# Math Help - Taylor Series expansion

1. ## Taylor Series expansion

After working out the poles to:
$
f(z)=\frac{1}{2z^{2}+(6-4i)z-12i}$

I have the answer z1=2i and z2=3

Given this how would we find the taylor series if:

$\left | z \right |< \left | z1 \right |$

2. Originally Posted by zizou1089
After working out the poles to:
$
f(z)=\frac{1}{2z^{2}+(6-4i)z-12i}$

I have the answer z1=2i and z2=3
Shouldn't we have $z_2=-3$, or have you given us the wrong $f(z)$? For the rest of this post I am assuming that your $f(z)$ is right, and that $z_2=-3$.

Given this how would we find the taylor series if:

$\left | z \right |< \left | z1 \right |$
First you do a partial fraction decomposition by requiring that
$f(z)=\frac{a}{2i-z}+\frac{b}{3+z}=-\frac{1}{2(2i-z)(3+z)}$
which gives, if I am not misaken, that $a=b=-\frac{1}{6+4i}$. (I might be mistaken, however, so please to the math yourself.)

Now you can develop each of these two fractions with the help of a geometric series like this:
$\frac{a}{2i-z}+\frac{b}{3+z} = \frac{a}{2i}\cdot \frac{1}{1-\frac{z}{2i}}+\frac{b}{3}\cdot \frac{1}{1-\left(-\frac{z}{3}\right)}$

$=\frac{a}{2i}\sum_{n=0}^\infty \left(\frac{z}{2i}\right)^n+\frac{b}{3}\sum_{n=0}^ \infty \left(-\frac{z}{3}\right)^n=\ldots$

3. Ok, wouldnt $\left |2i \right |$=2?

4. How would the answer change if $\left |z1 \right |< \left | z \right |< \left | z2 \right |$

5. Originally Posted by zizou1089
How would the answer change if $\left |z1 \right |< \left | z \right |< \left | z2 \right |$
In that case, you need a Laurent-Series development, because the geometric series that we used for the first term of the partial fraction decomposition no longer converges for such values of z.

You can get it, starting from the same partial fraction decomposition, like this

$\frac{a}{2i-z}+\frac{b}{3+z} = -\frac{a}{z}\cdot \frac{1}{1-\frac{2i}{z}}+\frac{b}{3}\cdot \frac{1}{1-\left(-\frac{z}{3}\right)}$

$=-\frac{a}{z}\sum_{n=0}^\infty \left(\frac{2i}{z}\right)^n+\frac{b}{3}\sum_{n=0}^ \infty \left(-\frac{z}{3}\right)^n=\ldots$

6. Originally Posted by Failure
In that case, you need a Laurent-Series development, because the geometric series that we used for the first term of the partial fraction decomposition no longer converges for such values of z.

You can get it, starting from the same partial fraction decomposition, like this

$\frac{a}{2i-z}+\frac{b}{3+z} = -\frac{a}{z}\cdot \frac{1}{1-\frac{2i}{z}}+\frac{b}{3}\cdot \frac{1}{1-\left(-\frac{z}{3}\right)}$

$=-\frac{a}{z}\sum_{n=0}^\infty \left(\frac{2i}{z}\right)^n+\frac{b}{3}\sum_{n=0}^ \infty \left(-\frac{z}{3}\right)^n=\ldots$
Okay Im starting to get the hang of this, its just aint $\left | 2i \right |=2$? For the first part when we divide $\left | z \right |< \left | z1 \right |$ by $\left | z1 \right |$ we have

$\frac{\left | z \right |}{\left | 2i \right |}< 1$

which is equal to $\frac{\left | z \right |}{2}< 1$
therefore:

$\frac{a}{2}\cdot \frac{1}{1-\frac{z}{2}}+\frac{b}{3}\cdot \frac{1}{1-\left(-\frac{z}{3}\right)}$??

7. Originally Posted by zizou1089
Okay Im starting to get the hang of this,
Great!

its just aint $\left | 2i \right |=2$? For the first part when we divide $\left | z \right |< \left | z1 \right |$ by $\left | z1 \right |$ we have

$\frac{\left | z \right |}{\left | 2i \right |}< 1$

which is equal to $\frac{\left | z \right |}{2}< 1$
therefore:

$\frac{a}{2}\cdot \frac{1}{1-\frac{z}{2}}+\frac{b}{3}\cdot \frac{1}{1-\left(-\frac{z}{3}\right)}$??
Well, yes, the condition for the geometric series expansion $\frac{1}{1-z}=\sum_{n=0}^\infty z^n$ to converge is that $|z|<1$ holds.
Therefore, if $|z| < |2i|$, we have that $\left|\frac{z}{2i}\right| < 1$, and thus the series $\sum_{n=0}^\infty \left(\frac{z}{2i}\right)^n$ converges. But if $|2i|< |z|$, we have $1<\left|\frac{z}{2i}\right|$, thus $\sum_{n=0}^\infty \left(\frac{z}{2i}\right)^n$ diverges.

