# C-R of Laurent Series

1. Apr 13, 2008

### hob

When doing complex contour integration one can use the C-R formula or the Laurent series and find the first coefficient of the priciple part. What are the selection criteria for choosing these methods?

Regards,

Hob

2. Apr 13, 2008

### HallsofIvy

Staff Emeritus
What do you mean by "use the Cauchy-Riemann equations to do a contour integral"? Are you referring to actually integrating around the contour as opposed to summing the residues inside the contour?

3. Apr 13, 2008

### hob

Hi, I mean summing within the contour.

$$2 \pi$$ x $$\sum residues$$ = Integral around the contour.

You can also use the Laurent Expansion and finding the first principle coefficient.

I am unsure what method you would use when presented with a complex integration,

Regards,

Last edited: Apr 13, 2008
4. Apr 13, 2008

### HallsofIvy

Staff Emeritus
??The "first principle coefficient" of the Laurent expansion, around pole $z_0$, by which I think you mean the coefficient of $z^{-1}$, is the residue at that $z_0$. They are the same method.

5. Apr 14, 2008

### zhentil

Correct me if I'm wrong, but I think you mean should you try to evaluate the integral directly, or do the residue by other means? Almost certainly the residue method. After all, if you could muscle your way through integrals, you wouldn't need contour integration.