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Homework Help: Can a 3x3 matrix equate to a 4x4 matrix?

  1. Feb 8, 2010 #1
    1. The problem statement, all variables and given/known data

    My question involves finding the determinant of a 4x4 matrix, which I know how to do.

    The matrix is

    0 1 2 3
    0 1 2 5
    0 3 5 6
    0 0 0 0

    Since the matrix has zeros in every row and column, its determinant will equal 0.

    However, I got to thinking that if I wrote out the matrix in equation form, I'd get

    0a + b + 2c + 3d
    0a + b + 2c + 5d
    0a + 3b + 5c +6d
    0 + 0 + 0 + 0

    Does the 4x4 matrix equate to the 3x3 matrix

    1 2 3
    1 2 5
    3 5 6 ?


    2. Relevant equations

    (see above for equations)


    3. The attempt at a solution

    I'm incline to say no, since the 3x3 matrix would have a determinant, but from the perspective of the equations, I don't see how that are different (at least in terms of the information they convey).

    Can anyone explain to me how the two matrices are different? Thanks.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 8, 2010 #2

    Mark44

    Staff: Mentor

    These are not equations; they are expressions that represent the product of your 4x4 matrix and a column vector [a b c d]. They are not equations because there is no equals sign.
    No. An n x n matrix can never be equal to an m x m matrix if n is different from m. The two matrices belong to different spaces, so cannot be compared. The same is true for vectors from different spaces.
    The two matrices are different because they have different numbers of elements.
     
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