Investigate whether the given transformation is a monomorphism / epimorphism. Find image and kernel
$begingroup$
I have serious doubts - I will be very grateful if someone will help me here
Investigate whether the given transformation is a monomorphism / epimorphism. Find its image and kernel.
$$ F in L(mathbb R[x]_3,mathbb R[x]_3), F(p)(t) = p(t+1) - p(t) $$
My try
Okay, let
$$ p(t) = ax^3 + bx^2 + cx + d $$
then
$$F(p)(t) = ... = 3ax^2 + 3ax + 2bx + 3 = x^2(3a)+x(3a+2b) +3 = a(3x^2+3x) + b(2x) + 3 $$
Is it monomorphism?
let $m_0 = 0 wedge m_1 = 3a wedge m_2 = 3a+b wedge m_3 = 3$
so $a = frac{m_1}{3} wedge b = frac{m_2-m_1}{2} $
so $a$ and $b$ are determined unambiguously so it is monomorphism
It is not epimorphism because $ x^3 inmathbb R[x]_3 $ but I can't get $x^3$ in use of $F$
$ker F = left{ 0 right} $ because of part $ 3 $ it will be never polynomial zero
If it comes to image, we checked that:
$$F(p)(t) = ... = a(3x^2+3x) + b(2x) + 3 $$
so $ im(F) = span(3x^2+3,2x,3) $
Moreover this system is lineary independent so $rank(F) = 3$
Have I done this correctly? Or there is something to fix?
linear-algebra monomorphisms
$endgroup$
add a comment |
$begingroup$
I have serious doubts - I will be very grateful if someone will help me here
Investigate whether the given transformation is a monomorphism / epimorphism. Find its image and kernel.
$$ F in L(mathbb R[x]_3,mathbb R[x]_3), F(p)(t) = p(t+1) - p(t) $$
My try
Okay, let
$$ p(t) = ax^3 + bx^2 + cx + d $$
then
$$F(p)(t) = ... = 3ax^2 + 3ax + 2bx + 3 = x^2(3a)+x(3a+2b) +3 = a(3x^2+3x) + b(2x) + 3 $$
Is it monomorphism?
let $m_0 = 0 wedge m_1 = 3a wedge m_2 = 3a+b wedge m_3 = 3$
so $a = frac{m_1}{3} wedge b = frac{m_2-m_1}{2} $
so $a$ and $b$ are determined unambiguously so it is monomorphism
It is not epimorphism because $ x^3 inmathbb R[x]_3 $ but I can't get $x^3$ in use of $F$
$ker F = left{ 0 right} $ because of part $ 3 $ it will be never polynomial zero
If it comes to image, we checked that:
$$F(p)(t) = ... = a(3x^2+3x) + b(2x) + 3 $$
so $ im(F) = span(3x^2+3,2x,3) $
Moreover this system is lineary independent so $rank(F) = 3$
Have I done this correctly? Or there is something to fix?
linear-algebra monomorphisms
$endgroup$
add a comment |
$begingroup$
I have serious doubts - I will be very grateful if someone will help me here
Investigate whether the given transformation is a monomorphism / epimorphism. Find its image and kernel.
$$ F in L(mathbb R[x]_3,mathbb R[x]_3), F(p)(t) = p(t+1) - p(t) $$
My try
Okay, let
$$ p(t) = ax^3 + bx^2 + cx + d $$
then
$$F(p)(t) = ... = 3ax^2 + 3ax + 2bx + 3 = x^2(3a)+x(3a+2b) +3 = a(3x^2+3x) + b(2x) + 3 $$
Is it monomorphism?
let $m_0 = 0 wedge m_1 = 3a wedge m_2 = 3a+b wedge m_3 = 3$
so $a = frac{m_1}{3} wedge b = frac{m_2-m_1}{2} $
so $a$ and $b$ are determined unambiguously so it is monomorphism
It is not epimorphism because $ x^3 inmathbb R[x]_3 $ but I can't get $x^3$ in use of $F$
$ker F = left{ 0 right} $ because of part $ 3 $ it will be never polynomial zero
If it comes to image, we checked that:
$$F(p)(t) = ... = a(3x^2+3x) + b(2x) + 3 $$
so $ im(F) = span(3x^2+3,2x,3) $
Moreover this system is lineary independent so $rank(F) = 3$
Have I done this correctly? Or there is something to fix?
