Linear transformation surjective and/ or injective?
$begingroup$
I am new to linear algebra, and would like some feedback regarding the following answer to a textbook question:
Let $T:M_{2times 3}(mathbb R) to M_{3times 3}(mathbb R)$ be a linear
transformation. Answer the following two questions:
- Is it possible for T to be surjective?
- Is it possible for T to be injective?
My answer is the following:
Yes, because it is possible for each element in $M_{3times 3}(mathbb R)$ to be the result of linear transformation $T$ applied to $M_{2times 3}(mathbb R)$.
No, because there are more rows in $M_{3times 3}(mathbf R)$ than in $M_{2times 3}(mathbf R)$, and therefore the elements in $M_{3times 3}(mathbb R)$ cannot be the exclusive result of the elements in $M_{2times3}(mathbb R)$.
I have no idea whether I am on the right track?
Thank you!
linear-algebra linear-transformations
$endgroup$
add a comment |
$begingroup$
I am new to linear algebra, and would like some feedback regarding the following answer to a textbook question:
Let $T:M_{2times 3}(mathbb R) to M_{3times 3}(mathbb R)$ be a linear
transformation. Answer the following two questions:
- Is it possible for T to be surjective?
- Is it possible for T to be injective?
My answer is the following:
Yes, because it is possible for each element in $M_{3times 3}(mathbb R)$ to be the result of linear transformation $T$ applied to $M_{2times 3}(mathbb R)$.
No, because there are more rows in $M_{3times 3}(mathbf R)$ than in $M_{2times 3}(mathbf R)$, and therefore the elements in $M_{3times 3}(mathbb R)$ cannot be the exclusive result of the elements in $M_{2times3}(mathbb R)$.
I have no idea whether I am on the right track?
Thank you!
linear-algebra linear-transformations
$endgroup$
$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
1
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59
add a comment |
$begingroup$
I am new to linear algebra, and would like some feedback regarding the following answer to a textbook question:
Let $T:M_{2times 3}(mathbb R) to M_{3times 3}(mathbb R)$ be a linear
transformation. Answer the following two questions:
- Is it possible for T to be surjective?
- Is it possible for T to be injective?
My answer is the following:
Yes, because it is possible for each element in $M_{3times 3}(mathbb R)$ to be the result of linear transformation $T$ applied to $M_{2times 3}(mathbb R)$.
No, because there are more rows in $M_{3times 3}(mathbf R)$ than in $M_{2times 3}(mathbf R)$, and therefore the elements in $M_{3times 3}(mathbb R)$ cannot be the exclusive result of the elements in $M_{2times3}(mathbb R)$.
I have no idea whether I am on the right track?
Thank you!
linear-algebra linear-transformations
$endgroup$
I am new to linear algebra, and would like some feedback regarding the following answer to a textbook question:
Let $T:M_{2times 3}(mathbb R) to M_{3times 3}(mathbb R)$ be a linear
transformation. Answer the following two questions:
- Is it possible for T to be surjective?
- Is it possible for T to be injective?
My answer is the following:
Yes, because it is possible for each element in $M_{3times 3}(mathbb R)$ to be the result of linear transformation $T$ applied to $M_{2times 3}(mathbb R)$.
No, because there are more rows in $M_{3times 3}(mathbf R)$ than in $M_{2times 3}(mathbf R)$, and therefore the elements in $M_{3times 3}(mathbb R)$ cannot be the exclusive result of the elements in $M_{2times3}(mathbb R)$.
I have no idea whether I am on the right track?
Thank you!
linear-algebra linear-transformations
linear-algebra linear-transformations
edited Jan 1 at 17:04
amWhy
1
1
asked Jan 1 at 16:43
daltadalta
1168
1168
$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
1
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59
add a comment |
$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
1
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59
$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
1
1
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Hint:
For 1. use the rank-nullity theorem for $T$.
For 2. consider the embedding of $M_{2,3}(K)$ into $M_{3,3}(K)$.
$endgroup$
add a comment |
$begingroup$
Let $A:Uto V$ be a linear operator between finite-dimensional vector spaces $U$ and $V$.
(1) $text{rank}(A)leqtext{dim}(V)$.
(2) $text{rank}(A)+text{nullity}(A)=text{dim}(U)$.
(3) $A$ is injective iff $text{nullity}(A)=0$.
(4) $A$ is surjective iff $text{rank}(A)=text{dim}(V)$
(5) The dimension of $M_{mtimes n}(mathbb{R})$ is $mn$.
That is more than enough to answer both questions.
$endgroup$
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
|
show 2 more comments
Your Answer
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2 Answers
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2 Answers
2
active
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$begingroup$
Hint:
For 1. use the rank-nullity theorem for $T$.
For 2. consider the embedding of $M_{2,3}(K)$ into $M_{3,3}(K)$.
$endgroup$
add a comment |
$begingroup$
Hint:
For 1. use the rank-nullity theorem for $T$.
For 2. consider the embedding of $M_{2,3}(K)$ into $M_{3,3}(K)$.
