Linear Transformations between 2 non-standard basis of Polynomials

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If
$$
A = beginpmatrix 1 & -1 & 2 \
-2 & 1 &-1 \ 1 & 2 & 3 endpmatrix
$$
is the matrix representation of a
linear transformation $T : P_3(x) to P_3(x)$ with respect to
bases $1-x,x(1-x),x(1+x)$ and $1,1+x,1+x^2$. Find T.




While i have worked with transforming non-standard to standard basis, this is the first one i am encountering with transformation between 2 non-standard polynomial basis. I am not sure if i am working out rightly.



$T[1-x] = 1(1) -2(1+x) +1(1+x^2)$



$T[x(1-x)] = -1(1) +1(1+x) +2(1+x^2)$



$T[x(1+x)] = 2(1) -1(1+x) +3(1+x^2)$



Therefore, $T[a(1-x)+b(x(1-x))+c(x(1+x))] = (a-b+2c)(1) + (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2)$



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    If
    $$
    A = beginpmatrix 1 & -1 & 2 \
    -2 & 1 &-1 \ 1 & 2 & 3 endpmatrix
    $$
    is the matrix representation of a
    linear transformation $T : P_3(x) to P_3(x)$ with respect to
    bases $1-x,x(1-x),x(1+x)$ and $1,1+x,1+x^2$. Find T.




    While i have worked with transforming non-standard to standard basis, this is the first one i am encountering with transformation between 2 non-standard polynomial basis. I am not sure if i am working out rightly.



    $T[1-x] = 1(1) -2(1+x) +1(1+x^2)$



    $T[x(1-x)] = -1(1) +1(1+x) +2(1+x^2)$



    $T[x(1+x)] = 2(1) -1(1+x) +3(1+x^2)$



    Therefore, $T[a(1-x)+b(x(1-x))+c(x(1+x))] = (a-b+2c)(1) + (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2)$



    Is this fine ?







    share|cite|improve this question























      up vote
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      up vote
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      If
      $$
      A = beginpmatrix 1 & -1 & 2 \
      -2 & 1 &-1 \ 1 & 2 & 3 endpmatrix
      $$
      is the matrix representation of a
      linear transformation $T : P_3(x) to P_3(x)$ with respect to
      bases $1-x,x(1-x),x(1+x)$ and $1,1+x,1+x^2$. Find T.




      While i have worked with transforming non-standard to standard basis, this is the first one i am encountering with transformation between 2 non-standard polynomial basis. I am not sure if i am working out rightly.



      $T[1-x] = 1(1) -2(1+x) +1(1+x^2)$



      $T[x(1-x)] = -1(1) +1(1+x) +2(1+x^2)$



      $T[x(1+x)] = 2(1) -1(1+x) +3(1+x^2)$



      Therefore, $T[a(1-x)+b(x(1-x))+c(x(1+x))] = (a-b+2c)(1) + (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2)$



      Is this fine ?







      share|cite|improve this question














      If
      $$
      A = beginpmatrix 1 & -1 & 2 \
      -2 & 1 &-1 \ 1 & 2 & 3 endpmatrix
      $$
      is the matrix representation of a
      linear transformation $T : P_3(x) to P_3(x)$ with respect to
      bases $1-x,x(1-x),x(1+x)$ and $1,1+x,1+x^2$. Find T.




      While i have worked with transforming non-standard to standard basis, this is the first one i am encountering with transformation between 2 non-standard polynomial basis. I am not sure if i am working out rightly.



      $T[1-x] = 1(1) -2(1+x) +1(1+x^2)$



      $T[x(1-x)] = -1(1) +1(1+x) +2(1+x^2)$



      $T[x(1+x)] = 2(1) -1(1+x) +3(1+x^2)$



      Therefore, $T[a(1-x)+b(x(1-x))+c(x(1+x))] = (a-b+2c)(1) + (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2)$



      Is this fine ?









