Laurent series expansion of Log(1+1/(z-1))

In summary, the given function has a singularity at z = 1 and the nearest other singularity is at z = 0. Therefore, the Laurent series expansion exists on the punctured disk centered at z = 1 with radius 1. However, it cannot be found in the region 0 < |z-1| < 1, and the only region where it is possible is for |z-1| > 1. This is due to the function not being analytic on the real line segment between 0 and 1.
  • #1
Boorglar
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Homework Statement


Find the Laurent series expansion of [itex]f(z) = \log\left(1+\frac{1}{z-1}\right)[/itex] in powers of [itex]\left(z-1\right)[/itex].

Homework Equations


The function has a singularity at z = 1, and the nearest other singularity is at z = 0 (where the Log function diverges). So in theory there should be a Laurent series on the punctured disk centered at z = 1 with radius 1.

The Attempt at a Solution


I can't seem to rewrite the expression in a form which can be used to find the Laurent series for [itex]0 < |z - 1| < 1 [/itex]. I can find the Laurent series for [itex] |z - 1| > 1 [/itex] by rewriting [itex]f(z) = -\log\left(\frac{1}{1+\frac{1}{z-1}}\right)[/itex] and then expanding using the series expansion of [itex]\log\left(\frac{1}{1-z'}\right)[/itex] and substituting [itex]z' = \frac{-1}{z-1}[/itex]. This does not work in the region 0 < |z-1| < 1, unfortunately.

Wolfram and the solutions manual did not give any Laurent series for this region either, which leads me to believe that such a series does not exist. But I don't see why, since the function appears to be analytic in the punctured disk, and Laurent's theorem should apply.
 
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  • #2
I just realized I made a rookie mistake. The function is not analytic anywhere on the real line segment between 0 and 1, which means there is no punctured disk throughout which the function is analytic. The only region where the Laurent series exists is for |z-1| > 1.
Sorry for this :P
 

FAQ: Laurent series expansion of Log(1+1/(z-1))

1. What is a Laurent series expansion?

A Laurent series expansion is a mathematical representation of a complex function as a sum of a polynomial and an infinite series of negative powers of the variable. It is used to extend the concept of a Taylor series to functions that have singularities in their domain.

2. Why is the Laurent series expansion of Log(1+1/(z-1)) important?

The Laurent series expansion of Log(1+1/(z-1)) is important because it allows us to approximate the complex logarithm function near the singularity at z=1. This is useful in many applications, such as in complex analysis and in solving differential equations.

3. How is the Laurent series expansion of Log(1+1/(z-1)) calculated?

The Laurent series expansion of Log(1+1/(z-1)) can be calculated using the Cauchy integral formula and the Cauchy integral theorem. The resulting series will have a negative power term for each integer from -∞ to 0, and a positive power term for each positive integer.

4. What is the radius of convergence for the Laurent series expansion of Log(1+1/(z-1))?

The radius of convergence for the Laurent series expansion of Log(1+1/(z-1)) is the distance from the center of the series, which is at z=1, to the nearest singularity. In this case, the nearest singularity is at z=1, so the radius of convergence is 1.

5. Can the Laurent series expansion of Log(1+1/(z-1)) be used to evaluate the complex logarithm at points outside the radius of convergence?

No, the Laurent series expansion of Log(1+1/(z-1)) is only valid within its radius of convergence, which is 1. Attempting to evaluate the complex logarithm at points outside this radius may result in incorrect or undefined values.

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