$endgroup$
add a comment |
$begingroup$
Hint:
For 1. use the rank-nullity theorem for $T$.
For 2. consider the embedding of $M_{2,3}(K)$ into $M_{3,3}(K)$.
$endgroup$
Hint:
For 1. use the rank-nullity theorem for $T$.
For 2. consider the embedding of $M_{2,3}(K)$ into $M_{3,3}(K)$.
answered Jan 1 at 16:54
Dietrich BurdeDietrich Burde
80.1k647104
80.1k647104
add a comment |
add a comment |
$begingroup$
Let $A:Uto V$ be a linear operator between finite-dimensional vector spaces $U$ and $V$.
(1) $text{rank}(A)leqtext{dim}(V)$.
(2) $text{rank}(A)+text{nullity}(A)=text{dim}(U)$.
(3) $A$ is injective iff $text{nullity}(A)=0$.
(4) $A$ is surjective iff $text{rank}(A)=text{dim}(V)$
(5) The dimension of $M_{mtimes n}(mathbb{R})$ is $mn$.
That is more than enough to answer both questions.
$endgroup$
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
|
show 2 more comments
$begingroup$
Let $A:Uto V$ be a linear operator between finite-dimensional vector spaces $U$ and $V$.
(1) $text{rank}(A)leqtext{dim}(V)$.
(2) $text{rank}(A)+text{nullity}(A)=text{dim}(U)$.
(3) $A$ is injective iff $text{nullity}(A)=0$.
(4) $A$ is surjective iff $text{rank}(A)=text{dim}(V)$
(5) The dimension of $M_{mtimes n}(mathbb{R})$ is $mn$.
That is more than enough to answer both questions.
$endgroup$
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
|
show 2 more comments
$begingroup$
Let $A:Uto V$ be a linear operator between finite-dimensional vector spaces $U$ and $V$.
(1) $text{rank}(A)leqtext{dim}(V)$.
(2) $text{rank}(A)+text{nullity}(A)=text{dim}(U)$.
(3) $A$ is injective iff $text{nullity}(A)=0$.
(4) $A$ is surjective iff $text{rank}(A)=text{dim}(V)$
(5) The dimension of $M_{mtimes n}(mathbb{R})$ is $mn$.
That is more than enough to answer both questions.
$endgroup$
Let $A:Uto V$ be a linear operator between finite-dimensional vector spaces $U$ and $V$.
(1) $text{rank}(A)leqtext{dim}(V)$.
(2) $text{rank}(A)+text{nullity}(A)=text{dim}(U)$.
(3) $A$ is injective iff $text{nullity}(A)=0$.
(4) $A$ is surjective iff $text{rank}(A)=text{dim}(V)$
(5) The dimension of $M_{mtimes n}(mathbb{R})$ is $mn$.
That is more than enough to answer both questions.
answered Jan 1 at 16:53
Ben WBen W
2,322615
2,322615
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
|
show 2 more comments
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Right, which is why I used a different symbol to denote an arbitrary linear operator between $U$ and $V$
$endgroup$
– Ben W
Jan 1 at 16:58
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Ordinarily I would agree, but $T$ was already taken. It would be like fixing $n=4$ and then trying to use the notation $sum_{n=1}^infty frac{1}{n^2}$ or something.
$endgroup$
– Ben W
Jan 1 at 17:00
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
Thank you for your answer! Just want to check that I got this right, because the answer it leads to is the opposite of what I had originally thought: according to the above, if A is surjective, then rank(A)=9, but rank(A)+nullity(A)=dim(U), giving 9+nullity(A)=6, which is impossible: so A cannot be surjective. However, if A is injective, Rank A = 6 (because dimKerT = 0), which is indeed less than 9, so there is no contradiction, and A can be injective… Did I understand this correctly?
$endgroup$
– dalta
Jan 1 at 18:07
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
You have the first part right. But for the second part, merely showing that it doesn't contradict the rank-nullity theorem isn't enough. You have to actually exhibit some kind of example, even if it's not explicit. For example, let ${u_1,cdots,u_6}$ be a basis for $M_{2times 3}$ and ${v_1,cdots,v_9}$ a basis for $M_{3times 3}$. Then define $T$ by the rule $T(u_i)=v_i$ for $i=1,cdots 6$.
$endgroup$
– Ben W
Jan 1 at 18:10
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
$begingroup$
and the remaining $v_7-v_9$ are simply unrelated to $M_{2x3}$?
$endgroup$
– dalta
Jan 1 at 18:16
|
show 2 more comments
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$begingroup$
@DietrichBurde criticism accepted, but can you give me a hint as to how to go about 1 and how to redefine 2?
$endgroup$
– dalta
Jan 1 at 16:49
1
$begingroup$
Vector spaces of matrices can be confusing because the vectors are the matrices. Notice that a two-by-three matrix is equivalent to a $6$-dimensional vector because it is described by $6$ real numbers. Change the problem to $T:mathbb R^6tomathbb R^9$ and then try to answer the questions.
$endgroup$
– John Douma
Jan 1 at 16:59