Is division of matrices possible?











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down vote

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Is it possible to divide a matrix by another? If yes, What will be the result of $dfrac AB$ if
$$
A = begin{pmatrix}
a & b \
c & d \
end{pmatrix},
B = begin{pmatrix}
w & x \
y & z \
end{pmatrix}?
$$










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    see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
    – Raymond Manzoni
    Nov 3 '12 at 16:43












  • To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
    – Martin Sleziak
    Nov 3 '12 at 16:49















up vote
16
down vote

favorite
6












Is it possible to divide a matrix by another? If yes, What will be the result of $dfrac AB$ if
$$
A = begin{pmatrix}
a & b \
c & d \
end{pmatrix},
B = begin{pmatrix}
w & x \
y & z \
end{pmatrix}?
$$










share|cite|improve this question




















  • 6




    see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
    – Raymond Manzoni
    Nov 3 '12 at 16:43












  • To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
    – Martin Sleziak
    Nov 3 '12 at 16:49













up vote
16
down vote

favorite
6









up vote
16
down vote

favorite
6






6





Is it possible to divide a matrix by another? If yes, What will be the result of $dfrac AB$ if
$$
A = begin{pmatrix}
a & b \
c & d \
end{pmatrix},
B = begin{pmatrix}
w & x \
y & z \
end{pmatrix}?
$$










share|cite|improve this question















Is it possible to divide a matrix by another? If yes, What will be the result of $dfrac AB$ if
$$
A = begin{pmatrix}
a & b \
c & d \
end{pmatrix},
B = begin{pmatrix}
w & x \
y & z \
end{pmatrix}?
$$







matrices divisibility






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edited Nov 3 '12 at 18:21







user2468

















asked Nov 3 '12 at 16:39









Pranit Bauva

4442614




4442614








  • 6




    see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
    – Raymond Manzoni
    Nov 3 '12 at 16:43












  • To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
    – Martin Sleziak
    Nov 3 '12 at 16:49














  • 6




    see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
    – Raymond Manzoni
    Nov 3 '12 at 16:43












  • To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
    – Martin Sleziak
    Nov 3 '12 at 16:49








6




6




see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
– Raymond Manzoni
Nov 3 '12 at 16:43






see for example here. You'll have to distinguish between $AB^{-1}$ and $B^{-1}A$.
– Raymond Manzoni
Nov 3 '12 at 16:43














To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
– Martin Sleziak
Nov 3 '12 at 16:49




To add various types of brackets around entries of a matrix, you can use pmatrix or bmatrix or other variants. See tutorial at meta.
– Martin Sleziak
Nov 3 '12 at 16:49










7 Answers
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oldest

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up vote
26
down vote



accepted










For ordinary numbers $frac{a}{b}$ means the solution to the equation $xb=a$. This is the same as $bx=a$, but since matrix multiplication is not commutative, there are two different possible generalizations of "division" to matrices.



If $B$ is invertible, then you can form $AB^{-1}$ or $B^{-1}A$, but these are not in general the same matrix. They are the solutions to $XB=A$ and $BX=A$ respectively.



If $B$ is not invertible, then $XB=A$ and $BX=A$ may have solutions, but the solutions will not be unique. So in that situation speaking of "matrix division" is even less warranted.






share|cite|improve this answer






























    up vote
    5
    down vote













    There is a way to performa sort of division , but I am not sure if it is the way you are looking for. For motivation ,consider the ordinary real numbers $mathbb{R}$ . We have that for two real numbers, $x/y$ is really the same as multiplying x and $y^{-1}=1/y$. We call $y^{-1}$ the inverse of y, and note that it has the property that $yy^{-1}=1.$



