Does there exist a $1-1$ ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ for $d <n$?
$begingroup$
Let $mathbb{F}$ be a field and $d,n$ be positive integers with $d <n.$ Then does there exist a injective ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ ? (BTW A ring map sends $1$ to $1$)
I failed to produce a $1-1$ ring map. This appears in the process of solving a field theory exercise from Dummit and Foote. For your ref. it is Problem number $19.(b)$ in sec. $13$ (Second Edition). Any help will be appreciated. Thanks.
Edited Later:Prob. $19.(b).,$ Section $13.2,$ from Abstract Algebra by Dummit and Foote(Second Edition) is stated below.
Let $K$ be an extension of $F$ of degree $n.$ Prove that $K$ is isomorphic to a subfield of the ring $M_n(F),$ so $M_n(F)$ contains an isomorphic copy of every extension of $F$ of degree $leq n.$
linear-algebra abstract-algebra matrices ring-theory field-theory
$endgroup$
add a comment |
$begingroup$
Let $mathbb{F}$ be a field and $d,n$ be positive integers with $d <n.$ Then does there exist a injective ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ ? (BTW A ring map sends $1$ to $1$)
I failed to produce a $1-1$ ring map. This appears in the process of solving a field theory exercise from Dummit and Foote. For your ref. it is Problem number $19.(b)$ in sec. $13$ (Second Edition). Any help will be appreciated. Thanks.
Edited Later:Prob. $19.(b).,$ Section $13.2,$ from Abstract Algebra by Dummit and Foote(Second Edition) is stated below.
Let $K$ be an extension of $F$ of degree $n.$ Prove that $K$ is isomorphic to a subfield of the ring $M_n(F),$ so $M_n(F)$ contains an isomorphic copy of every extension of $F$ of degree $leq n.$
linear-algebra abstract-algebra matrices ring-theory field-theory
$endgroup$
$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
1
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15
add a comment |
$begingroup$
Let $mathbb{F}$ be a field and $d,n$ be positive integers with $d <n.$ Then does there exist a injective ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ ? (BTW A ring map sends $1$ to $1$)
I failed to produce a $1-1$ ring map. This appears in the process of solving a field theory exercise from Dummit and Foote. For your ref. it is Problem number $19.(b)$ in sec. $13$ (Second Edition). Any help will be appreciated. Thanks.
Edited Later:Prob. $19.(b).,$ Section $13.2,$ from Abstract Algebra by Dummit and Foote(Second Edition) is stated below.
Let $K$ be an extension of $F$ of degree $n.$ Prove that $K$ is isomorphic to a subfield of the ring $M_n(F),$ so $M_n(F)$ contains an isomorphic copy of every extension of $F$ of degree $leq n.$
linear-algebra abstract-algebra matrices ring-theory field-theory
$endgroup$
Let $mathbb{F}$ be a field and $d,n$ be positive integers with $d <n.$ Then does there exist a injective ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ ? (BTW A ring map sends $1$ to $1$)
I failed to produce a $1-1$ ring map. This appears in the process of solving a field theory exercise from Dummit and Foote. For your ref. it is Problem number $19.(b)$ in sec. $13$ (Second Edition). Any help will be appreciated. Thanks.
Edited Later:Prob. $19.(b).,$ Section $13.2,$ from Abstract Algebra by Dummit and Foote(Second Edition) is stated below.
Let $K$ be an extension of $F$ of degree $n.$ Prove that $K$ is isomorphic to a subfield of the ring $M_n(F),$ so $M_n(F)$ contains an isomorphic copy of every extension of $F$ of degree $leq n.$
linear-algebra abstract-algebra matrices ring-theory field-theory
linear-algebra abstract-algebra matrices ring-theory field-theory
edited Dec 29 '18 at 9:22
user371231
asked Dec 29 '18 at 7:58
user371231user371231
417511
417511
$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
1
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15
add a comment |
$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
1
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15
$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
1
1
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
As Slade notes in a comment, this is true when $dmid n$, and the homomorphism from $M_d(mathbb F)$ to $M_n(mathbb F)$ can be defined by sending a $dtimes d$ matrix $A$ to a block diagonal matrix with $n/d$ copies of $A$.
