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Finding the Orthogonal Projection from a Matrix where $ a_ij = vec v_icdot vec v_j$



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)A question about subspacesEntries of a positive definite symmetric matrix as inner productsGoing from an arbitrary basis $mathbbB_u$ to an ON-basis $mathbbB_v$ in $mathbbR^3$Find orthonormal basis of $mathbbR^3$ with a given span of two basis vectorsName of outer “product” of two vectors with exponentiation instead of multiplicationFind the maximum possible $k$ s.t. $langle v_i,v_jrangleleq 0$ for all $ineq j$.If I have a diagonalized matrix with degenerate eigenvalues, how do I generate an orthonormal set of vectors?why is $ vec u bullet vec v = u_1v_1 + u_2v_2$?Prove that every $3$ of $ 4$ vectors of $2$- planes are linearly independent.Form of a matrix in a basis where some of the basis vectors are eigenvectors










1












$begingroup$


I am working on a problem that is asking to find $operatornameproj_Vv_3$ where the matrix $$A=beginbmatrix 3&5&11\ 5&9&20\ 11&20&49endbmatrix
$$

has entries $a_ij=vec v_icdot vec v_j$.
I though that the simplest way to answer this question was to use the formula
$$ (u_1,x)u_1+(u_2,x)u_2 = proj_vx .$$



Since the question also asks to express the solution as a linear combination of $ v_1$ and $ v_2 $, I assumed that I could just plug the numbers in. I knew I had to make the vectors unit vectors so my equation looked something like this:



$ 3(5)(v2)+ 1/11(v3) $ This answer is not the correct answer but I am wondering if I am reading the matrix in the wrong way or if I am confused about the usage of the formula. Does someone have a better idea?










share|cite|improve this question











$endgroup$











  • $begingroup$
    What is $v$ here?
    $endgroup$
    – amd
    Apr 1 at 1:16















1












$begingroup$


I am working on a problem that is asking to find $operatornameproj_Vv_3$ where the matrix $$A=beginbmatrix 3&5&11\ 5&9&20\ 11&20&49endbmatrix
$$

has entries $a_ij=vec v_icdot vec v_j$.
I though that the simplest way to answer this question was to use the formula
$$ (u_1,x)u_1+(u_2,x)u_2 = proj_vx .$$



Since the question also asks to express the solution as a linear combination of $ v_1$ and $ v_2 $, I assumed that I could just plug the numbers in. I knew I had to make the vectors unit vectors so my equation looked something like this:



$ 3(5)(v2)+ 1/11(v3) $ This answer is not the correct answer but I am wondering if I am reading the matrix in the wrong way or if I am confused about the usage of the formula. Does someone have a better idea?










share|cite|improve this question











$endgroup$











  • $begingroup$
    What is $v$ here?
    $endgroup$
    – amd
    Apr 1 at 1:16













1












1








1





$begingroup$


I am working on a problem that is asking to find $operatornameproj_Vv_3$ where the matrix $$A=beginbmatrix 3&5&11\ 5&9&20\ 11&20&49endbmatrix
$$

has entries $a_ij=vec v_icdot vec v_j$.
I though that the simplest way to answer this question was to use the formula
$$ (u_1,x)u_1+(u_2,x)u_2 = proj_vx .$$



Since the question also asks to express the solution as a linear combination of $ v_1$ and $ v_2 $, I assumed that I could just plug the numbers in. I knew I had to make the vectors unit vectors so my equation looked something like this:



$ 3(5)(v2)+ 1/11(v3) $ This answer is not the correct answer but I am wondering if I am reading the matrix in the wrong way or if I am confused about the usage of the formula. Does someone have a better idea?










share|cite|improve this question











$endgroup$




I am working on a problem that is asking to find $operatornameproj_Vv_3$ where the matrix $$A=beginbmatrix 3&5&11\ 5&9&20\ 11&20&49endbmatrix
$$

has entries $a_ij=vec v_icdot vec v_j$.
I though that the simplest way to answer this question was to use the formula
$$ (u_1,x)u_1+(u_2,x)u_2 = proj_vx .$$



Since the question also asks to express the solution as a linear combination of $ v_1$ and $ v_2 $, I assumed that I could just plug the numbers in. I knew I had to make the vectors unit vectors so my equation looked something like this:



$ 3(5)(v2)+ 1/11(v3) $ This answer is not the correct answer but I am wondering if I am reading the matrix in the wrong way or if I am confused about the usage of the formula. Does someone have a better idea?







linear-algebra matrices projection






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share|cite|improve this question













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share|cite|improve this question








edited Apr 1 at 2:03









Martin Argerami

130k1184185




130k1184185










asked Mar 31 at 19:56









user3471031user3471031

314




314











  • $begingroup$
    What is $v$ here?
    $endgroup$
    – amd
    Apr 1 at 1:16
















  • $begingroup$
    What is $v$ here?
    $endgroup$
    – amd
    Apr 1 at 1:16















$begingroup$
What is $v$ here?
$endgroup$
– amd
Apr 1 at 1:16




$begingroup$
What is $v$ here?
$endgroup$
– amd
Apr 1 at 1:16










1 Answer
1






active

oldest

votes


















0












$begingroup$

The formula as you wrote it makes no sense. The correct version, since $V=operatornamespanv_1,v_2$, is
$$
operatornameproj_V(v_3)=fracv_3cdot v_1v_1cdot v_1,v_1+fracv_3cdot v_2v_2cdot v_2,v_2.
$$

Reading the four numbers from the matrix we get
$$
operatornameproj_V(v_3)=fraca_13a_11,v_1+fraca_23a_22,v_2=frac113,v_1+frac59,v_2.
$$






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    1 Answer
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    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    0












    $begingroup$

    The formula as you wrote it makes no sense. The correct version, since $V=operatornamespanv_1,v_2$, is
    $$
    operatornameproj_V(v_3)=fracv_3cdot v_1v_1cdot v_1,v_1+fracv_3cdot v_2v_2cdot v_2,v_2.
    $$

    Reading the four numbers from the matrix we get
    $$
    operatornameproj_V(v_3)=fraca_13a_11,v_1+fraca_23a_22,v_2=frac113,v_1+frac59,v_2.
    $$






    share|cite|improve this answer









    $endgroup$

















      0












      $begingroup$

      The formula as you wrote it makes no sense. The correct version, since $V=operatornamespanv_1,v_2$, is
      $$
      operatornameproj_V(v_3)=fracv_3cdot v_1v_1cdot v_1,v_1+fracv_3cdot v_2v_2cdot v_2,v_2.
      $$

      Reading the four numbers from the matrix we get
      $$
      operatornameproj_V(v_3)=fraca_13a_11,v_1+fraca_23a_22,v_2=frac113,v_1+frac59,v_2.
      $$






      share|cite|improve this answer









      $endgroup$















        0












        0








        0





        $begingroup$

        The formula as you wrote it makes no sense. The correct version, since $V=operatornamespanv_1,v_2$, is
        $$
        operatornameproj_V(v_3)=fracv_3cdot v_1v_1cdot v_1,v_1+fracv_3cdot v_2v_2cdot v_2,v_2.
        $$

        Reading the four numbers from the matrix we get
        $$
        operatornameproj_V(v_3)=fraca_13a_11,v_1+fraca_23a_22,v_2=frac113,v_1+frac59,v_2.
        $$






        share|cite|improve this answer









        $endgroup$



        The formula as you wrote it makes no sense. The correct version, since $V=operatornamespanv_1,v_2$, is
        $$
        operatornameproj_V(v_3)=fracv_3cdot v_1v_1cdot v_1,v_1+fracv_3cdot v_2v_2cdot v_2,v_2.
        $$

        Reading the four numbers from the matrix we get
        $$
        operatornameproj_V(v_3)=fraca_13a_11,v_1+fraca_23a_22,v_2=frac113,v_1+frac59,v_2.
        $$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Apr 1 at 1:59









        Martin ArgeramiMartin Argerami

        130k1184185




        130k1184185



























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