linear-algebra monomorphisms
$endgroup$
I have serious doubts - I will be very grateful if someone will help me here
Investigate whether the given transformation is a monomorphism / epimorphism. Find its image and kernel.
$$ F in L(mathbb R[x]_3,mathbb R[x]_3), F(p)(t) = p(t+1) - p(t) $$
My try
Okay, let
$$ p(t) = ax^3 + bx^2 + cx + d $$
then
$$F(p)(t) = ... = 3ax^2 + 3ax + 2bx + 3 = x^2(3a)+x(3a+2b) +3 = a(3x^2+3x) + b(2x) + 3 $$
Is it monomorphism?
let $m_0 = 0 wedge m_1 = 3a wedge m_2 = 3a+b wedge m_3 = 3$
so $a = frac{m_1}{3} wedge b = frac{m_2-m_1}{2} $
so $a$ and $b$ are determined unambiguously so it is monomorphism
It is not epimorphism because $ x^3 inmathbb R[x]_3 $ but I can't get $x^3$ in use of $F$
$ker F = left{ 0 right} $ because of part $ 3 $ it will be never polynomial zero
If it comes to image, we checked that:
$$F(p)(t) = ... = a(3x^2+3x) + b(2x) + 3 $$
so $ im(F) = span(3x^2+3,2x,3) $
Moreover this system is lineary independent so $rank(F) = 3$
Have I done this correctly? Or there is something to fix?
linear-algebra monomorphisms
linear-algebra monomorphisms
asked Jan 4 at 12:42
VirtualUserVirtualUser
1,096116
1,096116
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
You have a contradiction in your solution: if the map is injective it must also be surjective, because it is a linear map of a finite dimensional vector space to itself.
It's much easier with matrices. If you consider the standard basis ${p_0=1,p_1=t,p_2=t^2,p_3=t^3}$, then you see that
$$
F(p_0)=0,quad
F(p_1)=1,quad
F(p_2)=2t+1quad
F(p_3)=3t^2+3t+1
$$
and therefore the matrix of $F$ is
begin{bmatrix}
0 & 1 & 1 & 1 \
0 & 0 & 2 & 3 \
0 & 0 & 0 & 3 \
0 & 0 & 0 & 0
end{bmatrix}
What's the rank of this matrix?
Where did you go wrong? The image of $p(t)=at^3+bt^2+ct+d$ is
begin{align}
p(t+1)-p(t)
&=at^3+3at^2+3at+a+bt^2+2bt+b+ct+d-at^3-bt^2-ct-d\
&=3at^2+(3a+2b)t+(a+b+c)
end{align}
which is zero when
begin{cases}
3a=0\
3a+2b=0\
a+b+c=0
end{cases}
so when $a=b=c=0$. But $d$ can be anything. Thus the nullity is $1$ and the rank is $3$.
$endgroup$
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3061619%2finvestigate-whether-the-given-transformation-is-a-monomorphism-epimorphism-fi%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
You have a contradiction in your solution: if the map is injective it must also be surjective, because it is a linear map of a finite dimensional vector space to itself.
It's much easier with matrices. If you consider the standard basis ${p_0=1,p_1=t,p_2=t^2,p_3=t^3}$, then you see that
$$
F(p_0)=0,quad
F(p_1)=1,quad
F(p_2)=2t+1quad
F(p_3)=3t^2+3t+1
$$
and therefore the matrix of $F$ is
begin{bmatrix}
0 & 1 & 1 & 1 \
0 & 0 & 2 & 3 \
0 & 0 & 0 & 3 \
0 & 0 & 0 & 0
end{bmatrix}
What's the rank of this matrix?
Where did you go wrong? The image of $p(t)=at^3+bt^2+ct+d$ is
begin{align}
p(t+1)-p(t)
&=at^3+3at^2+3at+a+bt^2+2bt+b+ct+d-at^3-bt^2-ct-d\
&=3at^2+(3a+2b)t+(a+b+c)
end{align}
which is zero when
begin{cases}
3a=0\
3a+2b=0\
a+b+c=0
end{cases}
so when $a=b=c=0$. But $d$ can be anything. Thus the nullity is $1$ and the rank is $3$.
$endgroup$
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
add a comment |
$begingroup$
You have a contradiction in your solution: if the map is injective it must also be surjective, because it is a linear map of a finite dimensional vector space to itself.
