# Simple Pole

## Main Question or Discussion Point

How exactly would one go about proving that

$$\frac{1}{1-2^{1-z}}$$ has a simple pole at $$z=1$$?

I've tried writing $$2^{1-z}$$ in terms of e to get a Taylor series for the denominator but can't quite figure out where to go from there.

Gib Z
Homework Helper
In complex analysis, a pole of a holomorphic function is a certain type of simple singularity that behaves like the singularity 1/z^n at z = 0. A pole of order 1 is called a simple pole as you know. Essentially you need to prove
z behaves like 1-2^(1-z) near zero. Perhaps if you showed that $$\lim_{z\to 0} \frac{z}{1-2^{1-z}} = 1$$...

Gib Z
Homework Helper
Scratch that, im not sure that helps..All you should need to show is that it approaches the same limit, so the first one, z, as z goes to 0, its just zero. the second one is also 0. That should do it.

If your not happy, show that the function Laurent series near z=1 below degree −n vanishes and the term in degree −n is not zero.

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OK so I've opted for the Laurant Series route. Where am I going wrong here:

$$\frac{1}{1-2^{1-z}} = \displaystyle\sum_{n=0}^{\infty}(2^{1-z})^n = \displaystyle\sum_{n=0}^{\infty}(e^{(1-z)log2})^n = \displaystyle\sum_{n=0}^{\infty}(\displaystyle\sum_{m=0}^{\infty}\frac{(1-z)^m(log2)^m}{m!})^n$$

This doesn't look like it has any singilarities.

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Zurtex
Homework Helper
I'm not sure using the geometric series is really valid for about z = 1, I can get an appropriate series on mathematica which shows it is a simple pole, but I have no idea how to generate it myself.

marcusl
Gold Member
If it has a simple pole, then the residue should be bounded and given by

$$R=\lim_{z\rightarrow 1} (z-1)f(z)=\lim_{z\rightarrow 1} \frac{z-1}{1-2^{1-z}}.$$

Substitute $$u=z-1$$ to get

$$R=\lim_{u\rightarrow 0} \frac{u}{1-2^{-u}}$$

Evaluating using l'Hospital's rule gives R=2.
EDIT: Accordingly, 2/(z-1) is a term in the Laurent series.
EDIT2: define u=z-1 instead of 1-z for clarity

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Hmmm, it's actually 1/log2 but thanks anyway. I'm really intrigued to know whether there is a reasonable way of calculating the Laurant series though. Surely there must be a bit of trickery that will work.

marcusl
Gold Member
Oops! I took the derivative wrong! Very sorry

mathwonk
Homework Helper
you want to show [z-1] over that function is bounded.

but you just want to show 2^(1-z) equals 1 simply when z=1, or that 2^z equals 1 simply when z=0, but thats sort of clear, since the derivative is not zero anywhere, 2^z has no multiple values at all.

marcusl
Gold Member
I'll repeat my earlier derivation, without the silly error.

$$f(z)=\frac{1}{1-2^{1-z}}=\frac{2^z}{2^z-2}$$

has a pole if the residue

$$R=\lim_{z\rightarrow 1} (z-1)f(z)=\lim_{z\rightarrow 1} \frac{2^z (z-1)}{2^z-2}$$.

is bounded. To evaluate via l'Hospital's rule, write

$$2^z=(e^{\ln2})^z=e^{z\ln2}$$

so the derivative is

$$\frac{d(2^z)}{dz}=2^z \ln(2).$$

Then

$$R=\lim_{z\rightarrow 1} \frac{2^z[1+(z-1)\ln(2)]}{2^z \ln2} = \frac{1}{\ln(2)} .$$

Hope I redeemed myself!

EDIT: It's even simpler to define u=z-1, then

$$R=\lim_{u\rightarrow 0} \frac{u}{1-2^{-u}}$$

and applying l'Hospital's rule gives

$$R=\lim_{u\rightarrow 0} \frac{1}{2^u \ln(2)}=\frac{1}{\ln(2)}.$$

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mathwonk
Homework Helper
didnt i make this trivial? or did i screw up?

marcusl
Gold Member
Can't answer 'cause I don't know what you mean by "2^(1-z) equals 1 simply when z=1"?