By a purely algebraic transformation of that first term of the partial fraction decomposition we can use the (for $|2i| < |z|$) convergent geometric series $\sum_{n=0}^\infty \left(\frac{2i}{z}\right)^n$, instead.

Simililarly, if we were told to develop the given function for $|-3|<|z|$, we would have to transform the second term of the partial fraction decomposition in such a way that we can develop it in a, for $\left|\frac{-3}{z}\right|<1$, convergent series of the form $\sum_{n=0}^\infty\left(-\frac{3}{z}\right)^n$.

8. Originally Posted by Failure
Great!

Well, yes, the condition for the geometric series expansion $\frac{1}{1-z}=\sum_{n=0}^\infty z^n$ to converge is that $|z|<1$ holds.
Therefore, if $|z| < |2i|$, we have that $\left|\frac{z}{2i}\right| < 1$, and thus the series $\sum_{n=0}^\infty \left(\frac{z}{2i}\right)^n$ converges. But if $|2i|< |z|$, we have $1<\left|\frac{z}{2i}\right|$, thus $\sum_{n=0}^\infty \left(\frac{z}{2i}\right)^n$ diverges.

By a purely algebraic transformation of that first term of the partial fraction decomposition we can use the (for $|2i| < |z|$) convergent geometric series $\sum_{n=0}^\infty \left(\frac{2i}{z}\right)^n$, instead.

Simililarly, if we were told to develop the given function for $|-3|<|z|$, we would have to transform the second term of the partial fraction decomposition in such a way that we can develop it in a, for $\left|\frac{-3}{z}\right|<1$, convergent series of the form $\sum_{n=0}^\infty\left(-\frac{3}{z}\right)^n$.
Ok I seem to be getting more questions correct, one last example Im having difficulty with is what if,

$|z-z1| < |z2-z1|$??

Thanks!

9. Originally Posted by zizou1089
Ok I seem to be getting more questions correct, one last example Im having difficulty with is what if,

$|z-z1| < |z2-z1|$??

Thanks!
In this case you cannot develop the series around $0$ anymore, but have instead to develop it around $z_1$: which means, that you want to develop your function into Laurent-series that do not have terms of the form $a_n z^n$ but terms of the form $a_n (z-z_1)^n$ instead.

The first term of your partial fraction decomposition is already (almost) in that form, namely
$\frac{a}{2i-z}=-a(z-2i)^{-1}$.

For the second term of your partial fraction decomposition you can substitute $(z-2i)+2i$ for $z$ and try to transform it into a power series in terms of $(z-2i)^n$.

10. Originally Posted by Failure
In this case you cannot develop the series around $0$ anymore, but have instead to develop it around $z_1$: which means, that you want to develop your function into Laurent-series that do not have terms of the form $a_n z^n$ but terms of the form $a_n (z-z_1)^n$ instead.

The first term of your partial fraction decomposition is already (almost) in that form, namely
$\frac{a}{2i-z}=-a(z-2i)^{-1}$.

For the second term of your partial fraction decomposition you can substitute $(z-2i)+2i$ for $z$ and try to transform it into a power series in terms of $(z-2i)^n$.
I dont seem to be getting the right answer for this last question, help needed please! Thanks in advance!

11. Originally Posted by zizou1089
I dont seem to be getting the right answer for this last question, help needed please! Thanks in advance!
I have only very little time for this (it's early in the morning here in Switzerland). So let me start with the partial fraction decomposition that we've already used successfully for the other cases and try to develop it as series of integral powers of $(z-z_1)$, that is $(z-2i)$:
$\frac{a}{2i-z}+\frac{b}{3+z} = -a(z-2i)^{-1}+\frac{b}{3+2i+(z-2i)} =-a(z-2i)^{-1}+\frac{b}{3+2i}\cdot\frac{1}{1-\left(-\frac{z-2i}{3+2i}\right)}$

$=-a(z-2i)^{-1}+\frac{b}{3+2i}\cdot\sum_{n=0}^\infty \left(-\frac{z-2i}{3+2i}\right)^n=\ldots$

Note that this series converges, as required, if $|z-2i| < |-3-2i|=|3+2i|$, because in that case we have that $\left|-\frac{z-2i}{3+2i}\right|<1$

12. Originally Posted by Failure
I have only very little time for this (it's early in the morning here in Switzerland). So let me start with the partial fraction decomposition that we've already used successfully for the other cases and try to develop it as series of integral powers of $(z-z_1)$, that is $(z-2i)$:
$\frac{a}{2i-z}+\frac{b}{3+z} = -a(z-2i)^{-1}+\frac{b}{3+2i+(z-2i)} =-a(z-2i)^{-1}-\frac{b}{3+2i}\cdot\frac{1}{1-\left(-\frac{z-2i}{3+2i}\right)}$

$=-a(z-2i)^{-1}+\frac{b}{3+2i}\cdot\sum_{n=0}^\infty \left(-\frac{z-2i}{3+2i}\right)^n=\ldots$

Note that this series converges, as required, if $|z-2i| < |-3-2i|=|3+2i|$, because in that case we have that $\left|-\frac{z-2i}{3+2i}\right|<1$

Thanks that helped alot.