However, it is not true for general $d$ and $n$. We can see this, for example, with $mathbb F=mathbb C$, $d=2$, $n=3$.
If we have such an injection, let $A$ be the image of
$$begin{pmatrix}0&1\0&0end{pmatrix}$$
and $B$ be the image of
$$begin{pmatrix}0&0\1&0end{pmatrix}.$$
Then $A^2=0$, $B^2=0$, and $(A+B)^2=I$.
We can see (e.g. using the Jordan form) that the only $3times 3$ matrix $A$ satisfying $Ane0$ but $A^2=0$ is, up to similarity,
$$begin{pmatrix}0&1&0\0&0&0\0&0&0end{pmatrix},$$
so $A$ must have rank $1$. Similarly $B$ also has rank $1$, so their sum $A+B$ has rank at most $2$. But this contradicts $(A+B)^2=I$, which would require $A+B$ to be invertible and so have full rank.
This proof uses a bit of linear algebra and doesn't generalize to show that the statement is false for all $dnotmid n$, although I expect that it is so.
$endgroup$
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
add a comment |
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$begingroup$
As Slade notes in a comment, this is true when $dmid n$, and the homomorphism from $M_d(mathbb F)$ to $M_n(mathbb F)$ can be defined by sending a $dtimes d$ matrix $A$ to a block diagonal matrix with $n/d$ copies of $A$.
However, it is not true for general $d$ and $n$. We can see this, for example, with $mathbb F=mathbb C$, $d=2$, $n=3$.
If we have such an injection, let $A$ be the image of
$$begin{pmatrix}0&1\0&0end{pmatrix}$$
and $B$ be the image of
$$begin{pmatrix}0&0\1&0end{pmatrix}.$$
Then $A^2=0$, $B^2=0$, and $(A+B)^2=I$.
We can see (e.g. using the Jordan form) that the only $3times 3$ matrix $A$ satisfying $Ane0$ but $A^2=0$ is, up to similarity,
$$begin{pmatrix}0&1&0\0&0&0\0&0&0end{pmatrix},$$
so $A$ must have rank $1$. Similarly $B$ also has rank $1$, so their sum $A+B$ has rank at most $2$. But this contradicts $(A+B)^2=I$, which would require $A+B$ to be invertible and so have full rank.
This proof uses a bit of linear algebra and doesn't generalize to show that the statement is false for all $dnotmid n$, although I expect that it is so.
$endgroup$
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
add a comment |
$begingroup$
As Slade notes in a comment, this is true when $dmid n$, and the homomorphism from $M_d(mathbb F)$ to $M_n(mathbb F)$ can be defined by sending a $dtimes d$ matrix $A$ to a block diagonal matrix with $n/d$ copies of $A$.
However, it is not true for general $d$ and $n$. We can see this, for example, with $mathbb F=mathbb C$, $d=2$, $n=3$.
If we have such an injection, let $A$ be the image of
$$begin{pmatrix}0&1\0&0end{pmatrix}$$
and $B$ be the image of
$$begin{pmatrix}0&0\1&0end{pmatrix}.$$
Then $A^2=0$, $B^2=0$, and $(A+B)^2=I$.
We can see (e.g. using the Jordan form) that the only $3times 3$ matrix $A$ satisfying $Ane0$ but $A^2=0$ is, up to similarity,
$$begin{pmatrix}0&1&0\0&0&0\0&0&0end{pmatrix},$$
so $A$ must have rank $1$. Similarly $B$ also has rank $1$, so their sum $A+B$ has rank at most $2$. But this contradicts $(A+B)^2=I$, which would require $A+B$ to be invertible and so have full rank.
This proof uses a bit of linear algebra and doesn't generalize to show that the statement is false for all $dnotmid n$, although I expect that it is so.
$endgroup$
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
add a comment |
$begingroup$
As Slade notes in a comment, this is true when $dmid n$, and the homomorphism from $M_d(mathbb F)$ to $M_n(mathbb F)$ can be defined by sending a $dtimes d$ matrix $A$ to a block diagonal matrix with $n/d$ copies of $A$.
However, it is not true for general $d$ and $n$. We can see this, for example, with $mathbb F=mathbb C$, $d=2$, $n=3$.
If we have such an injection, let $A$ be the image of
$$begin{pmatrix}0&1\0&0end{pmatrix}$$
and $B$ be the image of
$$begin{pmatrix}0&0\1&0end{pmatrix}.$$
Then $A^2=0$, $B^2=0$, and $(A+B)^2=I$.
We can see (e.g. using the Jordan form) that the only $3times 3$ matrix $A$ satisfying $Ane0$ but $A^2=0$ is, up to similarity,
$$begin{pmatrix}0&1&0\0&0&0\0&0&0end{pmatrix},$$
so $A$ must have rank $1$. Similarly $B$ also has rank $1$, so their sum $A+B$ has rank at most $2$. But this contradicts $(A+B)^2=I$, which would require $A+B$ to be invertible and so have full rank.
This proof uses a bit of linear algebra and doesn't generalize to show that the statement is false for all $dnotmid n$, although I expect that it is so.
$endgroup$
As Slade notes in a comment, this is true when $dmid n$, and the homomorphism from $M_d(mathbb F)$ to $M_n(mathbb F)$ can be defined by sending a $dtimes d$ matrix $A$ to a block diagonal matrix with $n/d$ copies of $A$.
However, it is not true for general $d$ and $n$. We can see this, for example, with $mathbb F=mathbb C$, $d=2$, $n=3$.
If we have such an injection, let $A$ be the image of
$$begin{pmatrix}0&1\0&0end{pmatrix}$$
and $B$ be the image of
$$begin{pmatrix}0&0\1&0end{pmatrix}.$$
Then $A^2=0$, $B^2=0$, and $(A+B)^2=I$.
We can see (e.g. using the Jordan form) that the only $3times 3$ matrix $A$ satisfying $Ane0$ but $A^2=0$ is, up to similarity,
$$begin{pmatrix}0&1&0\0&0&0\0&0&0end{pmatrix},$$
so $A$ must have rank $1$. Similarly $B$ also has rank $1$, so their sum $A+B$ has rank at most $2$. But this contradicts $(A+B)^2=I$, which would require $A+B$ to be invertible and so have full rank.
This proof uses a bit of linear algebra and doesn't generalize to show that the statement is false for all $dnotmid n$, although I expect that it is so.
answered Dec 29 '18 at 8:57
CarmeisterCarmeister
2,8692924
2,8692924
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
add a comment |
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
Nice answer, many thanks. And I guess the Dummit Foote problem can be solved without using this.
$endgroup$
– user371231
Dec 29 '18 at 9:20
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
$begingroup$
If extension of $F$ means that intermediate subfield of $F$ and $K$ then this answers the question.
$endgroup$
– user371231
Dec 29 '18 at 9:22
1
1
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Yes, I'm guessing the problem comes in with the last statement, "...contains an isomorphic copy of every extension of $F$ of degree $le n$" specifically the $<n$ part. I believe that may be an error in the exercise, and that actually it's only true for the $=n$ case, but I'm not sure - it's worth asking a separate question about that.
$endgroup$
– Carmeister
Dec 29 '18 at 9:32
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
$begingroup$
Ok then I'll ask a separate question, thanks for the nice counter example.
$endgroup$
– user371231
Dec 29 '18 at 9:38
add a comment |
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$begingroup$
Could you say what the Dummit and Foote exercise is?
$endgroup$
– Slade
Dec 29 '18 at 8:01
1
$begingroup$
And such a map does exist when $dmid n$: send $A$ to $n/d$ copies of $A$ in block diagonal form.
$endgroup$
– Slade
Dec 29 '18 at 8:04
$begingroup$
Yes, I stated the Problem from the abstract Algebra book by Dummit and Foote.
$endgroup$
– user371231
Dec 29 '18 at 9:15