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Gib Z
Homework Helper
>.<" Perhaps the "simply" is troubling you? Thats not a mathematical term he's using, hes just saying that it is simple.

For 2^n to equal 1, the only value n can be is 0. In this case, n is 1-z.
1-z=0. z=1.

What about $$z=1 + \frac{2\pi in}{log(2)}$$ for any integer n?

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Gib Z
Homework Helper
No need for that double post :P And we are dealing with the primary branch only, otherwise nothing we talk about here are functions anymore.

I'm sorry you've lost me now. Is it or is it not true that the function 2^(1-z) has poles at $$z=1 + \frac{2\pi in}{log(2)}$$?

What do you mean by primary branch?

Oh I see now, the primary branch of the complex log function. I forgot about that.

Still, do we have poles here or not?

Gib Z
Homework Helper
Yes! Think about the definiton of a simple pole, and what mathwonk said.

I don't understand why you said

>.<" Perhaps the "simply" is troubling you? Thats not a mathematical term he's using, hes just saying that it is simple.

For 2^n to equal 1, the only value n can be is 0. In this case, n is 1-z.
1-z=0. z=1.
when it is clearly wrong.

And what's all this talk about it not being a function? I have only ever alluded to 2^n being 1 whose imaginary part, if I am not mistaken, lies between + and - pi.

Gib Z
Homework Helper
"For 2^n to equal 1, the only value n can be is 0. In this case, n is 1-z.
1-z=0. z=1. "

I don't understand why that is clearly wrong...

For 2^n to equal 1, the only value n can be is 0.
Try $$n = \frac{2\pi i}{\log 2} \not = 0$$.

Gib Z
Homework Helper
Ok fine let me rephrase that then, the only value n can be, in the primary branch on the complex log function which is what we are dealing with me, is zero.

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Zurtex
Homework Helper
Ok fine let me rephrase that then, the only value n can be, IN THE PRIMARY BRANCH OF THE COMPLEX LOG FUNCTION WITH WHICH WE ARE DEALING WITH HERE, is zero.
Erm, o.k, I don't really understand what you are on about. You're talking about the solutions of a complex equation, there's no reason to think we should limit our search to only real numbers.

But furthermore I don't understand why you are talking about the primary branch of the complex log function, it doesn't seem to make sense to the context of the question. I don't think stating it in capitals makes it any more relevant, perhaps you could explain your motive for talking about this better.

Gib Z
Homework Helper
Im not limiting the search to real numbers, just the primary branch of the complex log. In this case, the solution happens to be a real number. And We needed to bring the primary branch of the complex log into this because otherwise we would have an infinite number of solutions to choose from to solve 2^(1-z)=1, as the OP noticed.

Ill edit my previous post so its not so capital, that was stupid i realize now.

I think we may have gone off on a bit of a tangent. Thank you for trying to expain Gib Z but I think a few of us do not understand mathwonk's motives for his method and hence don't understand your motives for limiting the number of solutions. Not a clue what's going on there. I myself have only seen two ways of showing that a function has simple poles, namely the limit method that marcusl demonstrated; and finding a Laurent series in order to show that the only nonzero coefficient of a negative power is that corresponding to the power -1. I can't see how mathwonk's method falls into either of these categories.

It would be nice to know though. And I'm still curious as to whether there is a nice way of deducing the exact form of the Laurent series since Zurtex claims that mathematica gives it fairly succinctly.

you want to show [z-1] over that function is bounded.

but you just want to show 2^(1-z) equals 1 simply when z=1, or that 2^z equals 1 simply when z=0, but thats sort of clear, since the derivative is not zero anywhere, 2^z has no multiple values at all.
Just don't know what he's trying to do here.