# Homework Help: All Laurent series expansion around 1.

1. Jul 14, 2010

### roy240

1. The problem statement, all variables and given/known data

Question is= Find all Laurent series expansion of f(z)=z^4/(3+z^2) around 1. I will be very very thankful if someone can help me to do this question.

2. Relevant equations

3. The attempt at a solution

can I assume (z-1=u) here and change the function in terms of $u$. then i will have

f(u+1)=(u+1)^4/(u+2)^2

It should have singularity at $u=-2$. now I m big confuse about in what regions I should compute Laurent series.

I have trouble of thinking that how many laurent series I will have and what will be the conditions on $z$.

Thanks !!!!

2. Jul 14, 2010

### lanedance

first, subbing in for z = u+1
g(u) = f(u+1)=(u+1)^4/(3+(u+1)^2) =(u+1)^4/(u^2+ 2u +4)

3. Jul 14, 2010

### lanedance

there will be a laurent series for each disk about z-1 = u = 0 defined by the poles of the function

4. Jul 14, 2010

### lanedance

5. Jul 14, 2010

### Gib Z

You've done the substitution to center the Laurent expansion around u=0, now draw a diagram, plotting the poles of the new function and draw the annuli these poles split the plane into. For each annuli, decide what the rule is for that region (eg 8< |u| < 23 or something).

The next part is a bit harder: Try to manipulate the expression you have ( $$\frac{ (u+1)^4}{(u+1)^2+3}$$ ) into the product of two terms, 1 a polynomial, and another that you can interpret as the sum of a geometric series. Then you can expand the geometric series into a sum, and multiply the polynomial into it to get the Laurent series.