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Thread: find the n transformation

  1. #1
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    find the n transformation

    let T\in\mathcal L(\mathbb R^3), such that T(x,y,z)=(ux+y,uy+z,uz). Compute T^n(x,y,z).
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  2. #2
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    obviously for each 0\ne u\in\mathbb R.

    as for the problem, compute the matrix associated to the transformation (it's the canonical basis for \mathbb R^3, so it's easy) and then compute the characteristic polynomial.

    use the division algorithm and put t^n=(u-\lambda)^3f(t)+at^2+bt+c, (1) where (u-\lambda)^3 is the characteristic polynomial and at^2+bt+c is the remainder (its degree is one less than the characteristic polynomial), now you gotta compute a,b,c.

    first put t=u, and differentiate once and evaluate again at t=u, differentiate again and evaluate at t=u, this will be useful to find a,b,c.

    once got those values, put them at (1) and then evaluate the associated matrix to the transformation; by the Cayley - Hamilton theorem, the matrix evaluated in the characteristic polynomial produces the null one so you'll end up (say the matrix is A) A^n=aA^2+bA+cI_3.

    finally, explicitly find A^n and multiply it by \left[ \begin{matrix}<br />
   x  \\<br />
   y  \\<br />
   z  <br />
\end{matrix} \right], and we're done!
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  3. #3
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    Quote Originally Posted by Morgan View Post
    let T\in\mathcal L(\mathbb R^3), such that T(x,y,z)=(ux+y,uy+z,uz). Compute T^n(x,y,z).
    a very easy induction on n will prove that: T^n(x,y,z)=(u^nx + nu^{n-1}y+ \frac{n(n-1)}{2}u^{n-2}z, \ u^ny + nu^{n-1}z, \ u^n z).
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  4. #4
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    Quote Originally Posted by Morgan View Post
    let T\in\mathcal L(\mathbb R^3), such that T(x,y,z)=(ux+y,uy+z,uz). Compute T^n(x,y,z).
    Write T as a matrix and calculate the first few powers of T. You should see the pattern pretty quickly.

    Or you could just calculate them directly from that formula, but I think it is easier to see with the matrix.
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