Rank of a matrix product of two vectors
$begingroup$
Let $lambda=(lambda_1, lambda_2,...,lambda_n)in Bbb R^n$ a vector no zero. Let $A=(a_{ij})$ the matrix of $ntimes n$ so that $a_{ij}=lambda_i lambda_j$. Determine the rank of the matrix $A$.
The matrix $A$ is $$ left(
begin{array}{ccccc}
lambda_1^2 & lambda_1lambda_2 & cdots & lambda_1lambda_n \
lambda_2lambda_1 & lambda_2^2 &cdots & lambda_2lambda_n \
vdots& vdots & ddots & vdots \
lambda_nlambda_1 & lambda_nlambda_2& cdots & lambda_n^2 \
end{array}
right)$$
matrices
$endgroup$
add a comment |
$begingroup$
Let $lambda=(lambda_1, lambda_2,...,lambda_n)in Bbb R^n$ a vector no zero. Let $A=(a_{ij})$ the matrix of $ntimes n$ so that $a_{ij}=lambda_i lambda_j$. Determine the rank of the matrix $A$.
The matrix $A$ is $$ left(
begin{array}{ccccc}
lambda_1^2 & lambda_1lambda_2 & cdots & lambda_1lambda_n \
lambda_2lambda_1 & lambda_2^2 &cdots & lambda_2lambda_n \
vdots& vdots & ddots & vdots \
lambda_nlambda_1 & lambda_nlambda_2& cdots & lambda_n^2 \
end{array}
right)$$
matrices
$endgroup$
add a comment |
$begingroup$
Let $lambda=(lambda_1, lambda_2,...,lambda_n)in Bbb R^n$ a vector no zero. