Does there exist a $1-1$ ring homomorphism from $M_d(mathbb{F})$ to $M_n(mathbb{F})$ for $d <n$?












4












$begingroup$


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 $leq n.$











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$endgroup$












  • $begingroup$
    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
















4












$begingroup$


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 $leq n.$











share|cite|improve this question











$endgroup$












  • $begingroup$
    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














4












4








4


1



$begingroup$


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 $leq n.$











share|cite|improve this question











$endgroup$




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






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share|cite|improve this question








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


















  • $begingroup$
    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










1 Answer
1






active

oldest

votes


















7












$begingroup$

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.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    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














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1 Answer
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1 Answer
1






active

oldest

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active

oldest

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active

oldest

votes









7












$begingroup$

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.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    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


















7












$begingroup$

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.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    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
















7












7








7





$begingroup$

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.






share|cite|improve this answer









$endgroup$



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.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










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




















  • $begingroup$
    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




















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