    The same goes for different algebraic structures. That is, for two elements x,y in this algebraic structure we define $x/y$ as $xy^{-1}$ (under some operation). Most notably, we have a notion of division in any division ring (hence the name!) . It turns out that if you consider invertible $n times n$ matrices with addition and ordinary matrix multiplication, there is a sensible way to define division since every invertible matrix has well, an inverse. So just to help you grip what an inverse is, say that you have a 2x2 matrix $$A= begin{bmatrix} a & b \ c & d end{bmatrix}.$$
    The inverse of A is then given by
    $$A^{-1} = dfrac{1}{(ad-bc)} begin{bmatrix} d & -b \ -c & a end{bmatrix}$$
    and you should check that $AA^{-1}=E$, the identity matrix. Now, for two matrices $B$ and $A$, $B/A = BA^{-1}$.






    share|cite|improve this answer





















    • I know this is not the most eloquent answer, but why the downvote?
      – Dedalus
      Apr 2 '14 at 13:02




















    up vote
    4
    down vote













    Normaly matrix division is defined as $frac{A}{B}=AB^{-1}$ where $B^{-1}$ stands for inverse matrix of $B$. In case when inverse doesn't exist so called pseudoinverse may be used.






    share|cite|improve this answer






























      up vote
      2
      down vote













      We can say $$frac{A}{B}=Atimes B^{-1}$$ where $B^{-1}$ is inverse matrice of $B$






      share|cite|improve this answer

















      • 1




        if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
        – user31280
        Nov 3 '12 at 18:29








      • 1




        That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
        – Adi Dani
        Nov 3 '12 at 19:02


















      up vote
      1
      down vote













      There are two issues: first, that matrices have divisors of zero; second, that matrix multiplication is in general not commutative.



      To give meaning to $A/B$, you need to give meaning to $I/B$ (because then $A/B=A(I/B)$. Now, no one ever writes $I/B$, people actually write $B^{-1}$. Anyway, what is $B^{-1}$? It should be a matrix such that multiplied by $B$ gives you the identity. Now, there exist nonzero matrices $C$, $B$ with $BC=0$. If $B$ had an inverse $B^{-1}$, we would have
      $$
      0=B^{-1}0=B^{-1}BC=C,
      $$
      a contradiction. So such a matrix $B$ cannot have an inverse, i.e. "$I/B$" does not make sense.



      The invertible matrices are exactly those with nonzero determinant. So, if $det Bne0$, then $AB^{-1}$ does make sense.



      In you case, that would be the condition $wz-yxne0$. In that case,
      $$
      begin{bmatrix}w&x\ y&zend{bmatrix}^{-1}=frac1{wz-yx}begin{bmatrix}z&-x\ -y&wend{bmatrix}
      $$



      The second issue is a non-issue, because it can be proven that, for matrices, if $B^{-1}A=I$, then $AB^{-1}=I$.






      share|cite|improve this answer




























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        0
        down vote













        Do you know why matrix multiplication is defined in such a weird way ? It is defined thus, so that the effect on a column vector of left-multiplying it by one matrix, and then left-multiplying the result by another matrix, is exactly the same as the effect of left-multiplying by a single matrix that is the product of those two matrices. That is, matrix multiplication corresponds to composition of linear operators. If you didn't know that already, then you should try to convince yourself that it's true, in a simple class of special cases. I would recommend trying a 2 by 1 column vector with variables for entries, and two 2 by 2 matrices, also with variables for entries. So "dividing by" a matrix would correspond to "undoing" the effect of a linear operator. For instance, rotating the plane clockwise by a certain angle "undoes" the anticlockwise rotation of the plane by the same angle. However, there are plenty of linear operators that can't be undone (because they have squashed something flat), and the matrices representing these operators (with respect to any given basis) must therefore be not-dividable-by. This is unlike the situation with the real numbers, for instance, in which there is only one of them by which you can't divide, namely zero.






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          Element-wise division is frequently used to normalize matrices in R o MatLab. For instance, in R if dtm is a contingency matrix counting occurrences of something, you can normalize the rows so that all the rows sum 1.0 as:



          dtm.proportions <- dtm / rowSums(dtm)


          Or normalize the columns transposing the matrix first:



          dtm.proportions <- t(t(dtm)/colSums(dtm))





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            7 Answers
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            7 Answers
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            active

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            active

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            up vote
            26
            down vote



            accepted










            For ordinary numbers $frac{a}{b}$ means the solution to the equation $xb=a$. This is the same as $bx=a$, but since matrix multiplication is not commutative, there are two different possible generalizations of "division" to matrices.



            If $B$ is invertible, then you can form $AB^{-1}$ or $B^{-1}A$, but these are not in general the same matrix. They are the solutions to $XB=A$ and $BX=A$ respectively.



            If $B$ is not invertible, then $XB=A$ and $BX=A$ may have solutions, but the solutions will not be unique. So in that situation speaking of "matrix division" is even less warranted.






            share|cite|improve this answer



























              up vote
              26
              down vote



              accepted










              For ordinary numbers $frac{a}{b}$ means the solution to the equation $xb=a$. This is the same as $bx=a$, but since matrix multiplication is not commutative, there are two different possible generalizations of "division" to matrices.



              If $B$ is invertible, then you can form $AB^{-1}$ or $B^{-1}A$, but these are not in general the same matrix. They are the solutions to $XB=A$ and $BX=A$ respectively.



              If $B$ is not invertible, then $XB=A$ and $BX=A$ may have solutions, but the solutions will not be unique. So in that situation speaking of "matrix division" is even less warranted.






              share|cite|improve this answer

























                up vote
                26
                down vote



                accepted







                up vote
                26
                down vote



                accepted






                For ordinary numbers $frac{a}{b}$ means the solution to the equation $xb=a$. This is the same as $bx=a$, but since matrix multiplication is not commutative, there are two different possible generalizations of "division" to matrices.



                If $B$ is invertible, then you can form $AB^{-1}$ or $B^{-1}A$, but these are not in general the same matrix. They are the solutions to $XB=A$ and $BX=A$ respectively.



                If $B$ is not invertible, then $XB=A$ and $BX=A$ may have solutions, but the solutions will not be unique. So in that situation speaking of "matrix division" is even less warranted.






                share|cite|improve this answer














                For ordinary numbers $frac{a}{b}$ means the solution to the equation $xb=a$. This is the same as $bx=a$, but since matrix multiplication is not commutative, there are two different possible generalizations of "division" to matrices.



                If $B$ is invertible, then you can form $AB^{-1}$ or $B^{-1}A$, but these are not in general the same matrix. They are the solutions to $XB=A$ and $BX=A$ respectively.



                If $B$ is not invertible, then $XB=A$ and $BX=A$ may have solutions, but the solutions will not be unique. So in that situation speaking of "matrix division" is even less warranted.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited Nov 3 '12 at 16:56

























                answered Nov 3 '12 at 16:46









                Henning Makholm

                236k16300534




                236k16300534






















                    up vote
                    5
                    down vote













                    There is a way to performa sort of division , but I am not sure if it is the way you are looking for. For motivation ,consider the ordinary real numbers $mathbb{R}$ . We have that for two real numbers, $x/y$ is really the same as multiplying x and $y^{-1}=1/y$. We call $y^{-1}$ the inverse of y, and note that it has the property that $yy^{-1}=1.$



                    The same goes for different algebraic structures. That is, for two elements x,y in this algebraic structure we define $x/y$ as $xy^{-1}$ (under some operation). Most notably, we have a notion of division in any division ring (hence the name!) . It turns out that if you consider invertible $n times n$ matrices with addition and ordinary matrix multiplication, there is a sensible way to define division since every invertible matrix has well, an inverse. So just to help you grip what an inverse is, say that you have a 2x2 matrix $$A= begin{bmatrix} a & b \ c & d end{bmatrix}.$$
                    The inverse of A is then given by
                    $$A^{-1} = dfrac{1}{(ad-bc)} begin{bmatrix} d & -b \ -c & a end{bmatrix}$$
                    and you should check that $AA^{-1}=E$, the identity matrix. Now, for two matrices $B$ and $A$, $B/A = BA^{-1}$.






