# Partial fraction decomposition

1. Sep 24, 2011

### bryansteele

Yes, another of these. How do you decompose (z3+1)/(z(1-z)2) ?

I've tried A/z + B/(1-z) + (Cz+D)/(1-z)2 and a slew of others that don't work.

Thanks a bunch!

Last edited: Sep 24, 2011
2. Sep 24, 2011

### BloodyFrozen

(z3+1)/z(1-z2)

Do you mean:

(z3+1)/(z(1-z2))

or

((z3+1)/z)(1-z2)

3. Sep 24, 2011

### Dickfore

First of all, the power of the numerator is the same as the power of the denominator. You have to make long division of polynomials:

$$\frac{z^{3} + 1}{z(1 - z^{2})} = \frac{z^{3} + 1}{-z^{3} + z} = \frac{z^{3} - z + z + 1}{-z^{3} + z} = -1 - \frac{z + 1}{z^{3} - z}$$

Next, the denominator factors into linear factors:
$$z^{3} - z = z (z^{2} - 1) = z (z - 1) (z + 1)$$
so, you have:
$$\frac{z + 1}{z^{3} - z} = \frac{z + 1}{z ( z - 1) (z + 1)} = \frac{A}{z} + \frac{B}{z - 1} + \frac{C}{z + 1}$$
You find A, B and C.

4. Sep 24, 2011

### bryansteele

Hey, I made a quick edit in the problem. I meant the quantity squared. Sorry for the confusion.

5. Sep 24, 2011

### Dickfore

Again, your numerator and denominator have the same power. Do the long division:

$$(z^3 + 1) \colon (z^3 - 2 z^2 + z) = ?$$

first. What is the quotient and what is the remainder?

6. Sep 24, 2011

### bryansteele

It comes out to ((z+1)(z2-z+1))/(z(z-1)2)

7. Sep 24, 2011

### Dickfore

no, this is not long division.

8. Sep 24, 2011

### bryansteele

Right. It is 1 + (2z2-z+1)/(z3-2z2+z)

Last edited: Sep 24, 2011
9. Sep 24, 2011

### Dickfore

How?

$$1 \cdot (z^{3} - 2 z^{2} + z) + (2 z^{2} - z) = z^{3} - 2 z^{2} + z + 2 z^{2} - z = z^{3} \neq z^{3} + 1$$

10. Sep 24, 2011

### bryansteele

right again. I've edited my last reply. I forgot the +1 in the numerator.

11. Sep 24, 2011

### Dickfore

Ok, now look at the normal fraction (power of numerator is lower than power of denominator). The partional fraction decomposition is:

$$\frac{2 z^{2} - z + 1}{z ( z - 1)^{2}} = \frac{A}{z} + \frac{B}{z - 1} + \frac{C}{(z - 1)^{2}}$$

If you multiply by $z (z - 1)^{2}$, and compare coefficients in front of like powers of z, you will get 3 equations for A, B and C. Solve them and you are done.

12. Sep 24, 2011

### bryansteele

Great, appreciate the patience. Imagine, I've made it to a graduate-level complex analysis class without ever doing partial fractions...