Fundamental Theorem of Linear Algebra over Complex Field

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How is the fundamental theorem of linear algebra stated when the inducing matrix has elements from the complex field? For example, does the usual transpose become a Hermitian transpose in a statement like this: $(nullspace(A)) = (rangespace(A^T))^perp$







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  • If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
    – Alex
    Sep 30 '15 at 23:21















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How is the fundamental theorem of linear algebra stated when the inducing matrix has elements from the complex field? For example, does the usual transpose become a Hermitian transpose in a statement like this: $(nullspace(A)) = (rangespace(A^T))^perp$







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  • If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
    – Alex
    Sep 30 '15 at 23:21













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How is the fundamental theorem of linear algebra stated when the inducing matrix has elements from the complex field? For example, does the usual transpose become a Hermitian transpose in a statement like this: $(nullspace(A)) = (rangespace(A^T))^perp$







share|cite|improve this question












How is the fundamental theorem of linear algebra stated when the inducing matrix has elements from the complex field? For example, does the usual transpose become a Hermitian transpose in a statement like this: $(nullspace(A)) = (rangespace(A^T))^perp$









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asked Sep 30 '15 at 22:19









Alex

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  • If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
    – Alex
    Sep 30 '15 at 23:21

















  • If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
    – Alex
    Sep 30 '15 at 23:21
















If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
– Alex
Sep 30 '15 at 23:21





If A=$beginbmatrixi&1\0&0endbmatrix$ and B = $beginbmatrix-i&1\0&0endbmatrix$the conjugate of A, then A and B have a different null-space because A$beginbmatrixi\1endbmatrix$ =$beginbmatrix0\0endbmatrix$, but B $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
– Alex
Sep 30 '15 at 23:21











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Essentially, yes.



Let's say $mathbfA$ is $ntimes k$. To avoid ambiguity, $mathbfA^top$ will denote the ordinary transpose of $mathbfA$, that is, the matrix obtained by simply swapping the entries of $mathbfA$ across its main diagonal. $overlinemathbfA$ will denote the complex conjugate of $mathbfA$, that is, the matrix obtained by taking the complex conjugate of the entries of $mathbfA$. Finally, $mathbfA^*$ will denote the conjugate transpose of $mathbfA$, that is,
$$mathbfA^*=overlinemathbfA^topmbox.$$
By definition, the row space of a matrix is the span of its rows, while the column space is the span of its columns. So the row space of $mathbfA$ is always the same as the column space of $mathbfA^top$. It doesn't matter if $mathbfA$ has complex entries.



If you want to talk about the orthogonal complement of the row space of $mathbfA$, or equivalently, the orthogonal complement of the column space of $mathbfA^top$, then it does matter if $mathbfA$ has complex entries. Let's suppose it does. So the rows of $mathbfA$ lie in $mathbbC^k$, which uses the Euclidean inner product
$$left<mathbfv,mathbfwright>=mathbfv^topoverlinemathbfwmbox.$$



where $mathbfv$ and $mathbfw$ are $ktimes1$ column matrices. It doesn't matter too much if you use row vectors or column vectors. What's important is that the complex conjugate of $mathbfw$ is taken prior to the dot product. This ensures that $left<cdot,cdotright>$ satisfies all the defining properties of an inner product.



With this inner product in mind, a column vector $mathbfwinmathbbC^k$ is orthogonal to each of the rows of $mathbfA$ if and only if
$$mathbfAoverlinemathbfw=mathbf0$$
if and only if
$$overlinemathbfAmathbfw=mathbf0mbox,$$
from which we see that $mathbfw$ is in the null space of $overlinemathbfA$ (as opposed to the null space of $mathbfA$). So
$$mboxnulloverlinemathbfA=left(mboxrangemathbfA^topright)^perpmbox.$$
Replacing $mathbfA$ with $overlinemathbfA$ in the last equation shows
$$mboxnullmathbfA=left(mboxrangemathbfA^*right)^perp$$
as you suspected. Similarly, we get
$$mboxnullmathbfA^top=left(mboxrangeoverlinemathbfAright)^perpmbox.$$
Thus, if the FTLA were stated for matrices in $mathbbC^ntimes k$, then it would say something along the lines of: “The null space of $mathbfA$ is the orthogonal complement of the row space of $overlinemathbfA$, while the left null space of $mathbfA$ is the orthogonal complement of the column space of $overlinemathbfA$.”