                    share|cite|improve this answer





















                    • I know this is not the most eloquent answer, but why the downvote?
                      – Dedalus
                      Apr 2 '14 at 13:02

















                    up vote
                    5
                    down vote













                    There is a way to performa sort of division , but I am not sure if it is the way you are looking for. For motivation ,consider the ordinary real numbers $mathbb{R}$ . We have that for two real numbers, $x/y$ is really the same as multiplying x and $y^{-1}=1/y$. We call $y^{-1}$ the inverse of y, and note that it has the property that $yy^{-1}=1.$



                    The same goes for different algebraic structures. That is, for two elements x,y in this algebraic structure we define $x/y$ as $xy^{-1}$ (under some operation). Most notably, we have a notion of division in any division ring (hence the name!) . It turns out that if you consider invertible $n times n$ matrices with addition and ordinary matrix multiplication, there is a sensible way to define division since every invertible matrix has well, an inverse. So just to help you grip what an inverse is, say that you have a 2x2 matrix $$A= begin{bmatrix} a & b \ c & d end{bmatrix}.$$
                    The inverse of A is then given by
                    $$A^{-1} = dfrac{1}{(ad-bc)} begin{bmatrix} d & -b \ -c & a end{bmatrix}$$
                    and you should check that $AA^{-1}=E$, the identity matrix. Now, for two matrices $B$ and $A$, $B/A = BA^{-1}$.






                    share|cite|improve this answer





















                    • I know this is not the most eloquent answer, but why the downvote?
                      – Dedalus
                      Apr 2 '14 at 13:02















                    up vote
                    5
                    down vote










                    up vote
                    5
                    down vote









                    There is a way to performa sort of division , but I am not sure if it is the way you are looking for. For motivation ,consider the ordinary real numbers $mathbb{R}$ . We have that for two real numbers, $x/y$ is really the same as multiplying x and $y^{-1}=1/y$. We call $y^{-1}$ the inverse of y, and note that it has the property that $yy^{-1}=1.$



                    The same goes for different algebraic structures. That is, for two elements x,y in this algebraic structure we define $x/y$ as $xy^{-1}$ (under some operation). Most notably, we have a notion of division in any division ring (hence the name!) . It turns out that if you consider invertible $n times n$ matrices with addition and ordinary matrix multiplication, there is a sensible way to define division since every invertible matrix has well, an inverse. So just to help you grip what an inverse is, say that you have a 2x2 matrix $$A= begin{bmatrix} a & b \ c & d end{bmatrix}.$$
                    The inverse of A is then given by
                    $$A^{-1} = dfrac{1}{(ad-bc)} begin{bmatrix} d & -b \ -c & a end{bmatrix}$$
                    and you should check that $AA^{-1}=E$, the identity matrix. Now, for two matrices $B$ and $A$, $B/A = BA^{-1}$.






                    share|cite|improve this answer












                    There is a way to performa sort of division , but I am not sure if it is the way you are looking for. For motivation ,consider the ordinary real numbers $mathbb{R}$ . We have that for two real numbers, $x/y$ is really the same as multiplying x and $y^{-1}=1/y$. We call $y^{-1}$ the inverse of y, and note that it has the property that $yy^{-1}=1.$



                    The same goes for different algebraic structures. That is, for two elements x,y in this algebraic structure we define $x/y$ as $xy^{-1}$ (under some operation). Most notably, we have a notion of division in any division ring (hence the name!) . It turns out that if you consider invertible $n times n$ matrices with addition and ordinary matrix multiplication, there is a sensible way to define division since every invertible matrix has well, an inverse. So just to help you grip what an inverse is, say that you have a 2x2 matrix $$A= begin{bmatrix} a & b \ c & d end{bmatrix}.$$
                    The inverse of A is then given by
                    $$A^{-1} = dfrac{1}{(ad-bc)} begin{bmatrix} d & -b \ -c & a end{bmatrix}$$
                    and you should check that $AA^{-1}=E$, the identity matrix. Now, for two matrices $B$ and $A$, $B/A = BA^{-1}$.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Nov 3 '12 at 16:55









                    Dedalus

                    1,99711936




                    1,99711936












                    • I know this is not the most eloquent answer, but why the downvote?
                      – Dedalus
                      Apr 2 '14 at 13:02




















                    • I know this is not the most eloquent answer, but why the downvote?
                      – Dedalus
                      Apr 2 '14 at 13:02


















                    I know this is not the most eloquent answer, but why the downvote?
                    – Dedalus
                    Apr 2 '14 at 13:02






                    I know this is not the most eloquent answer, but why the downvote?
                    – Dedalus
                    Apr 2 '14 at 13:02












                    up vote
                    4
                    down vote













                    Normaly matrix division is defined as $frac{A}{B}=AB^{-1}$ where $B^{-1}$ stands for inverse matrix of $B$. In case when inverse doesn't exist so called pseudoinverse may be used.






                    share|cite|improve this answer



























                      up vote
                      4
                      down vote













                      Normaly matrix division is defined as $frac{A}{B}=AB^{-1}$ where $B^{-1}$ stands for inverse matrix of $B$. In case when inverse doesn't exist so called pseudoinverse may be used.






                      share|cite|improve this answer

























                        up vote
                        4
                        down vote










                        up vote
                        4
                        down vote









                        Normaly matrix division is defined as $frac{A}{B}=AB^{-1}$ where $B^{-1}$ stands for inverse matrix of $B$. In case when inverse doesn't exist so called pseudoinverse may be used.






                        share|cite|improve this answer














                        Normaly matrix division is defined as $frac{A}{B}=AB^{-1}$ where $B^{-1}$ stands for inverse matrix of $B$. In case when inverse doesn't exist so called pseudoinverse may be used.







                        share|cite|improve this answer














                        share|cite|improve this answer



                        share|cite|improve this answer








                        edited Nov 3 '12 at 17:35









                        Martin Sleziak

                        44.5k7115268




                        44.5k7115268










                        answered Nov 3 '12 at 16:45









                        Mykolas

                        692616




                        692616






















                            up vote
                            2
                            down vote













                            We can say $$frac{A}{B}=Atimes B^{-1}$$ where $B^{-1}$ is inverse matrice of $B$






                            share|cite|improve this answer

















                            • 1




                              if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                              – user31280
                              Nov 3 '12 at 18:29








                            • 1




                              That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                              – Adi Dani
                              Nov 3 '12 at 19:02















                            up vote
                            2
                            down vote













                            We can say $$frac{A}{B}=Atimes B^{-1}$$ where $B^{-1}$ is inverse matrice of $B$






                            share|cite|improve this answer

















                            • 1




                              if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                              – user31280
                              Nov 3 '12 at 18:29








                            • 1




                              That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                              – Adi Dani
                              Nov 3 '12 at 19:02













                            up vote
                            2
                            down vote










                            up vote
                            2
                            down vote









                            We can say $$frac{A}{B}=Atimes B^{-1}$$ where $B^{-1}$ is inverse matrice of $B$






                            share|cite|improve this answer












                            We can say $$frac{A}{B}=Atimes B^{-1}$$ where $B^{-1}$ is inverse matrice of $B$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Nov 3 '12 at 16:46









                            Adi Dani

                            15.3k32046




                            15.3k32046








                            • 1




                              if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                              – user31280
                              Nov 3 '12 at 18:29








                            • 1




                              That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                              – Adi Dani
                              Nov 3 '12 at 19:02














                            • 1




                              if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                              – user31280
                              Nov 3 '12 at 18:29








                            • 1




                              That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                              – Adi Dani
                              Nov 3 '12 at 19:02








                            1




                            1




                            if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                            – user31280
                            Nov 3 '12 at 18:29






                            if $ cfrac AB = A times B^{-1}$ then what is $B^{-1} times A$?
                            – user31280
                            Nov 3 '12 at 18:29






                            1




                            1




                            That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                            – Adi Dani
                            Nov 3 '12 at 19:02




                            That is an other possibility since product of matrices is not commutative.So division of matrices can not be defined in an unique way
                            – Adi Dani
                            Nov 3 '12 at 19:02










                            up vote
                            1
                            down vote













                            There are two issues: first, that matrices have divisors of zero; second, that matrix multiplication is in general not commutative.



