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Find a kernel and image basis of a linear transformation

  1. Jun 19, 2012 #1
    1. The problem statement, all variables and given/known data

    Find a kernel and image basis of the linear transformation having:

    [tex] \displaystyle T:{{\mathbb{R}}^{3}}\to {{\mathbb{R}}^{3}}[/tex] so that

    [tex] \displaystyle _{B}{{\left( T \right)}_{B}}=\left( \begin{matrix}
    1 & 2 & 1 \\
    2 & 4 & 2 \\
    0 & 0 & 0 \\
    \end{matrix} \right)[/tex]

    [tex] \displaystyle B=\left\{ \left( 1,1,0 \right),\left( 0,2,0 \right),\left( 2,0,-1 \right) \right\}[/tex]

    2. Relevant equations


    3. The attempt at a solution

    For the image basis it is easy given the fact that the rank of the associated matrix is 1 so, the image is generated by one column.

    The problem comes when finding the Kernel basis. My idea is to save the general fromula of the linear map which would work for sure but I wanted to know if there's a quicker way of doing it without finding the general formula of the linear map.


    Thanks!
     
  2. jcsd
  3. Jun 19, 2012 #2

    micromass

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    To find the kernel, you'll need to find the (x,y,z) such that

    [tex]\left(\begin{array}{ccc} 1 & 2 & 1\\ 2 & 4 & 2\\ 0 & 0 & 0\end{array}\right)\left(\begin{array}{c} x\\ y\\ z\\\end{array}\right)=\left(\begin{array}{c} 0\\ 0\\ 0\\\end{array}\right)[/tex]

    This is a system of equations whose solution is not that hard to find.

    (note: you know the rank of the matrix, so the dimension of the kernel is easy to find out)
     
  4. Jun 19, 2012 #3
    Yeah, the dimension of the kernel should be 2. I also thought about that system, but I've got some doubts with it.

    We have:

    $$\left(\begin{array}{ccc} 1 & 2 & 1\\ 2 & 4 & 2\\ 0 & 0 & 0\end{array}\right)\left(\begin{array}{c} x\\ y\\ z\\\end{array}\right)=\left(\begin{array}{c} 0\\ 0\\ 0\\\end{array}\right)$$

    But there, what is (x,y,z)? Are they coordinates or is it a vector? What I understand is that if I multiply the associated matrix with the COORDINATES of a vector in base B then the result is the coordinates of the vector TRANSFORMATION. Is that right?

    Thanks!
     
  5. Jun 19, 2012 #4

    micromass

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    The (x,y,z) are coordinates of a vector with respect to base B. If you want to find the corresponding vector, this can easily be done.
     
  6. Jun 19, 2012 #5
    Ahhm, I see. So we get that:

    [tex]Ker\left( T \right) = \left\{ {\left( {x,y, - x - 2y} \right)} \right\}[/tex]

    Right? So there we have the set of the coordinates of the vectors in basis B whose transformation is 0. Or are they vectors?

    Thanks
     
  7. Jun 19, 2012 #6

    micromass

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    They are coordinates for vectors. You can first take special values for x and y to find two special coordinates in the kernel. Then you have to find the corresponding vector.
     
  8. Jun 19, 2012 #7
    Alright so, we have the set of the coordinates of the vectors whose images are the null vector.
    To find the generator of the coordinates:

    [tex]\left( {x,y, - x - 2y} \right) = x\left( {1, - 1,0} \right) + y\left( {0,1, - 2} \right)[/tex]

    So: [tex]\left\{ {\left( {1, - 1,0} \right),\left( {0,1, - 2} \right)} \right\}[/tex] is a generator (basis) of the coordinates. Now, how can I get the basis of the actual vectors but not in base B?


    Thanks!
     
  9. Jun 19, 2012 #8

    micromass

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    Do you know what coordinates are?? How are they defined??
     
  10. Jun 19, 2012 #9
    Yes, they are the coefficients of the linear combination of the base. I got that

    $$\left\{ {\left( {1, - 1,0} \right),\left( {0,1, - 2} \right)} \right\}$$

    is a basis of the kernel. But those elements are vectors or what?
     
  11. Jun 19, 2012 #10

    micromass

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    Ok, so take the linear combination.

    For example, the coordinates (1,2,3) would correspond to the vector

    [tex](1,1,0)+2(0,2,0)+3(2,0,-1)[/tex]
     
  12. Jun 19, 2012 #11
    Ahhh I get it now by that way! So:

    [tex]x\left( {1,1,0} \right) + y\left( {0,2,0} \right) + \left( { - x - 2y} \right)\left( {2,0, - 1} \right) = \left( { - x - 4y,x + 2y,x + 2y} \right)[/tex]

    And:

    [tex]\left( { - x - 4y,x + 2y,x + 2y} \right) = x\left( { - 1,1,1} \right) + y\left( { - 4,2,2} \right)[/tex]

    So a basis of the kernel is:
    [tex]{\left( { - 1,1,1} \right),\left( { - 4,2,2} \right)}[/tex]

    Now just a last question. What was wrong about:

    $$\left( {x,y, - x - 2y} \right) = x\left( {1, - 1,0} \right) + y\left( {0,1, - 2} \right)$$

    How could I solve the problem following that way? It would be the same, wouldnt it?


    Thanks!!
     
  13. Jun 19, 2012 #12

    micromass

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    Nothing was wrong about that. It just gives you coordinates of a vector. That is, (1,-1,0) and (0,1,-2) are coordinates of a certain vector. Take linear combinations to find the actual vector.
     
  14. Jun 19, 2012 #13
    Thank you very much for your efficiency and quickness for helping (the best combination) :)
     
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