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I don't really understand how one can find the type of singularities for a given function. Say if $$f(z) = \frac{z}{e^z-1}$$ then I know that the singularities are at $z = 2n\pi i$

However, how do I find the type? If I try to write out Laurent series for this by using $$e^z = 1 + z + z^2/2! + z^3/3! + \cdots $$, then I got a summation in the denominator, which I don't know how to rearrange as a Laurent series.

Can anyone please give me a hint?

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Then $z=0$ is removable. –  беркай May 31 '13 at 8:12
    
Oh great! I see it now. Thanks. –  Shar May 31 '13 at 8:14
    
See here. –  Mhenni Benghorbal May 31 '13 at 9:48
    
Related problem. –  Mhenni Benghorbal May 31 '13 at 9:56
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2 Answers

up vote 3 down vote accepted

On $z=0$ you can remove the singularity.

For the other use that a singularity at $z_0$ is a pole of order $k$ when $f(z)\cdot (z-z_0)^k$ does have a removable singularity at $z_0$.

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That is, to study the singularity at $2n\pi i$, you merely have to compute the order of the zero in the denominator. –  GEdgar May 31 '13 at 13:56
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Some of key points to decide the type of the singularity

I am not claiming that these points are complete. But certainly these point may help you to decide about the types of singularity of a function at a point.

1: A singular point $z_0 $is called an isolated singular point of an analytic function $f(z)$ if there exists a deleted $\epsilon$-spherical neighborhood of $z_0$ that contains no singularity. If no such neighborhood can be found, $z_0 $is called a non-isolated singular point. Thus an isolated singular point is a singular point that stands completely by itself, embedded in regular points.

2. An isolated singular point $z_0$ such that $f$ can be defined, or redefined, at $z_0$ in such a way as to be make it analytic at $z_0$ is called removable singularity. A singular point $z_0$ is removable if $\lim _{z\to z_{0}} f(z)$ exists. For example The singular point $z = 0$ is a removable singularity of $f(z) = \frac{\sin z}{z}$ since $\lim _{z\to z_{0}} \frac{\sin z}{z} = 1$. Dominic's answer says about when a singular point $z_0$ is a pole of order $k$.

3. A singular point that is not a pole or removable singularity is called an essential singular point.

4 Isolated essential singularity

Limit points of the sequence of a zeros of the nonzero analytic function $f(z)$ is an isolated singularity. Since it is not a pole it is an isolated essential singularity. For example consider the function $\sin \frac{1}{z}$ it has zeros at $z = \frac{1}{k\pi}$ . Limit points of the zeros is the zeros is the point $z =0$ hence, $z = 0$ is an isolated essential singularity of $f(z)$.

5 . Non -isolated essential singularity

Let $z_1, z_2,\ldots, z_n, \ldots$ be the set of points having limit point $z_0$ in a region $D$. Let $f$ be analytic in $D$ except at the poles $z_1, z_2,\ldots, z_n, \ldots$, then $z_0$ is also singularity of $f(z)$. The reason is that $f$ is unbounded in the neighborhood of $z_0$. But $z_0$ is not isolated because it is limit points of the poles. Hence $f(z)$ has a non-isolated singularity at $z_0$. For example consider the function $\frac{1}{\cos(1/z)}$ this function has poles at $k = \frac{2}{(2k+1)\pi}$ the point $z =0$ is the limit point of these poles hence given function has non -isolated essential singularity at $z = 0$.

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@srijan.... nice explanation sir... –  monalisa May 31 '13 at 9:22
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Thank you for the explanation. –  Shar Jun 11 '13 at 3:45
    
you are welcome. –  srijan Jun 12 '13 at 17:22
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