                            To give meaning to $A/B$, you need to give meaning to $I/B$ (because then $A/B=A(I/B)$. Now, no one ever writes $I/B$, people actually write $B^{-1}$. Anyway, what is $B^{-1}$? It should be a matrix such that multiplied by $B$ gives you the identity. Now, there exist nonzero matrices $C$, $B$ with $BC=0$. If $B$ had an inverse $B^{-1}$, we would have
                            $$
                            0=B^{-1}0=B^{-1}BC=C,
                            $$
                            a contradiction. So such a matrix $B$ cannot have an inverse, i.e. "$I/B$" does not make sense.



                            The invertible matrices are exactly those with nonzero determinant. So, if $det Bne0$, then $AB^{-1}$ does make sense.



                            In you case, that would be the condition $wz-yxne0$. In that case,
                            $$
                            begin{bmatrix}w&x\ y&zend{bmatrix}^{-1}=frac1{wz-yx}begin{bmatrix}z&-x\ -y&wend{bmatrix}
                            $$



                            The second issue is a non-issue, because it can be proven that, for matrices, if $B^{-1}A=I$, then $AB^{-1}=I$.






                            share|cite|improve this answer

























                              up vote
                              1
                              down vote













                              There are two issues: first, that matrices have divisors of zero; second, that matrix multiplication is in general not commutative.



                              To give meaning to $A/B$, you need to give meaning to $I/B$ (because then $A/B=A(I/B)$. Now, no one ever writes $I/B$, people actually write $B^{-1}$. Anyway, what is $B^{-1}$? It should be a matrix such that multiplied by $B$ gives you the identity. Now, there exist nonzero matrices $C$, $B$ with $BC=0$. If $B$ had an inverse $B^{-1}$, we would have
                              $$
                              0=B^{-1}0=B^{-1}BC=C,
                              $$
                              a contradiction. So such a matrix $B$ cannot have an inverse, i.e. "$I/B$" does not make sense.



                              The invertible matrices are exactly those with nonzero determinant. So, if $det Bne0$, then $AB^{-1}$ does make sense.



                              In you case, that would be the condition $wz-yxne0$. In that case,
                              $$
                              begin{bmatrix}w&x\ y&zend{bmatrix}^{-1}=frac1{wz-yx}begin{bmatrix}z&-x\ -y&wend{bmatrix}
                              $$



                              The second issue is a non-issue, because it can be proven that, for matrices, if $B^{-1}A=I$, then $AB^{-1}=I$.






                              share|cite|improve this answer























                                up vote
                                1
                                down vote










                                up vote
                                1
                                down vote









                                There are two issues: first, that matrices have divisors of zero; second, that matrix multiplication is in general not commutative.



                                To give meaning to $A/B$, you need to give meaning to $I/B$ (because then $A/B=A(I/B)$. Now, no one ever writes $I/B$, people actually write $B^{-1}$. Anyway, what is $B^{-1}$? It should be a matrix such that multiplied by $B$ gives you the identity. Now, there exist nonzero matrices $C$, $B$ with $BC=0$. If $B$ had an inverse $B^{-1}$, we would have
                                $$
                                0=B^{-1}0=B^{-1}BC=C,
                                $$
                                a contradiction. So such a matrix $B$ cannot have an inverse, i.e. "$I/B$" does not make sense.



                                The invertible matrices are exactly those with nonzero determinant. So, if $det Bne0$, then $AB^{-1}$ does make sense.



                                In you case, that would be the condition $wz-yxne0$. In that case,
                                $$
                                begin{bmatrix}w&x\ y&zend{bmatrix}^{-1}=frac1{wz-yx}begin{bmatrix}z&-x\ -y&wend{bmatrix}
                                $$



                                The second issue is a non-issue, because it can be proven that, for matrices, if $B^{-1}A=I$, then $AB^{-1}=I$.






