Laurent series of the function f(z)=1/z^2 for |z-a|>|a|

In summary, the conversation is about finding the Laurent series for the function f(z) = 1/z^2 for |z-a| > |a|, and also for the function F(z) = exp(z+1/z) around zero. The participants discuss different approaches, including using a formula for the coefficients, rewriting the function as a sum of geometric series, and considering the derivative of -1/z. There is also a mention of the difficulty of dealing with infinite sums.
  • #1
heman
361
0
Hi all,,

I have an Exam tommorow and this question is irritating me...Pls help

Laurent series of the function f(z)=1/z^2 for |z-a|>|a| .a is not equal to zero...
I am waiting for yours responses...I will be highly thankful to you.
 
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  • #2
Don't you have a formula for the coefficients, that involves integrating over the annulus |z - a| > |a|?

Or, maybe you could look at it as:

|1 / (z-a)| < 1/|a|

And try to rewrite 1/z^2 so it looks like the sum of a geometric series in 1/(z-a)?
 
  • #3
Omg,,,I did not use the formula for the coefficients,it did not seemed to strike me,,,yeah then its easy...and can be solved...

I was trying to do it by bringing in the form of geometric series..
Hurkyl,
Pls tell me how can i write 1/z^2 in that apt form as here the power of z is 2...
I seem to moving forward but then 2 series will be multiplied acc. to me,won't it be tedious..?
 
  • #4
Hrm, yah, writing it as a geometric series won't work... but maybe the sum of two geometric series?

I guess it would be easier to observe that d/dz (-1/z) = 1/z^2, so if you had a Laurent series for -1/z...
 
  • #5
Oh yes,,,i got it ...That will work...Thanks Hurkyl ,I feel much better now!

But in writing the Laurentz Series for F(z)=exp(z+1/z) around zero ,i think i have got no escape ...i think i am bound to multiply the series for e^z and e^(1/z),,,But how will i write the coefficient for each term,,or there can be any better approach...
 
  • #6
I don't see the difficulty in multiplying the series... I guess you don't like the fact that each coefficient will be an infinite sum. :smile: (Though, it wouldn't surprise me if there's a clever way to do this that gives you a closed form)
 
  • #7
Hurkyl said:
I don't see the difficulty in multiplying the series... I guess you don't like the fact that each coefficient will be an infinite sum. :smile: (Though, it wouldn't surprise me if there's a clever way to do this that gives you a closed form)


"Each coefficient will be an infinite sum."...that i can see but where it will start that seems out of my reach...limits are worrying me...i am able to see it mechanically only...
I think i sincerely need help on this!
 
  • #8
Have you taken an analysis or advanced calculus course? The relevant theorems should be in there.

The main thing here, though, is that the series involved are absolutely convergent, meaning that you can rearrange terms in any way you please.
 

1. What is a Laurent series?

A Laurent series is a type of mathematical series that represents a complex function as an infinite sum of terms. It is commonly used to study functions in complex analysis.

2. What is the function f(z)=1/z^2?

The function f(z)=1/z^2 is a complex function that maps every point in the complex plane to its reciprocal squared. This means that for any complex number z, f(z) is equal to 1 divided by z squared.

3. What does |z-a|>|a| mean in the context of the Laurent series of f(z)=1/z^2?

This notation indicates that the Laurent series is being evaluated at a point a, which is outside the circle of convergence (i.e. |z-a|>|a|). In other words, the series is being evaluated at a point where the function is not analytic.

4. How do you find the Laurent series of f(z)=1/z^2 for |z-a|>|a|?

To find the Laurent series of f(z)=1/z^2 for |z-a|>|a|, you can use the method of partial fractions. First, express the function as a sum of two terms (1/z and 1/z^2), and then expand each term using their respective power series. The resulting series will be the Laurent series of f(z)=1/z^2 for |z-a|>|a|.

5. What is the significance of studying the Laurent series of f(z)=1/z^2 for |z-a|>|a|?

The Laurent series of f(z)=1/z^2 for |z-a|>|a| is important because it allows us to study the behavior of the function at points where it is not analytic. This can provide insights into the singularities and poles of the function, and how it behaves near these points.

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