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      edited Jul 21 at 16:06









      mvw

      30.4k22250




      30.4k22250









      asked Jul 21 at 15:41









      Sarkar

      52




      52




















          3 Answers
          3






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          up vote
          0
          down vote













          Everything you've written is correct, although I suspect the problem is asking you to find $T[a + bx + cx^2]$.






          share|cite|improve this answer




























            up vote
            0
            down vote













            What you did is fine, but now you have to compute $T[alpha+beta x+gamma x^2]$ for arbitrary $alpha,beta,gammainmathbb R$. In order to do that, solve the equation$$alpha+beta x+gamma x^2=a-b+2c+ (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2).$$That is, solve the system$$left{beginarrayla-b+2c=alpha\-2a+b-c=beta\a+2b+3c=gamma.endarrayright.$$






            share|cite|improve this answer





















            • then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
              – Sarkar
              Jul 21 at 16:12











            • This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
              – José Carlos Santos
              Jul 21 at 16:19











            • Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
              – Sarkar
              Jul 21 at 16:29

















            up vote
            0
            down vote













            I also suspect the real question is to find the image of $a+bx+cx^2$ in the canonical basis.



            I would do it in a formal way first: denote $X, Y$, &c. the column vectors of polynomials in the canonical basis, $X_1, Y_1$, &c. their column vectors in the first basis and $X_2, Y_2$, &c. their column vectors in the second basis.



            We're given the matrix $A$ of a linear transformation $T$ from $(P_2(x),mathcal B_1)$ to $(P_2(x),mathcal B_2)$, i.e. we have a matrix relation
            $$Y_2=AX_1$$
            and asked for the matrix of this same linear transformation from $(P_2(x),mathcal B_textcanon)$ to itself, i.e. we're asked for the matrix $T$ such that
            $$Y=TX.$$
            Now that's easy, given the change of basis matrices:
            $$P_1=beginbmatrix!!beginarrayrrc
            1&0&0\-1&1&1\0&:llap-1&1
            endarrayendbmatrix,qquad P_2=beginbmatrix
            1&1&1\0&1&0\0&0&1
            endbmatrix.$$
            We have $Y=P_2Y_2$, $X=P_1X_1$, so
            $$Y=P_2Y_2=P_2AX_1=(underbraceP_2AP_1^-1_T)X.$$
            There remains to find the inverse of $P_1$, which is standard by row reduction.






            share|cite|improve this answer





















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              3 Answers
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              3 Answers
              3






              active

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              active

              oldest

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              up vote
              0
              down vote













              Everything you've written is correct, although I suspect the problem is asking you to find $T[a + bx + cx^2]$.






              share|cite|improve this answer

























                up vote
                0
                down vote













                Everything you've written is correct, although I suspect the problem is asking you to find $T[a + bx + cx^2]$.






                share|cite|improve this answer























                  up vote
                  0
                  down vote










                  up vote
                  0
                  down vote









                  Everything you've written is correct, although I suspect the problem is asking you to find $T[a + bx + cx^2]$.






                  share|cite|improve this answer













                  Everything you've written is correct, although I suspect the problem is asking you to find $T[a + bx + cx^2]$.







                  share|cite|improve this answer













                  share|cite|improve this answer



                  share|cite|improve this answer











                  answered Jul 21 at 15:44









                  Marcus M

                  8,1731847




                  8,1731847




















                      up vote
                      0
                      down vote













                      What you did is fine, but now you have to compute $T[alpha+beta x+gamma x^2]$ for arbitrary $alpha,beta,gammainmathbb R$. In order to do that, solve the equation$$alpha+beta x+gamma x^2=a-b+2c+ (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2).$$That is, solve the system$$left{beginarrayla-b+2c=alpha\-2a+b-c=beta\a+2b+3c=gamma.endarrayright.$$






                      share|cite|improve this answer





















                      • then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                        – Sarkar
                        Jul 21 at 16:12











                      • This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                        – José Carlos Santos
                        Jul 21 at 16:19











                      • Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                        – Sarkar
                        Jul 21 at 16:29














                      up vote
                      0
                      down vote













                      What you did is fine, but now you have to compute $T[alpha+beta x+gamma x^2]$ for arbitrary $alpha,beta,gammainmathbb R$. In order to do that, solve the equation$$alpha+beta x+gamma x^2=a-b+2c+ (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2).$$That is, solve the system$$left{beginarrayla-b+2c=alpha\-2a+b-c=beta\a+2b+3c=gamma.endarrayright.$$






                      share|cite|improve this answer





















                      • then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                        – Sarkar
                        Jul 21 at 16:12











