# Thread: Limit point and cluster point

1. ## Limit point and cluster point

My professor vaguely defined a cluster point and I'm a bit confused on the difference between limit point and cluster point.

He said that

" Suppose a_n converges to alpha.

Sequence {a_n} has a cluster point, if a subsequence of a_n converges to alpha.

If a is a cluster point, it is a limit point."

Can you clarify the difference between cluster point and limit point.

I am currently in a introductory analysis class.

Thank you.

2. ## Re: Limit point and cluster point

Originally Posted by rokman54
My professor vaguely defined a cluster point and I'm a bit confused on the difference between limit point and cluster point. He said that
" Suppose a_n converges to alpha.
Sequence {a_n} has a cluster point, if a subsequence of a_n converges to alpha.
If a is a cluster point, it is a limit point."
Can you clarify the difference between cluster point and limit point.
I am currently in a introductory analysis class.
Look at this webpage.

There is a very common misconception here. The limit of a sequence is a cluster point of the sequence but a cluster of a sequence may not be a limit of a sequence. The sequence $\left( {{{\left( { - 1} \right)}^n} + \frac{1}{n}} \right)$ has two cluster points, $1~\&~-1$ but the sequence does not converge so it has no limit although it does have two limit points.

3. ## Re: Limit point and cluster point

What is the formal definition of cluster point?

4. ## Re: Limit point and cluster point

Originally Posted by rokman54
What is the formal definition of cluster point?
Did you look at that web page? Cluster point is just another type of limit point.

5. ## Re: Limit point and cluster point

The Wikipedia page Plato links to gives the definition of a "cluster point":
A point x ∈ X is a cluster point or accumulation point of a sequence $(x_n)$, n ∈ N, if, for every neighbourhood V of x, there are infinitely many natural numbers n such that $x_n$ ∈ V.

It also says "The set of all cluster points of a sequence is sometimes called a limit set." implying that "cluster point" and "limit point" are names for the same thing. However, Plato's point, before, was that "cluster point" or "limit point" of a sequence may not be a limit of that sequence.