# Systems of ODE's - Complex Eigenvalues

1. Apr 28, 2010

### gabriels-horn

1. The problem statement, all variables and given/known data
Find the general solution of the given system.

The given matrix is X' = (1st row (1,-1,2) 2nd row (-1,1,0) 3rd row (-1,0,1))X

2. The attempt at a solution

The eigenvalue determinant = (1st row (1-λ,-1,2) second row (-1,1-λ,0) 3rd row (-1,0,1-λ)

Solving the determinant gives -(λ-1)(λ^2-2λ+2)

So λ1,2 = 1+i and 1-i and λ3 = 1

Now my question is how do I solve the 3x3 matrix with copmlex numbers in it? I used gaussian elimination to solve for the real value λ3.

(1st row (-i,-1,2) 2nd row (-1,-i,0) 3rd row (-1,0,-i)) multiplied by the column (k1,k2,k3) = 0

Last edited: Apr 28, 2010
2. Apr 28, 2010

### gabbagabbahey

Your two complex eigenvalues have a sign error. The roots of $\lambda^2-2\lambda+2=0$ are $\lambda=1\pm i$.

You can still use Gaussian elimination to solve for the eigenvectors.

3. Apr 28, 2010

### gabriels-horn

Can someone show me how?

4. Apr 28, 2010

### gabbagabbahey

You do it exactly the same way you did it for your real eigenvalue....try it out and post your work if you get stuck.

5. Apr 28, 2010

### gabriels-horn

Is multiplying a row by a complex number a valid operation?

6. Apr 28, 2010

### gabbagabbahey

Yes. It's exactly the same as multiplying both sides of an equation by a complex number.

7. Apr 28, 2010

### gabriels-horn

The corrected eigenvalue matrix is
[-i,-1,2]
[-1,-i,0]
[-1,0,-i]

using the operations 1) i*R1, 2) R1+R2, 3) R1+R3, 4) (-1/2)R2+R3, 5) -2*R2+R1 gives

[1,-i,0]
[0,-i, i]
[0,0,0]

which says k2=k3; picking k1=1 and k2=k3= i gives one solution,k=
[1]

which is split into the real and imaginary parts B1 and B2, respectively

[1]
[0]
[0]

[0]
[1]
[1]

The general solution is c1[B1*cos(t)-B2sin(t)]e^t + c2[B2*cos(t)-B1sin(t)]e^t +c3
([0])e^t
([1])
([2])

Last edited: Apr 28, 2010