Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Eigvector/value problem (need validation of my proof)

  1. Jun 6, 2008 #1

    rock.freak667

    User Avatar
    Homework Helper

    [SOLVED] eigvector/value problem (need validation of my proof)

    1. The problem statement, all variables and given/known data
    Show that if [itex]\lambda[/itex] is an e.value of a square matrix A with e as a corresponding e.vector, and [ites]\mu[/itex] is an e.value of the square matrix B for which e is also a corresponding e.vector,the [itex]\lambda + \mu[/itex] is an e.value of the matrix A+B with e as a corresponding e.vector


    2. Relevant equations



    3. The attempt at a solution

    From the def'n of an e.vector

    [tex]Ax= \lambda x[/tex]

    [tex]Ax+Bx= \lambda x + \mu x[/tex]

    [tex](A+B)x= (\lambda +\mu)x[/tex]

    hence [itex]\lambda +\mu[/itex] is an e.value of A+B
     
  2. jcsd
  3. Jun 6, 2008 #2
    I don't see anything wrong with your proof. Although since the problem defines [tex]e[/tex] as the eigenvector, it'd probably be more prudent (albeit arbitrary) to use that instead of [tex]x[/tex].
     
  4. Jun 6, 2008 #3

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I would consider the fact he used x when he should have been using e an actual error in his proof. Yes, it might simply be a persistent typographical error (which is still an error!) -- but it might also be a symptom of a deeper misunderstanding of how to manipulate mathematical statements.

    However, I do completely agreement with you that the original poster's proof idea is correct.
     
  5. Jun 6, 2008 #4

    rock.freak667

    User Avatar
    Homework Helper

    ah yeah, a typo, supposed to be e since that is the e.vector.
     
  6. Jun 6, 2008 #5

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Ah, there we go. Mystery solved. :smile:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook