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thdrbird

  • 3 years ago

Let A = [-3,2,6 0,-1,6 0,0,-3] Find an invertible matrix P and a diagonal matrix D such that D = P^{-1}AP. find P and D I worked out the polynomial -7x^3-7x^2-15x-9 to give x= -1,-3,-3 My answer for P= [1,0,0 1,-3,-3 0,1,1] and D= -1,0,0 0,-3,0 0,0,-3 something's wrong!

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  1. electrokid
    • 3 years ago
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    what type of decomposition are you doing here ?

  2. Hoa
    • 3 years ago
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    I think you get something wrong at P, since eigenvalue -3 has 2 dimensions and they are not yours! recheck

  3. electrokid
    • 3 years ago
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    you should get D=dia(-3,-1,-3)

  4. Hoa
    • 3 years ago
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    yes, surely, because they are eigenvalues of A, what I need is checking the order this guy arrange in P to have corresponding of numbers, like (-3,-1,-3) or (-1,-3,-3)

  5. thdrbird
    • 3 years ago
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    ok I will try!

  6. Hoa
    • 3 years ago
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    We cannot guess, right, kid? if we mess the order up, although we know it is the Diagonal matrix of A but the P is in different order of eigenvectors, We will be crazy when checking what wrong is, I had that experience. It made me crazy

  7. electrokid
    • 3 years ago
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    tr(A)=-3-1-3=-7 A11+A22+A33=3+9+3=15 |A|=-3(3)=-9 so, the characteristic polynomial would be\[\Delta(t)=t^3+7t^2+15t+9\]

  8. electrokid
    • 3 years ago
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    well, as long as the eigen vector columns are arranged in proper order, you should be good.

  9. thdrbird
    • 3 years ago
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    still no luck...

  10. thdrbird
    • 3 years ago
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    no

  11. electrokid
    • 3 years ago
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    eigen vectors=? for t=-1: \[-2x+2y+6z=0\\6z=0\\-3z=0\implies v_1=(1,1,0)^T\]

  12. thdrbird
    • 3 years ago
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    yup

  13. thdrbird
    • 3 years ago
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    and v2 and v3 I put at (0,-3,1)

  14. electrokid
    • 3 years ago
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    for t= -3: \[ 2y+6z=0\\ 2y+6z=0\\ v_{2,3}=(\alpha,-3\beta,\beta)^T \]

  15. electrokid
    • 3 years ago
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    @thdrbird you cannot have them two identical

  16. thdrbird
    • 3 years ago
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    oh,ok..

  17. electrokid
    • 3 years ago
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    you can use v2 when a=0,b=1 v3 when a=1,b=1

  18. electrokid
    • 3 years ago
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    or v3 when a=1,b=0 ANY combination of alpha and beta that do not give the same vectors..

  19. thdrbird
    • 3 years ago
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    can you give me an example?

  20. thdrbird
    • 3 years ago
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    got it!

  21. thdrbird
    • 3 years ago
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    I forgot to set alpha!

  22. thdrbird
    • 3 years ago
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    thanks @electrokid, great help!

  23. thdrbird
    • 3 years ago
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    @Hoa as well! Thanks!

  24. Hoa
    • 3 years ago
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    I did nothing guy!!!

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