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    1 Answer
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    active

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    1 Answer
    1






    active

    oldest

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    active

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    active

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    up vote
    1
    down vote



    accepted










    Essentially, yes.



    Let's say $mathbfA$ is $ntimes k$. To avoid ambiguity, $mathbfA^top$ will denote the ordinary transpose of $mathbfA$, that is, the matrix obtained by simply swapping the entries of $mathbfA$ across its main diagonal. $overlinemathbfA$ will denote the complex conjugate of $mathbfA$, that is, the matrix obtained by taking the complex conjugate of the entries of $mathbfA$. Finally, $mathbfA^*$ will denote the conjugate transpose of $mathbfA$, that is,
    $$mathbfA^*=overlinemathbfA^topmbox.$$
    By definition, the row space of a matrix is the span of its rows, while the column space is the span of its columns. So the row space of $mathbfA$ is always the same as the column space of $mathbfA^top$. It doesn't matter if $mathbfA$ has complex entries.



    If you want to talk about the orthogonal complement of the row space of $mathbfA$, or equivalently, the orthogonal complement of the column space of $mathbfA^top$, then it does matter if $mathbfA$ has complex entries. Let's suppose it does. So the rows of $mathbfA$ lie in $mathbbC^k$, which uses the Euclidean inner product
    $$left<mathbfv,mathbfwright>=mathbfv^topoverlinemathbfwmbox.$$



    where $mathbfv$ and $mathbfw$ are $ktimes1$ column matrices. It doesn't matter too much if you use row vectors or column vectors. What's important is that the complex conjugate of $mathbfw$ is taken prior to the dot product. This ensures that $left<cdot,cdotright>$ satisfies all the defining properties of an inner product.



    With this inner product in mind, a column vector $mathbfwinmathbbC^k$ is orthogonal to each of the rows of $mathbfA$ if and only if
    $$mathbfAoverlinemathbfw=mathbf0$$
    if and only if
    $$overlinemathbfAmathbfw=mathbf0mbox,$$
    from which we see that $mathbfw$ is in the null space of $overlinemathbfA$ (as opposed to the null space of $mathbfA$). So
    $$mboxnulloverlinemathbfA=left(mboxrangemathbfA^topright)^perpmbox.$$
    Replacing $mathbfA$ with $overlinemathbfA$ in the last equation shows
    $$mboxnullmathbfA=left(mboxrangemathbfA^*right)^perp$$
    as you suspected. Similarly, we get
    $$mboxnullmathbfA^top=left(mboxrangeoverlinemathbfAright)^perpmbox.$$
    Thus, if the FTLA were stated for matrices in $mathbbC^ntimes k$, then it would say something along the lines of: “The null space of $mathbfA$ is the orthogonal complement of the row space of $overlinemathbfA$, while the left null space of $mathbfA$ is the orthogonal complement of the column space of $overlinemathbfA$.”






    share|cite|improve this answer
























      up vote
      1
      down vote



      accepted










      Essentially, yes.



      Let's say $mathbfA$ is $ntimes k$. To avoid ambiguity, $mathbfA^top$ will denote the ordinary transpose of $mathbfA$, that is, the matrix obtained by simply swapping the entries of $mathbfA$ across its main diagonal. $overlinemathbfA$ will denote the complex conjugate of $mathbfA$, that is, the matrix obtained by taking the complex conjugate of the entries of $mathbfA$. Finally, $mathbfA^*$ will denote the conjugate transpose of $mathbfA$, that is,
      $$mathbfA^*=overlinemathbfA^topmbox.$$
      By definition, the row space of a matrix is the span of its rows, while the column space is the span of its columns. So the row space of $mathbfA$ is always the same as the column space of $mathbfA^top$. It doesn't matter if $mathbfA$ has complex entries.



