- #1
thelema418
- 132
- 4
My task is to solve the integral [itex]\frac{1}{\cos 2z}[/itex] on the contour [itex]z=|1|[/itex] using a Laurent series.
The easy part of this is the geometric part. I drew a picture of the problem. I see that there are two singularity points that occur within the contour region at [itex]\pm \frac{\pi}{4}[/itex]. I realize that one of the problems with a Taylor series expansion of sec z is that it is only convergent for [itex]\pm \frac{\pi}{2}[/itex].
I have tried several methods of determining the Laurent series, but I am having difficulty with this problem. I have tried two methods, and I am not certain what might be a better strategy -- or if I am going down the wrong road completely. In one method (a), I create a Taylor series, and then construct a geometric series. In the other (b), I use a trigonometric identity and derive partial fractions.
In method (a) I considered [itex]u=2z[/itex], [itex]\frac{1}{\cos{u}}[/itex]. I changed the cosine to a Taylor Series, and then recognized the geometric series, so that:
[itex]\sum_{j=0}^\infty \left( \sum_{k=0}^\infty \frac{z^{(2k+2)}}{(2k+1)!} \right)^j[/itex]
In method (b) I got
[itex] \frac{1}{2} \left( \frac{1}{1-\sqrt{2} \sin z} - \frac{1}{1+\sqrt{2} \sin z}\right)[/itex]
Is one of these methods better to keep following -- or am I on the wrong track altogether? Also, sorry to be not as thoroughly detailed... I wrote a lot, but the system logged me out during one of my previews and I lost everything.
Thanks
The easy part of this is the geometric part. I drew a picture of the problem. I see that there are two singularity points that occur within the contour region at [itex]\pm \frac{\pi}{4}[/itex]. I realize that one of the problems with a Taylor series expansion of sec z is that it is only convergent for [itex]\pm \frac{\pi}{2}[/itex].
I have tried several methods of determining the Laurent series, but I am having difficulty with this problem. I have tried two methods, and I am not certain what might be a better strategy -- or if I am going down the wrong road completely. In one method (a), I create a Taylor series, and then construct a geometric series. In the other (b), I use a trigonometric identity and derive partial fractions.
In method (a) I considered [itex]u=2z[/itex], [itex]\frac{1}{\cos{u}}[/itex]. I changed the cosine to a Taylor Series, and then recognized the geometric series, so that:
[itex]\sum_{j=0}^\infty \left( \sum_{k=0}^\infty \frac{z^{(2k+2)}}{(2k+1)!} \right)^j[/itex]
In method (b) I got
[itex] \frac{1}{2} \left( \frac{1}{1-\sqrt{2} \sin z} - \frac{1}{1+\sqrt{2} \sin z}\right)[/itex]
Is one of these methods better to keep following -- or am I on the wrong track altogether? Also, sorry to be not as thoroughly detailed... I wrote a lot, but the system logged me out during one of my previews and I lost everything.
Thanks