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Finding eigenvectors for complex eigenvalues

  • Thread starter Dusty912
  • Start date
  • #1
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Homework Statement


So I have been having trouble with finding the proper eigen vector for a complex eigen value
for the matrix A=(-3 -5)
. .........................(3 1)

had a little trouble with formating this matrix sorry
The eigen values are -1+i√11 and -1-i√11

The Attempt at a Solution


using AYY=0 (where Y is a vector (x, y) I obtain two different equations :
3x+y-(-1+i√11)y=0 and -3x-5y-(-1+i√11)x=0

these simplify too: 3x=(i√11 -2)y and 5y=(-2-i√11)x

no I know that the right eigen vecotor is (i√11 -2,3) but why isnt (5, -2-i√11) one? they both work when I plug them in.
 

Answers and Replies

  • #2
Samy_A
Science Advisor
Homework Helper
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Homework Statement


So I have been having trouble with finding the proper eigen vector for a complex eigen value
for the matrix A=(-3 -5)
. .........................(3 1)

had a little trouble with formating this matrix sorry
The eigen values are -1+i√11 and -1-i√11

The Attempt at a Solution


using AYY=0 (where Y is a vector (x, y) I obtain two different equations :
3x+y-(-1+i√11)y=0 and -3x-5y-(-1+i√11)x=0

these simplify too: 3x=(i√11 -2)y and 5y=(-2-i√11)x

no I know that the right eigen vecotor is (i√11 -2,3) but why isnt (5, -2-i√11) one? they both work when I plug them in.
If ##\vec x## is an eigenvector of ##A## with eigenvalue ##\lambda##, then so is ##k \vec x##, where ##k \in \mathbb C, k\neq 0##.
Check if ##(i\sqrt {11}-2,3)## is a (complex) scalar multiple of ##(5,-2-i\sqrt{11})##.
 
  • #3
144
1
well are you saying that both of these are eigen vectors for this matrix?
 
  • #4
144
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They appear to be scalar multiples of each other
 
  • #5
Samy_A
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well are you saying that both of these are eigen vectors for this matrix?
If they are non zero scalar multiples of each other, yes. As said, any non zero scalar multiple of an eigenvector is an eigenvector.
They appear to be scalar multiples of each other
Indeed. So either answer is correct.
 
  • #6
144
1
but when i try to compute the general solution it looks different.
 
  • #7
144
1
computing the general solution with eλt*V where V is the eigen vector
 
  • #8
Samy_A
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but when i try to compute the general solution it looks different.
computing the general solution with eλt*V where V is the eigen vector
It may be me, but I don't understand what you are saying here.
 

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