      If you want to talk about the orthogonal complement of the row space of $mathbfA$, or equivalently, the orthogonal complement of the column space of $mathbfA^top$, then it does matter if $mathbfA$ has complex entries. Let's suppose it does. So the rows of $mathbfA$ lie in $mathbbC^k$, which uses the Euclidean inner product
      $$left<mathbfv,mathbfwright>=mathbfv^topoverlinemathbfwmbox.$$



      where $mathbfv$ and $mathbfw$ are $ktimes1$ column matrices. It doesn't matter too much if you use row vectors or column vectors. What's important is that the complex conjugate of $mathbfw$ is taken prior to the dot product. This ensures that $left<cdot,cdotright>$ satisfies all the defining properties of an inner product.



      With this inner product in mind, a column vector $mathbfwinmathbbC^k$ is orthogonal to each of the rows of $mathbfA$ if and only if
      $$mathbfAoverlinemathbfw=mathbf0$$
      if and only if
      $$overlinemathbfAmathbfw=mathbf0mbox,$$
      from which we see that $mathbfw$ is in the null space of $overlinemathbfA$ (as opposed to the null space of $mathbfA$). So
      $$mboxnulloverlinemathbfA=left(mboxrangemathbfA^topright)^perpmbox.$$
      Replacing $mathbfA$ with $overlinemathbfA$ in the last equation shows
      $$mboxnullmathbfA=left(mboxrangemathbfA^*right)^perp$$
      as you suspected. Similarly, we get
      $$mboxnullmathbfA^top=left(mboxrangeoverlinemathbfAright)^perpmbox.$$
      Thus, if the FTLA were stated for matrices in $mathbbC^ntimes k$, then it would say something along the lines of: “The null space of $mathbfA$ is the orthogonal complement of the row space of $overlinemathbfA$, while the left null space of $mathbfA$ is the orthogonal complement of the column space of $overlinemathbfA$.”






      share|cite|improve this answer






















        up vote
        1
        down vote



        accepted







        up vote
        1
        down vote



        accepted






        Essentially, yes.



        Let's say $mathbfA$ is $ntimes k$. To avoid ambiguity, $mathbfA^top$ will denote the ordinary transpose of $mathbfA$, that is, the matrix obtained by simply swapping the entries of $mathbfA$ across its main diagonal. $overlinemathbfA$ will denote the complex conjugate of $mathbfA$, that is, the matrix obtained by taking the complex conjugate of the entries of $mathbfA$. Finally, $mathbfA^*$ will denote the conjugate transpose of $mathbfA$, that is,
        $$mathbfA^*=overlinemathbfA^topmbox.$$
        By definition, the row space of a matrix is the span of its rows, while the column space is the span of its columns. So the row space of $mathbfA$ is always the same as the column space of $mathbfA^top$. It doesn't matter if $mathbfA$ has complex entries.



        If you want to talk about the orthogonal complement of the row space of $mathbfA$, or equivalently, the orthogonal complement of the column space of $mathbfA^top$, then it does matter if $mathbfA$ has complex entries. Let's suppose it does. So the rows of $mathbfA$ lie in $mathbbC^k$, which uses the Euclidean inner product
        $$left<mathbfv,mathbfwright>=mathbfv^topoverlinemathbfwmbox.$$



        where $mathbfv$ and $mathbfw$ are $ktimes1$ column matrices. It doesn't matter too much if you use row vectors or column vectors. What's important is that the complex conjugate of $mathbfw$ is taken prior to the dot product. This ensures that $left<cdot,cdotright>$ satisfies all the defining properties of an inner product.