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                                There are two issues: first, that matrices have divisors of zero; second, that matrix multiplication is in general not commutative.



                                To give meaning to $A/B$, you need to give meaning to $I/B$ (because then $A/B=A(I/B)$. Now, no one ever writes $I/B$, people actually write $B^{-1}$. Anyway, what is $B^{-1}$? It should be a matrix such that multiplied by $B$ gives you the identity. Now, there exist nonzero matrices $C$, $B$ with $BC=0$. If $B$ had an inverse $B^{-1}$, we would have
                                $$
                                0=B^{-1}0=B^{-1}BC=C,
                                $$
                                a contradiction. So such a matrix $B$ cannot have an inverse, i.e. "$I/B$" does not make sense.



                                The invertible matrices are exactly those with nonzero determinant. So, if $det Bne0$, then $AB^{-1}$ does make sense.



                                In you case, that would be the condition $wz-yxne0$. In that case,
                                $$
                                begin{bmatrix}w&x\ y&zend{bmatrix}^{-1}=frac1{wz-yx}begin{bmatrix}z&-x\ -y&wend{bmatrix}
                                $$



                                The second issue is a non-issue, because it can be proven that, for matrices, if $B^{-1}A=I$, then $AB^{-1}=I$.







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                                answered Nov 3 '12 at 16:50









                                Martin Argerami

                                122k1174172




                                122k1174172






















                                    up vote
                                    0
                                    down vote













                                    Do you know why matrix multiplication is defined in such a weird way ? It is defined thus, so that the effect on a column vector of left-multiplying it by one matrix, and then left-multiplying the result by another matrix, is exactly the same as the effect of left-multiplying by a single matrix that is the product of those two matrices. That is, matrix multiplication corresponds to composition of linear operators. If you didn't know that already, then you should try to convince yourself that it's true, in a simple class of special cases. I would recommend trying a 2 by 1 column vector with variables for entries, and two 2 by 2 matrices, also with variables for entries. So "dividing by" a matrix would correspond to "undoing" the effect of a linear operator. For instance, rotating the plane clockwise by a certain angle "undoes" the anticlockwise rotation of the plane by the same angle. However, there are plenty of linear operators that can't be undone (because they have squashed something flat), and the matrices representing these operators (with respect to any given basis) must therefore be not-dividable-by. This is unlike the situation with the real numbers, for instance, in which there is only one of them by which you can't divide, namely zero.






                                    share|cite|improve this answer

























                                      up vote
                                      0
                                      down vote













                                      Do you know why matrix multiplication is defined in such a weird way ? It is defined thus, so that the effect on a column vector of left-multiplying it by one matrix, and then left-multiplying the result by another matrix, is exactly the same as the effect of left-multiplying by a single matrix that is the product of those two matrices. That is, matrix multiplication corresponds to composition of linear operators. If you didn't know that already, then you should try to convince yourself that it's true, in a simple class of special cases. I would recommend trying a 2 by 1 column vector with variables for entries, and two 2 by 2 matrices, also with variables for entries. So "dividing by" a matrix would correspond to "undoing" the effect of a linear operator. For instance, rotating the plane clockwise by a certain angle "undoes" the anticlockwise rotation of the plane by the same angle. However, there are plenty of linear operators that can't be undone (because they have squashed something flat), and the matrices representing these operators (with respect to any given basis) must therefore be not-dividable-by. This is unlike the situation with the real numbers, for instance, in which there is only one of them by which you can't divide, namely zero.