                      • This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                        – José Carlos Santos
                        Jul 21 at 16:19











                      • Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                        – Sarkar
                        Jul 21 at 16:29












                      up vote
                      0
                      down vote










                      up vote
                      0
                      down vote









                      What you did is fine, but now you have to compute $T[alpha+beta x+gamma x^2]$ for arbitrary $alpha,beta,gammainmathbb R$. In order to do that, solve the equation$$alpha+beta x+gamma x^2=a-b+2c+ (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2).$$That is, solve the system$$left{beginarrayla-b+2c=alpha\-2a+b-c=beta\a+2b+3c=gamma.endarrayright.$$






                      share|cite|improve this answer













                      What you did is fine, but now you have to compute $T[alpha+beta x+gamma x^2]$ for arbitrary $alpha,beta,gammainmathbb R$. In order to do that, solve the equation$$alpha+beta x+gamma x^2=a-b+2c+ (-2a+b-c)(1+x) +(a+2b+3c)(1+x^2).$$That is, solve the system$$left{beginarrayla-b+2c=alpha\-2a+b-c=beta\a+2b+3c=gamma.endarrayright.$$







                      share|cite|improve this answer













                      share|cite|improve this answer



                      share|cite|improve this answer











                      answered Jul 21 at 15:46









                      José Carlos Santos

                      114k1698177




                      114k1698177











                      • then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                        – Sarkar
                        Jul 21 at 16:12











                      • This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                        – José Carlos Santos
                        Jul 21 at 16:19











                      • Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                        – Sarkar
                        Jul 21 at 16:29
















                      • then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                        – Sarkar
                        Jul 21 at 16:12











                      • This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                        – José Carlos Santos
                        Jul 21 at 16:19











                      • Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                        – Sarkar
                        Jul 21 at 16:29















                      then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                      – Sarkar
                      Jul 21 at 16:12





                      then it becomes a $ T:[(1,x,x^2) -> (1,1+x,1+x^2)]$ ?
                      – Sarkar
                      Jul 21 at 16:12













                      This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                      – José Carlos Santos
                      Jul 21 at 16:19





                      This doesn't make sense. The elements of the domain and of the codomain of $T$ are polynomials with degree smaller than or equal to $2$.
                      – José Carlos Santos
                      Jul 21 at 16:19













                      Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                      – Sarkar
                      Jul 21 at 16:29




                      Okay. i get it. So now the coefficients of basis $(1,1+x,1+x^2)$ is standardised using standard basis$(1,x,x^2)$ coefficients
                      – Sarkar
                      Jul 21 at 16:29










                      up vote
                      0
                      down vote













                      I also suspect the real question is to find the image of $a+bx+cx^2$ in the canonical basis.



                      I would do it in a formal way first: denote $X, Y$, &c. the column vectors of polynomials in the canonical basis, $X_1, Y_1$, &c. their column vectors in the first basis and $X_2, Y_2$, &c. their column vectors in the second basis.



                      We're given the matrix $A$ of a linear transformation $T$ from $(P_2(x),mathcal B_1)$ to $(P_2(x),mathcal B_2)$, i.e. we have a matrix relation
                      $$Y_2=AX_1$$
                      and asked for the matrix of this same linear transformation from $(P_2(x),mathcal B_textcanon)$ to itself, i.e. we're asked for the matrix $T$ such that
                      $$Y=TX.$$
                      Now that's easy, given the change of basis matrices:
                      $$P_1=beginbmatrix!!beginarrayrrc
                      1&0&0\-1&1&1\0&:llap-1&1
                      endarrayendbmatrix,qquad P_2=beginbmatrix
                      1&1&1\0&1&0\0&0&1
                      endbmatrix.$$
                      We have $Y=P_2Y_2$, $X=P_1X_1$, so
                      $$Y=P_2Y_2=P_2AX_1=(underbraceP_2AP_1^-1_T)X.$$
                      There remains to find the inverse of $P_1$, which is standard by row reduction.






                      share|cite|improve this answer

























                        up vote
                        0
                        down vote













                        I also suspect the real question is to find the image of $a+bx+cx^2$ in the canonical basis.



