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Null matrix can be an idempotent matrix?

  1. Jan 2, 2015 #1
    Friends!
    Is there anything wrong in the statement"A null matrix is idempotent".
     
  2. jcsd
  3. Jan 2, 2015 #2

    Mark44

    Staff: Mentor

    What do you think? How is the term "idempotent" defined?
     
  4. Jan 2, 2015 #3
    A square equal A. So if A is zero matrix then it satsfies this condition.
     
  5. Jan 2, 2015 #4

    Mark44

    Staff: Mentor

    Any square zero matrix A is idempotent, since A2 = A. So, in regard to your question in the first post, not all zero matrices are idempotent, since A2 might not be defined. For example,
    $$A = \begin{bmatrix} 0 & 0 & 0 \\ 0 & 0 & 0\end{bmatrix}$$
     
  6. Jan 2, 2015 #5
    Thank You.Doubt had arisen while showing B square not equal to I to be not always true when given B equal I minus A and A is idempotent.
     
  7. Jan 2, 2015 #6

    Mark44

    Staff: Mentor

    I'm not sure what you're trying to say here. Writing an equation would make it clearer.

    Are you saying that B = I - A, and A is idempotent? And you need to show that B2 ≠ I?

    If so, that's easy to show -- just expand B2 = (I - A)2.
     
  8. Jan 3, 2015 #7
    I needed to show that( B squarenot equai to I) is not always true.
     
  9. Jan 3, 2015 #8

    Mark44

    Staff: Mentor

    Like I said in my previous post, this is easy to do.

    Also, please take some time to learn how to write proper math notation. To indicate the square of something, at the very least you can write B^2. Better yet, write the exponent as a superscript, like this: B2. The x2 icon in the menu bar can be used to write exponents of all kinds. The ##\Sigma## icon in the menu bar has many math symbols, such as ≠ and ∞ and many others.
     
  10. Jan 4, 2015 #9
    Thank You Mark44 for the valuable help and suggestions!
     
  11. Jan 4, 2015 #10

    Mark44

    Staff: Mentor

    You're welcome!
     
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