# Convergence and Divergence of a series (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

#### mohabitar

The series from n=1 to infinity log(n/(n+1)). This was on my quiz, which I got wrong. Here's what I did:
lim n-->infinity of log(n/(n+1))
so then that becomes: log(lim n-->infinity n/(n+1))
which becomes the log1, which is 0, so it converges.

Whats wrong with my steps?

#### rakalakalili

What you showed was that the sequence converges to 0, not the series. The sequence has to converge to 0 for the series to converge, but it does not guarantee that it does. A good example of this is the harmonic series
$$\sum_{k=1}^{\infty} \frac{1){k}$$
The limit as k goes to infinity of 1/k is zero, but the series still diverges.

#### Mark44

Mentor
If you have a series $\sum a_n$, and lim an is not 0, or the limit doesn't exist, then you know that your series diverges.

If lim an = 0, then you really can't say much at all about your series. That's what all the tests (comparison, ratio, root, integral, limit comparison, etc.) are about.

#### mohabitar

I thought what I did was a test?? Ahhh this stuff is so confusing! So many times in my book it said if the lim of an=0, then it converges. I dont get whats going on.

#### Mark44

Mentor
I thought what I did was a test?? Ahhh this stuff is so confusing! So many times in my book it said if the lim of an=0, then it converges. I dont get whats going on.
I suspect that you are confusing a sequence, {an}, with a series, $\sum a_n$. If lim an = 0 (or any specific number), the sequence converges, but nothing can be said about the series $\sum a_n$.

The harmonic series that rakalakalili gave and this series $$\sum_{k=1}^{\infty} \frac{1}{k^2}$$
are such that lim an = 0, but the harmonic series diverges and the other series converges.

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving