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: $(nullspace(A)) = (rangespace(A^T))^perp$
linear-algebra complex-numbers
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up vote
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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: $(nullspace(A)) = (rangespace(A^T))^perp$
linear-algebra complex-numbers
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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â Alex
Sep 30 '15 at 23:21
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up vote
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up vote
2
down vote
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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: $(nullspace(A)) = (rangespace(A^T))^perp$
linear-algebra complex-numbers
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: $(nullspace(A)) = (rangespace(A^T))^perp$
linear-algebra complex-numbers
asked Sep 30 '15 at 22:19
Alex
10410
10410
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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â Alex
Sep 30 '15 at 23:21
add a comment |Â
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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â 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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â 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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â Alex
Sep 30 '15 at 23:21
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1 Answer
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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: âÂÂThe 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 $overlinemathbfA$.âÂÂ
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
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: âÂÂThe 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 $overlinemathbfA$.âÂÂ
add a comment |Â
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: âÂÂThe 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 $overlinemathbfA$.âÂÂ
add a comment |Â
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: âÂÂThe 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 $overlinemathbfA$.âÂÂ
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: âÂÂThe 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 $overlinemathbfA$.âÂÂ
answered Aug 23 at 6:24
Marc Nye
564
564
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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 $beginbmatrixi\1endbmatrix$ = $beginbmatrix2\0endbmatrix$.
â Alex
Sep 30 '15 at 23:21