Residues/Classifying singularities

In summary, The terms essential singularity, isolated singularity, removable singularity, and pole of order m are used to classify singularities in complex analysis. An essential singularity is a singularity where the function is not analytic and cannot be removed by a change of definition. An isolated singularity is a singularity that is not surrounded by any other singularities. A removable singularity can be removed by a change of definition. A pole of order m is a singularity where the function has a Laurent expansion with a non-zero coefficient at degree m. Examples of functions with these types of singularities include $e^{\frac{1}{z}}$, $\frac{1}{z-1}$, $\frac{1}{z
  • #1
nacho-man
171
0
Hi,
so first of all
I am not entirely confident with the terminology when it comes to classifying singularities.
Could someone give me an example of the different types, or explain what they mean? My confusion stems from the terms:
essential singularity, isolated singularity, removable singularity and pole of order m
I would prefer a non-mathematical jargonised definition, as these are readily available in my textbook, however I find them difficult to comprehend.

I know that $e^{\frac{1}{Z}}$ has an essential singularity at $z=0$
But I don't see what really distinguishes this and any other type of singularity. Except that you can't let z approach 0 and 'remove' it, i guess.

Whereas $\frac{1}{z-1}$ has a removable singularity at z=1. Is this also known as an isolated singularity?

$\frac{1}{z^2}$ does this have a pole of order 2 at z=0
WHat about $\sin{\frac{1}{z^2}}$ ?
Secondly, I have attached a residue question.
so far, I want to know if i am on the correct track.

$\cosh z = \frac{e^Z+e^{-Z}}{2}$
This is analytic everywhere, so we need not worry about this.

The poles are:

$z = 2$ is a removable pole? (i don't know if this is correct terminology)
$z=1$ is a pole of order 2
$z=0$ is a removable pole.

Am I on the right track so far?

I feel i might have missed something with the treatment $\cosh z$ part

Any help is very much appreciated, thanks!
 

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  • #2
nacho said:
Hi,
so first of all
I am not entirely confident with the terminology when it comes to classifying singularities.
Could someone give me an example of the different types, or explain what they mean? My confusion stems from the terms:
essential singularity, isolated singularity, removable singularity and pole of order m

I would prefer a non-mathematical jargonised definition, as these are readily available in my textbook, however I find them difficult to comprehend.

I know that $e^{\frac{1}{Z}}$ has an essential singularity at $z=0$

But I don't see what really distinguishes this and any other type of singularity. Except that you can't let z approach 0 and 'remove' it, i guess.

Whereas $\frac{1}{z-1}$ has a removable singularity at z=1. Is this also known as an isolated singularity?

$\frac{1}{z^2}$ does this have a pole of order 2 at z=0

WHat about $\sin{\frac{1}{z^2}}$?...

I agree with You about the fact that in most textbooks the argument 'singularities' is trated with great superficiality. Generally specking a function f(z) has a singularity in $z=z_{0}$ if in that point it is not analytical. A singularity is said to be isolated if in all the points where is $|z - z_{0}| < r, r > 0$ there aren't other singularities. A singularity is said to be removable if with a different definition of f(*) in $z=z_{0}$ the singularity disappears. In the other cases the singularity is [or better should be...] classified according to its Laurent expansion around $z_{0}$... $\displaystyle f(z) = \sum_{n = - \infty}^{ + \infty} c_{n}\ (z - z_{0})^{n}\ (1)$

If $\displaystyle \lim_{z \rightarrow z_{0}} f(z)\ (z-z_{0})^{k} = c_{- k} \ne 0$ then $z_{0}$ is a pole of order k. If n isn't bounded in its negative values then $z_{0}$ is an essential singularity. This definition however don't take into account that an f(z) can be non analytic in $z= z_{0}$ and the Laurent expansion of f(z) around $z = z_{0}$ may not exists... Kind regards $\chi$ $\sigma$
 
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FAQ: Residues/Classifying singularities

What are residues in mathematics?

Residues are a mathematical concept used in complex analysis to understand the behavior of a function around its singularities. They are essentially the "leftover" of a function after all the poles (or singularities) have been taken into account.

How are residues calculated?

Residues can be calculated using the formula Res(f, z0) = limz→z0 (z-z0)f(z), where z0 is a singularity of the function f(z). This means taking the limit of the function as it approaches the singularity, multiplied by the difference between the function and the singularity.

What is the purpose of classifying singularities?

Classifying singularities helps us understand the behavior of a function near that singularity. It tells us whether the function is bounded or unbounded, and how fast it approaches infinity. It also helps in finding residues and evaluating complex integrals.

What are the different types of singularities?

There are three types of singularities: removable, poles, and essential. A removable singularity is a point where the function can be extended to be continuous. A pole is a point where the function becomes unbounded, and an essential singularity is a point where the function oscillates infinitely.

How do I know which type of singularity a function has?

To determine the type of singularity, we can look at the behavior of the function near that point. If the function approaches a finite value, it is a removable singularity. If it approaches infinity, it is a pole. If the function oscillates, it is an essential singularity. We can also use the Laurent series expansion to determine the type of singularity.

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