                        I would do it in a formal way first: denote $X, Y$, &c. the column vectors of polynomials in the canonical basis, $X_1, Y_1$, &c. their column vectors in the first basis and $X_2, Y_2$, &c. their column vectors in the second basis.



                        We're given the matrix $A$ of a linear transformation $T$ from $(P_2(x),mathcal B_1)$ to $(P_2(x),mathcal B_2)$, i.e. we have a matrix relation
                        $$Y_2=AX_1$$
                        and asked for the matrix of this same linear transformation from $(P_2(x),mathcal B_textcanon)$ to itself, i.e. we're asked for the matrix $T$ such that
                        $$Y=TX.$$
                        Now that's easy, given the change of basis matrices:
                        $$P_1=beginbmatrix!!beginarrayrrc
                        1&0&0\-1&1&1\0&:llap-1&1
                        endarrayendbmatrix,qquad P_2=beginbmatrix
                        1&1&1\0&1&0\0&0&1
                        endbmatrix.$$
                        We have $Y=P_2Y_2$, $X=P_1X_1$, so
                        $$Y=P_2Y_2=P_2AX_1=(underbraceP_2AP_1^-1_T)X.$$
                        There remains to find the inverse of $P_1$, which is standard by row reduction.






                        share|cite|improve this answer























                          up vote
                          0
                          down vote










                          up vote
                          0
                          down vote









                          I also suspect the real question is to find the image of $a+bx+cx^2$ in the canonical basis.



                          I would do it in a formal way first: denote $X, Y$, &c. the column vectors of polynomials in the canonical basis, $X_1, Y_1$, &c. their column vectors in the first basis and $X_2, Y_2$, &c. their column vectors in the second basis.



                          We're given the matrix $A$ of a linear transformation $T$ from $(P_2(x),mathcal B_1)$ to $(P_2(x),mathcal B_2)$, i.e. we have a matrix relation
                          $$Y_2=AX_1$$
                          and asked for the matrix of this same linear transformation from $(P_2(x),mathcal B_textcanon)$ to itself, i.e. we're asked for the matrix $T$ such that
                          $$Y=TX.$$
                          Now that's easy, given the change of basis matrices:
                          $$P_1=beginbmatrix!!beginarrayrrc
                          1&0&0\-1&1&1\0&:llap-1&1
                          endarrayendbmatrix,qquad P_2=beginbmatrix
                          1&1&1\0&1&0\0&0&1
                          endbmatrix.$$
                          We have $Y=P_2Y_2$, $X=P_1X_1$, so
                          $$Y=P_2Y_2=P_2AX_1=(underbraceP_2AP_1^-1_T)X.$$
                          There remains to find the inverse of $P_1$, which is standard by row reduction.






                          share|cite|improve this answer













                          I also suspect the real question is to find the image of $a+bx+cx^2$ in the canonical basis.



                          I would do it in a formal way first: denote $X, Y$, &c. the column vectors of polynomials in the canonical basis, $X_1, Y_1$, &c. their column vectors in the first basis and $X_2, Y_2$, &c. their column vectors in the second basis.



                          We're given the matrix $A$ of a linear transformation $T$ from $(P_2(x),mathcal B_1)$ to $(P_2(x),mathcal B_2)$, i.e. we have a matrix relation
                          $$Y_2=AX_1$$
                          and asked for the matrix of this same linear transformation from $(P_2(x),mathcal B_textcanon)$ to itself, i.e. we're asked for the matrix $T$ such that
                          $$Y=TX.$$
                          Now that's easy, given the change of basis matrices:
                          $$P_1=beginbmatrix!!beginarrayrrc
                          1&0&0\-1&1&1\0&:llap-1&1
                          endarrayendbmatrix,qquad P_2=beginbmatrix
                          1&1&1\0&1&0\0&0&1
                          endbmatrix.$$
                          We have $Y=P_2Y_2$, $X=P_1X_1$, so
                          $$Y=P_2Y_2=P_2AX_1=(underbraceP_2AP_1^-1_T)X.$$
                          There remains to find the inverse of $P_1$, which is standard by row reduction.







                          share|cite|improve this answer













                          share|cite|improve this answer



                          share|cite|improve this answer











                          answered Jul 21 at 16:35









                          Bernard

                          110k635103




                          110k635103






















                               

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