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Linear transformation and its matrix

  1. Jan 13, 2009 #1
    Hello everybody,
    I have a problem. There is a linear trasformation [tex]\xi:\mathbb{R}^2\mapsto\mathbb{R}^2[/tex] and:
    How to find a matrix for this linear transformation for basis [tex]\mathcal{B}=\left\{\begin{pmatrix}1\\2\end{pmatrix},\begin{pmatrix}2\\1\end{pmatrix}\right\}[/tex]

    Thank you so much :-)
  2. jcsd
  3. Jan 13, 2009 #2
    There are a number of ways to do this. What have you tried so far? In general if you have a finite dimensional vector space V with a basis [tex]\{ e_{1},e_{2},...,e_{n}\}[/tex] the components of a linear transformation [tex]T:V\rightarrow V[/tex] are uniquely determined by [tex]Te_{i}=T^{j}_{i}e_{j}[/tex] where the sum over j from 1 to n is implied. Furthermore, if you have a second basis [tex]\{f_1,f_2,...,f_n\}[/tex] then the new and old basis vectors are related through [tex]e_{i}=A^{j}_{i}f_{j}[/tex].
  4. Jan 14, 2009 #3


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    Write each of the vectors in B in terms of the first basis, <3, 1>, and <1, 1>, so that you can apply the linear transformation to them. For example, <1, 2>= a<3, 1>+ b<1, 1>= <3a+ b, a+ b>. 3a+ b= 1, a+ b= 2. Subtracting the second equation from the first, 2a= -1 so a= -1/2. Then b= 5/2: <1, 2>= (-1/2)<3,1>+ (5/2)<1, 1>. Applying the [itex]\xi[/itex] to that, [itex]\xi[/itex]<1, 2>= (-1/2)[itex]\xi[/itex]<3,1>+ (5/2)[itex]\xi[/itex]<1,1>= (-1/2)<2, -4>+ (5/2)<0, 2>= <-1, 1>. Now write that in terms of <1, 2> and <2, 1>: <-1, 1>= a<1, 2>+ b<2, 1>= <a+ 2b, 2a+ b>. a+ 2b= -1 and 2a+ b= 1. Subtracting twice the first equation from the second, -3b= 3 so b= -1. Then a= 1. [itex]\xi[/itex]<1, 2>= (1)<1, 2>+ (-1)<2, 1> so the first column of the matrix representing [itex]\xi[/itex] in this basis is [1 -1].

    Do the same with <2, 1> to find the second column.
  5. Jan 14, 2009 #4
    Thank you both very much :-)
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