        With this inner product in mind, a column vector $mathbfwinmathbbC^k$ is orthogonal to each of the rows of $mathbfA$ if and only if
        $$mathbfAoverlinemathbfw=mathbf0$$
        if and only if
        $$overlinemathbfAmathbfw=mathbf0mbox,$$
        from which we see that $mathbfw$ is in the null space of $overlinemathbfA$ (as opposed to the null space of $mathbfA$). So
        $$mboxnulloverlinemathbfA=left(mboxrangemathbfA^topright)^perpmbox.$$
        Replacing $mathbfA$ with $overlinemathbfA$ in the last equation shows
        $$mboxnullmathbfA=left(mboxrangemathbfA^*right)^perp$$
        as you suspected. Similarly, we get
        $$mboxnullmathbfA^top=left(mboxrangeoverlinemathbfAright)^perpmbox.$$
        Thus, if the FTLA were stated for matrices in $mathbbC^ntimes k$, then it would say something along the lines of: “The null space of $mathbfA$ is the orthogonal complement of the row space of $overlinemathbfA$, while the left null space of $mathbfA$ is the orthogonal complement of the column space of $overlinemathbfA$.”






        share|cite|improve this answer












        Essentially, yes.



        Let's say $mathbfA$ is $ntimes k$. To avoid ambiguity, $mathbfA^top$ will denote the ordinary transpose of $mathbfA$, that is, the matrix obtained by simply swapping the entries of $mathbfA$ across its main diagonal. $overlinemathbfA$ will denote the complex conjugate of $mathbfA$, that is, the matrix obtained by taking the complex conjugate of the entries of $mathbfA$. Finally, $mathbfA^*$ will denote the conjugate transpose of $mathbfA$, that is,
        $$mathbfA^*=overlinemathbfA^topmbox.$$
        By definition, the row space of a matrix is the span of its rows, while the column space is the span of its columns. So the row space of $mathbfA$ is always the same as the column space of $mathbfA^top$. It doesn't matter if $mathbfA$ has complex entries.



        If you want to talk about the orthogonal complement of the row space of $mathbfA$, or equivalently, the orthogonal complement of the column space of $mathbfA^top$, then it does matter if $mathbfA$ has complex entries. Let's suppose it does. So the rows of $mathbfA$ lie in $mathbbC^k$, which uses the Euclidean inner product
        $$left<mathbfv,mathbfwright>=mathbfv^topoverlinemathbfwmbox.$$



        where $mathbfv$ and $mathbfw$ are $ktimes1$ column matrices. It doesn't matter too much if you use row vectors or column vectors. What's important is that the complex conjugate of $mathbfw$ is taken prior to the dot product. This ensures that $left<cdot,cdotright>$ satisfies all the defining properties of an inner product.



        With this inner product in mind, a column vector $mathbfwinmathbbC^k$ is orthogonal to each of the rows of $mathbfA$ if and only if
        $$mathbfAoverlinemathbfw=mathbf0$$
        if and only if
        $$overlinemathbfAmathbfw=mathbf0mbox,$$
        from which we see that $mathbfw$ is in the null space of $overlinemathbfA$ (as opposed to the null space of $mathbfA$). So
        $$mboxnulloverlinemathbfA=left(mboxrangemathbfA^topright)^perpmbox.$$
        Replacing $mathbfA$ with $overlinemathbfA$ in the last equation shows
        $$mboxnullmathbfA=left(mboxrangemathbfA^*right)^perp$$
        as you suspected. Similarly, we get
        $$mboxnullmathbfA^top=left(mboxrangeoverlinemathbfAright)^perpmbox.$$
        Thus, if the FTLA were stated for matrices in $mathbbC^ntimes k$, then it would say something along the lines of: “The null space of $mathbfA$ is the orthogonal complement of the row space of $overlinemathbfA$, while the left null space of $mathbfA$ is the orthogonal complement of the column space of $overlinemathbfA$.”







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        answered Aug 23 at 6:24









        Marc Nye

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