It's much easier with matrices. If you consider the standard basis ${p_0=1,p_1=t,p_2=t^2,p_3=t^3}$, then you see that
$$
F(p_0)=0,quad
F(p_1)=1,quad
F(p_2)=2t+1quad
F(p_3)=3t^2+3t+1
$$
and therefore the matrix of $F$ is
begin{bmatrix}
0 & 1 & 1 & 1 \
0 & 0 & 2 & 3 \
0 & 0 & 0 & 3 \
0 & 0 & 0 & 0
end{bmatrix}
What's the rank of this matrix?
Where did you go wrong? The image of $p(t)=at^3+bt^2+ct+d$ is
begin{align}
p(t+1)-p(t)
&=at^3+3at^2+3at+a+bt^2+2bt+b+ct+d-at^3-bt^2-ct-d\
&=3at^2+(3a+2b)t+(a+b+c)
end{align}
which is zero when
begin{cases}
3a=0\
3a+2b=0\
a+b+c=0
end{cases}
so when $a=b=c=0$. But $d$ can be anything. Thus the nullity is $1$ and the rank is $3$.
$endgroup$
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
add a comment |
$begingroup$
You have a contradiction in your solution: if the map is injective it must also be surjective, because it is a linear map of a finite dimensional vector space to itself.
It's much easier with matrices. If you consider the standard basis ${p_0=1,p_1=t,p_2=t^2,p_3=t^3}$, then you see that
$$
F(p_0)=0,quad
F(p_1)=1,quad
F(p_2)=2t+1quad
F(p_3)=3t^2+3t+1
$$
and therefore the matrix of $F$ is
begin{bmatrix}
0 & 1 & 1 & 1 \
0 & 0 & 2 & 3 \
0 & 0 & 0 & 3 \
0 & 0 & 0 & 0
end{bmatrix}
What's the rank of this matrix?
Where did you go wrong? The image of $p(t)=at^3+bt^2+ct+d$ is
begin{align}
p(t+1)-p(t)
&=at^3+3at^2+3at+a+bt^2+2bt+b+ct+d-at^3-bt^2-ct-d\
&=3at^2+(3a+2b)t+(a+b+c)
end{align}
which is zero when
begin{cases}
3a=0\
3a+2b=0\
a+b+c=0
end{cases}
so when $a=b=c=0$. But $d$ can be anything. Thus the nullity is $1$ and the rank is $3$.
$endgroup$
You have a contradiction in your solution: if the map is injective it must also be surjective, because it is a linear map of a finite dimensional vector space to itself.
It's much easier with matrices. If you consider the standard basis ${p_0=1,p_1=t,p_2=t^2,p_3=t^3}$, then you see that
$$
F(p_0)=0,quad
F(p_1)=1,quad
F(p_2)=2t+1quad
F(p_3)=3t^2+3t+1
$$
and therefore the matrix of $F$ is
begin{bmatrix}
0 & 1 & 1 & 1 \
0 & 0 & 2 & 3 \
0 & 0 & 0 & 3 \
0 & 0 & 0 & 0
end{bmatrix}
What's the rank of this matrix?
Where did you go wrong? The image of $p(t)=at^3+bt^2+ct+d$ is
begin{align}
p(t+1)-p(t)
&=at^3+3at^2+3at+a+bt^2+2bt+b+ct+d-at^3-bt^2-ct-d\
&=3at^2+(3a+2b)t+(a+b+c)
end{align}
which is zero when
begin{cases}
3a=0\
3a+2b=0\
a+b+c=0
end{cases}
so when $a=b=c=0$. But $d$ can be anything. Thus the nullity is $1$ and the rank is $3$.
edited Jan 4 at 16:35
answered Jan 4 at 15:50
egregegreg
184k1486205
184k1486205
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
add a comment |
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
rank of this matrix is 3. Can you point me which poin twhere exactly do I have the mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:30
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
@VirtualUser The rank is $3$.
$endgroup$
– egreg
Jan 4 at 16:31
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
Yes, stupid mistake, I had on mind $3$. But what with the moment of mistake?
$endgroup$
– VirtualUser
Jan 4 at 16:34
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
@VirtualUser I added the analysis.
$endgroup$
– egreg
Jan 4 at 16:36
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
$begingroup$
Ahh, there is mistake but in calculating. It is not good but still better than mistake in understanding the problem. Anyway, thanks for tip and analysis
$endgroup$
– VirtualUser
Jan 4 at 16:45
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3061619%2finvestigate-whether-the-given-transformation-is-a-monomorphism-epimorphism-fi%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown