Orthogonal Matrix with a specific row












0














I have an assignment with the following question:



Does an Orthogonal Matrix exist such that its first row consists of the

following values:


($1$/$sqrt{3}$, $1$/$sqrt{3}$, $1$/$sqrt{3}$)



If there is, find one.


I know I can solve this question with the Gram Schmidt algorithm, but it includes a lot of complicated calculations.



Is there any other way to prove this statement without the Gram Schmidt algorithm?



Thanks,



Alan










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  • There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
    – LouisB
    Nov 23 '14 at 6:32










  • @Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
    – Alan
    Nov 23 '14 at 6:34












  • I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
    – LouisB
    Nov 23 '14 at 7:01










  • Similar: math.stackexchange.com/questions/2241414/….
    – StubbornAtom
    May 7 '17 at 10:56










  • An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
    – CyclotomicField
    Jul 2 '18 at 4:31
















0














I have an assignment with the following question:



Does an Orthogonal Matrix exist such that its first row consists of the

following values:


($1$/$sqrt{3}$, $1$/$sqrt{3}$, $1$/$sqrt{3}$)



If there is, find one.


I know I can solve this question with the Gram Schmidt algorithm, but it includes a lot of complicated calculations.



Is there any other way to prove this statement without the Gram Schmidt algorithm?



Thanks,



Alan










share|cite|improve this question
























  • There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
    – LouisB
    Nov 23 '14 at 6:32










  • @Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
    – Alan
    Nov 23 '14 at 6:34












  • I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
    – LouisB
    Nov 23 '14 at 7:01










  • Similar: math.stackexchange.com/questions/2241414/….
    – StubbornAtom
    May 7 '17 at 10:56










  • An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
    – CyclotomicField
    Jul 2 '18 at 4:31














0












0








0


1





I have an assignment with the following question:



Does an Orthogonal Matrix exist such that its first row consists of the

following values:


($1$/$sqrt{3}$, $1$/$sqrt{3}$, $1$/$sqrt{3}$)



If there is, find one.


I know I can solve this question with the Gram Schmidt algorithm, but it includes a lot of complicated calculations.



Is there any other way to prove this statement without the Gram Schmidt algorithm?



Thanks,



Alan










share|cite|improve this question















I have an assignment with the following question:



Does an Orthogonal Matrix exist such that its first row consists of the

following values:


($1$/$sqrt{3}$, $1$/$sqrt{3}$, $1$/$sqrt{3}$)



If there is, find one.


I know I can solve this question with the Gram Schmidt algorithm, but it includes a lot of complicated calculations.



Is there any other way to prove this statement without the Gram Schmidt algorithm?



Thanks,



Alan







linear-algebra orthonormal orthogonality






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share|cite|improve this question




share|cite|improve this question








edited Nov 23 '14 at 6:27

























asked Nov 23 '14 at 6:13









Alan

1,3671021




1,3671021












  • There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
    – LouisB
    Nov 23 '14 at 6:32










  • @Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
    – Alan
    Nov 23 '14 at 6:34












  • I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
    – LouisB
    Nov 23 '14 at 7:01










  • Similar: math.stackexchange.com/questions/2241414/….
    – StubbornAtom
    May 7 '17 at 10:56










  • An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
    – CyclotomicField
    Jul 2 '18 at 4:31


















  • There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
    – LouisB
    Nov 23 '14 at 6:32










  • @Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
    – Alan
    Nov 23 '14 at 6:34












  • I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
    – LouisB
    Nov 23 '14 at 7:01










  • Similar: math.stackexchange.com/questions/2241414/….
    – StubbornAtom
    May 7 '17 at 10:56










  • An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
    – CyclotomicField
    Jul 2 '18 at 4:31
















There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
– LouisB
Nov 23 '14 at 6:32




There is another way. You should be able to show that each row (column) of an orthogonal matrix is orthonormal to the other rows (columns) of the matrix. That fact should be enough to answer the original question.
– LouisB
Nov 23 '14 at 6:32












@Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
– Alan
Nov 23 '14 at 6:34






@Louis, can you please elaborate, as I amnot sure how to prove that? Thank you in advance.
– Alan
Nov 23 '14 at 6:34














I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
– LouisB
Nov 23 '14 at 7:01




I misread the question! I managed to read $1, {sqrt{3}}$ instead of $1/{sqrt{3}}$. Sorry about that.
– LouisB
Nov 23 '14 at 7:01












Similar: math.stackexchange.com/questions/2241414/….
– StubbornAtom
May 7 '17 at 10:56




Similar: math.stackexchange.com/questions/2241414/….
– StubbornAtom
May 7 '17 at 10:56












An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
– CyclotomicField
Jul 2 '18 at 4:31




An orthogonal matrix will have mutually perpendicular rows of unit vectors so as long as the vector is a unit vector you're have sufficient conditions for the existence of such a matrix.
– CyclotomicField
Jul 2 '18 at 4:31










3 Answers
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0














You need to choose two vectors which are orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ and make sure they are also orthogonal to each other.



Vectors orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ lie in the plane $x+y+z=0$.



For example choose $(1,-1,0)$ to be the second row of your matrix.



Now you need a third vector which satisfies the equations:



$$x+y+z=0 ~~~ text{[is orthogonal to}~left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)]$$
$$x-y=0 ~~~ text{[is orthogonal to}~(1,-1,0)]$$



Any such vector can form the third row of your orthogonal matrix.






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  • One way to find a 3rd vector is to take the cross product of the first two vectors.
    – Gerry Myerson
    Nov 23 '14 at 10:57



















0














i answered a similar question before, but i don't know how to make a link to that.



so i will repeat the way to find a symmetric reflection matrix whose row is your unit vector. reflect $((1,1,1)^T/sqrt 3$ to $(1,0,0)^T$ on the mirror in the direction of their bisector $a = (1 + sqrt 3, 1, 1)^T.$ form the matrix $q = 2aa^T/a^Ta - I;$ that will satisfy your requirement.






share|cite|improve this answer





























    0














    $(frac1{sqrt3},frac1{sqrt3},frac1{sqrt3})$ is a unit vector, so you're off to a good start. (That is, there is such a matrix.)



    Next just choose a perpendicular vector, and normalize it. So, say, $(frac1{sqrt2},-frac1{sqrt2},0)$.



    Finally you need a vector perpendicular to the first two, and again of unit length. The cross-product will produce such a vector. Or, looking at the first two, I think I can produce one out of the blue. Namely, $(frac1{sqrt6},frac1{sqrt6},-frac2{sqrt6})$. You can easily check that the dot products with the other two vectors are zero. And it has unit length.



    So one such matrix is: $$begin{pmatrix}frac1{sqrt3}& frac1{sqrt3}& frac1{sqrt3}\frac1{sqrt2}&-frac1{sqrt2}&0\frac1{sqrt6}&frac1{sqrt6}&-frac2{sqrt6}end{pmatrix}$$.






    share|cite|improve this answer





















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      3 Answers
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      active

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      3 Answers
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      active

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      0














      You need to choose two vectors which are orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ and make sure they are also orthogonal to each other.



      Vectors orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ lie in the plane $x+y+z=0$.



      For example choose $(1,-1,0)$ to be the second row of your matrix.



      Now you need a third vector which satisfies the equations:



      $$x+y+z=0 ~~~ text{[is orthogonal to}~left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)]$$
      $$x-y=0 ~~~ text{[is orthogonal to}~(1,-1,0)]$$



      Any such vector can form the third row of your orthogonal matrix.






      share|cite|improve this answer





















      • One way to find a 3rd vector is to take the cross product of the first two vectors.
        – Gerry Myerson
        Nov 23 '14 at 10:57
















      0














      You need to choose two vectors which are orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ and make sure they are also orthogonal to each other.



      Vectors orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ lie in the plane $x+y+z=0$.



      For example choose $(1,-1,0)$ to be the second row of your matrix.



      Now you need a third vector which satisfies the equations:



      $$x+y+z=0 ~~~ text{[is orthogonal to}~left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)]$$
      $$x-y=0 ~~~ text{[is orthogonal to}~(1,-1,0)]$$



      Any such vector can form the third row of your orthogonal matrix.






      share|cite|improve this answer





















      • One way to find a 3rd vector is to take the cross product of the first two vectors.
        – Gerry Myerson
        Nov 23 '14 at 10:57














      0












      0








      0






      You need to choose two vectors which are orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ and make sure they are also orthogonal to each other.



      Vectors orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ lie in the plane $x+y+z=0$.



      For example choose $(1,-1,0)$ to be the second row of your matrix.



      Now you need a third vector which satisfies the equations:



      $$x+y+z=0 ~~~ text{[is orthogonal to}~left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)]$$
      $$x-y=0 ~~~ text{[is orthogonal to}~(1,-1,0)]$$



      Any such vector can form the third row of your orthogonal matrix.






      share|cite|improve this answer












      You need to choose two vectors which are orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ and make sure they are also orthogonal to each other.



      Vectors orthogonal to $left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)$ lie in the plane $x+y+z=0$.



      For example choose $(1,-1,0)$ to be the second row of your matrix.



      Now you need a third vector which satisfies the equations:



      $$x+y+z=0 ~~~ text{[is orthogonal to}~left(frac{1}{sqrt{3}},frac{1}{sqrt{3}},frac{1}{sqrt{3}}right)]$$
      $$x-y=0 ~~~ text{[is orthogonal to}~(1,-1,0)]$$



      Any such vector can form the third row of your orthogonal matrix.







      share|cite|improve this answer












      share|cite|improve this answer



      share|cite|improve this answer










      answered Nov 23 '14 at 6:39









      RHP

      1,40811525




      1,40811525












      • One way to find a 3rd vector is to take the cross product of the first two vectors.
        – Gerry Myerson
        Nov 23 '14 at 10:57


















      • One way to find a 3rd vector is to take the cross product of the first two vectors.
        – Gerry Myerson
        Nov 23 '14 at 10:57
















      One way to find a 3rd vector is to take the cross product of the first two vectors.
      – Gerry Myerson
      Nov 23 '14 at 10:57




      One way to find a 3rd vector is to take the cross product of the first two vectors.
      – Gerry Myerson
      Nov 23 '14 at 10:57











      0














      i answered a similar question before, but i don't know how to make a link to that.



      so i will repeat the way to find a symmetric reflection matrix whose row is your unit vector. reflect $((1,1,1)^T/sqrt 3$ to $(1,0,0)^T$ on the mirror in the direction of their bisector $a = (1 + sqrt 3, 1, 1)^T.$ form the matrix $q = 2aa^T/a^Ta - I;$ that will satisfy your requirement.






      share|cite|improve this answer


























        0














        i answered a similar question before, but i don't know how to make a link to that.



        so i will repeat the way to find a symmetric reflection matrix whose row is your unit vector. reflect $((1,1,1)^T/sqrt 3$ to $(1,0,0)^T$ on the mirror in the direction of their bisector $a = (1 + sqrt 3, 1, 1)^T.$ form the matrix $q = 2aa^T/a^Ta - I;$ that will satisfy your requirement.






        share|cite|improve this answer
























          0












          0








          0






          i answered a similar question before, but i don't know how to make a link to that.



          so i will repeat the way to find a symmetric reflection matrix whose row is your unit vector. reflect $((1,1,1)^T/sqrt 3$ to $(1,0,0)^T$ on the mirror in the direction of their bisector $a = (1 + sqrt 3, 1, 1)^T.$ form the matrix $q = 2aa^T/a^Ta - I;$ that will satisfy your requirement.






          share|cite|improve this answer












          i answered a similar question before, but i don't know how to make a link to that.



          so i will repeat the way to find a symmetric reflection matrix whose row is your unit vector. reflect $((1,1,1)^T/sqrt 3$ to $(1,0,0)^T$ on the mirror in the direction of their bisector $a = (1 + sqrt 3, 1, 1)^T.$ form the matrix $q = 2aa^T/a^Ta - I;$ that will satisfy your requirement.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 23 '14 at 15:50









          abel

          26.5k11948




          26.5k11948























              0














              $(frac1{sqrt3},frac1{sqrt3},frac1{sqrt3})$ is a unit vector, so you're off to a good start. (That is, there is such a matrix.)



              Next just choose a perpendicular vector, and normalize it. So, say, $(frac1{sqrt2},-frac1{sqrt2},0)$.



              Finally you need a vector perpendicular to the first two, and again of unit length. The cross-product will produce such a vector. Or, looking at the first two, I think I can produce one out of the blue. Namely, $(frac1{sqrt6},frac1{sqrt6},-frac2{sqrt6})$. You can easily check that the dot products with the other two vectors are zero. And it has unit length.



              So one such matrix is: $$begin{pmatrix}frac1{sqrt3}& frac1{sqrt3}& frac1{sqrt3}\frac1{sqrt2}&-frac1{sqrt2}&0\frac1{sqrt6}&frac1{sqrt6}&-frac2{sqrt6}end{pmatrix}$$.






              share|cite|improve this answer


























                0














                $(frac1{sqrt3},frac1{sqrt3},frac1{sqrt3})$ is a unit vector, so you're off to a good start. (That is, there is such a matrix.)



                Next just choose a perpendicular vector, and normalize it. So, say, $(frac1{sqrt2},-frac1{sqrt2},0)$.



                Finally you need a vector perpendicular to the first two, and again of unit length. The cross-product will produce such a vector. Or, looking at the first two, I think I can produce one out of the blue. Namely, $(frac1{sqrt6},frac1{sqrt6},-frac2{sqrt6})$. You can easily check that the dot products with the other two vectors are zero. And it has unit length.



                So one such matrix is: $$begin{pmatrix}frac1{sqrt3}& frac1{sqrt3}& frac1{sqrt3}\frac1{sqrt2}&-frac1{sqrt2}&0\frac1{sqrt6}&frac1{sqrt6}&-frac2{sqrt6}end{pmatrix}$$.






                share|cite|improve this answer
























                  0












                  0








                  0






                  $(frac1{sqrt3},frac1{sqrt3},frac1{sqrt3})$ is a unit vector, so you're off to a good start. (That is, there is such a matrix.)



                  Next just choose a perpendicular vector, and normalize it. So, say, $(frac1{sqrt2},-frac1{sqrt2},0)$.



                  Finally you need a vector perpendicular to the first two, and again of unit length. The cross-product will produce such a vector. Or, looking at the first two, I think I can produce one out of the blue. Namely, $(frac1{sqrt6},frac1{sqrt6},-frac2{sqrt6})$. You can easily check that the dot products with the other two vectors are zero. And it has unit length.



                  So one such matrix is: $$begin{pmatrix}frac1{sqrt3}& frac1{sqrt3}& frac1{sqrt3}\frac1{sqrt2}&-frac1{sqrt2}&0\frac1{sqrt6}&frac1{sqrt6}&-frac2{sqrt6}end{pmatrix}$$.






                  share|cite|improve this answer












                  $(frac1{sqrt3},frac1{sqrt3},frac1{sqrt3})$ is a unit vector, so you're off to a good start. (That is, there is such a matrix.)



                  Next just choose a perpendicular vector, and normalize it. So, say, $(frac1{sqrt2},-frac1{sqrt2},0)$.



                  Finally you need a vector perpendicular to the first two, and again of unit length. The cross-product will produce such a vector. Or, looking at the first two, I think I can produce one out of the blue. Namely, $(frac1{sqrt6},frac1{sqrt6},-frac2{sqrt6})$. You can easily check that the dot products with the other two vectors are zero. And it has unit length.



                  So one such matrix is: $$begin{pmatrix}frac1{sqrt3}& frac1{sqrt3}& frac1{sqrt3}\frac1{sqrt2}&-frac1{sqrt2}&0\frac1{sqrt6}&frac1{sqrt6}&-frac2{sqrt6}end{pmatrix}$$.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Jan 2 at 7:29









                  Chris Custer

                  10.9k3824




                  10.9k3824






























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