                                      share|cite|improve this answer























                                        up vote
                                        0
                                        down vote










                                        up vote
                                        0
                                        down vote









                                        Do you know why matrix multiplication is defined in such a weird way ? It is defined thus, so that the effect on a column vector of left-multiplying it by one matrix, and then left-multiplying the result by another matrix, is exactly the same as the effect of left-multiplying by a single matrix that is the product of those two matrices. That is, matrix multiplication corresponds to composition of linear operators. If you didn't know that already, then you should try to convince yourself that it's true, in a simple class of special cases. I would recommend trying a 2 by 1 column vector with variables for entries, and two 2 by 2 matrices, also with variables for entries. So "dividing by" a matrix would correspond to "undoing" the effect of a linear operator. For instance, rotating the plane clockwise by a certain angle "undoes" the anticlockwise rotation of the plane by the same angle. However, there are plenty of linear operators that can't be undone (because they have squashed something flat), and the matrices representing these operators (with respect to any given basis) must therefore be not-dividable-by. This is unlike the situation with the real numbers, for instance, in which there is only one of them by which you can't divide, namely zero.






                                        share|cite|improve this answer












                                        Do you know why matrix multiplication is defined in such a weird way ? It is defined thus, so that the effect on a column vector of left-multiplying it by one matrix, and then left-multiplying the result by another matrix, is exactly the same as the effect of left-multiplying by a single matrix that is the product of those two matrices. That is, matrix multiplication corresponds to composition of linear operators. If you didn't know that already, then you should try to convince yourself that it's true, in a simple class of special cases. I would recommend trying a 2 by 1 column vector with variables for entries, and two 2 by 2 matrices, also with variables for entries. So "dividing by" a matrix would correspond to "undoing" the effect of a linear operator. For instance, rotating the plane clockwise by a certain angle "undoes" the anticlockwise rotation of the plane by the same angle. However, there are plenty of linear operators that can't be undone (because they have squashed something flat), and the matrices representing these operators (with respect to any given basis) must therefore be not-dividable-by. This is unlike the situation with the real numbers, for instance, in which there is only one of them by which you can't divide, namely zero.







                                        share|cite|improve this answer












                                        share|cite|improve this answer



                                        share|cite|improve this answer










                                        answered Aug 17 at 18:23









                                        Simon

                                        556411




                                        556411






















                                            up vote
                                            0
                                            down vote













                                            Element-wise division is frequently used to normalize matrices in R o MatLab. For instance, in R if dtm is a contingency matrix counting occurrences of something, you can normalize the rows so that all the rows sum 1.0 as:



                                            dtm.proportions <- dtm / rowSums(dtm)


                                            Or normalize the columns transposing the matrix first:



                                            dtm.proportions <- t(t(dtm)/colSums(dtm))





                                            share|cite|improve this answer

























                                              up vote
                                              0
                                              down vote













                                              Element-wise division is frequently used to normalize matrices in R o MatLab. For instance, in R if dtm is a contingency matrix counting occurrences of something, you can normalize the rows so that all the rows sum 1.0 as:



                                              dtm.proportions <- dtm / rowSums(dtm)


                                              Or normalize the columns transposing the matrix first:



                                              dtm.proportions <- t(t(dtm)/colSums(dtm))





                                              share|cite|improve this answer























                                                up vote
                                                0
                                                down vote










                                                up vote
                                                0
                                                down vote









                                                Element-wise division is frequently used to normalize matrices in R o MatLab. For instance, in R if dtm is a contingency matrix counting occurrences of something, you can normalize the rows so that all the rows sum 1.0 as:



                                                dtm.proportions <- dtm / rowSums(dtm)


                                                Or normalize the columns transposing the matrix first:



                                                dtm.proportions <- t(t(dtm)/colSums(dtm))





                                                share|cite|improve this answer












                                                Element-wise division is frequently used to normalize matrices in R o MatLab. For instance, in R if dtm is a contingency matrix counting occurrences of something, you can normalize the rows so that all the rows sum 1.0 as:



                                                dtm.proportions <- dtm / rowSums(dtm)


                                                Or normalize the columns transposing the matrix first:



                                                dtm.proportions <- t(t(dtm)/colSums(dtm))






                                                share|cite|improve this answer












                                                share|cite|improve this answer



                                                share|cite|improve this answer










                                                answered Dec 2 at 9:55









                                                Freeman

                                                1184




                                                1184

















                                                    protected by Community Apr 1 '14 at 11:51



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