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Linear algebra - squaring via transformations

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data
    I have a transformation (not linear! that is what I have to show) F given by:

    F : P_4 -> P_7 (P_7 is the vector-space spanned by polynomials less than degree 7). I also know that F(p(x)) = (p(x))^2.

    The matrix A representing F with respect to the two basis is the one I get by taking the transformation F on P_4's elements [x^3, x^2, x, 1] and expressing by P_7's elements. I get a 7x4-matrix with 4 zeroes and the rest are zero-entries.

    3. The attempt at a solution
    This matrix is the matrix A in L(x) = Ax. So if I take a polynomial in P_4 and multiply with A, it should be squared:

    A*(a_1*x^3, a_2*x^2, a_3*x, a_0)^T.

    But this doesn't make a_1*x^3 go to (a_1)^2*x^6 and so on? Where am I going wrong?

    I hope you understand my questions.

  2. jcsd
  3. Jan 8, 2008 #2


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    Homework Helper

    I don't really understand what you're asking. You're saying that F is not linear, but then you say it's represented by the matrix A.

    What are you trying to do exactly? Prove that the mapping F:P_4->P_7 (where P_n is the space of polynomials over R of degree <n) given by F(p(x))=(p(x))^2 is linear? Well, it's not! For example: F(x+x) = F(2x) = 4x^2, which is not equal to F(x)+F(x)=2x^2.
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