit exists. My question was about this difference in saying limit exists and does not exist. $\endgroup$
    – 007resu
    Commented Dec 25, 2012 at 22:01
  • 1
    $\begingroup$ @user1710036: Did my edit answer your question? $\endgroup$
    – Asaf Karagila
    Commented Dec 25, 2012 at 22:08
  • $\begingroup$ Yes it does. Thanks. $\endgroup$
    – 007resu
    Commented Dec 25, 2012 at 22:17
1
$\begingroup$

A limit that equals to infinity does exist: it is infinity. Infinity as a result of a limit doesn't mean limit doesn't exist.

A limit that doesn´t exist is a limit that can't be determined. By definition, a global limit exists only if limits of both sides be the same, or, using your variables, this equation should be truth $$\lim_{j\to \infty+}=\lim_{j\to \infty-}$$

but it isn't!

So that limit, actually, doesn't exist!

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .