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Performing row reductions

  1. Nov 10, 2014 #1
    If you have two arbitrary matrices, A and B, I was wondering if row operations can be performed in any order to produce the same results.

    For example, you perform elementary row operations on A to produce A', then do A' - B, then also produce a new matrix through elementary row operations on this new matrix to produce a new matrix C.

    Can matrix C still be obtained by doing A (not A') - B, and then performing row operations? By performing row operations, the matrix is still remained intact, correct? There's no distortion of the row space, right?
     
  2. jcsd
  3. Nov 15, 2014 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
     
  4. Nov 15, 2014 #3

    Mark44

    Staff: Mentor

    Let's limit the discussion to a single matrix A. You can do elementary row operations in any order. At each step along the way, the new matrix will be equivalent to the one you started with.
    Why? When you perform an elementary row operation on A, you get a new matrix A' that is equivalent to A, but not equal to it. Some of the entries in the new matrix are different from those in A.

    The basic idea is that if Ax = 0, for instance, then A'x = 0 as well, even though A ##\neq## A'. Subtracting one matrix from another is not an elementary row operation, so I don't get the point of your question.
     
  5. Nov 15, 2014 #4
    Okay, thank you for the response. I was trying to get at the this question specifically: A' is a reduced form of A, then is (A' + B)' = (A + B)', where (A+B)' is some reduction of A+B. Does the above equation hold true? Note: the reductions on each matrix may involve completely different operations, there's just some kind of row operation being done.
     
  6. Nov 15, 2014 #5

    Mark44

    Staff: Mentor

    Why don't you try a couple of simple examples - say 2 x 2 matrices or 3 x 3 matrices?

    What you wrote is, I think, garbled.
    Did you mean (A' + B') = (A + B)'?

    Also, by "=" do you mean "is row equivalent to" or "equals"? A professor I had in a 400-level linear algebra class was always very careful to write ##\equiv## when he was doing row operations, a habit that I've followed ever since.
     
  7. Nov 15, 2014 #6
    Sorry, I meant "equals to." By " ' " I am just referring to any random sequence of elementary row operations (I am not sure if there is better notation). And the ' used for one matrix isn't necessarily the same exact sequence of row operations done on the other matrices.

    The only case I can necessarily find where it is not true is when A = B, since then A - B = 0, and no row operations can be used to derive A' - B. I don't seem to be able to prove if it's true for the nontrivial case, though. It does seem to stand, though.
     
  8. Nov 15, 2014 #7

    Mark44

    Staff: Mentor

    Did you try it with a few specific examples, as I suggested? I don't think the statement is true at all.
     
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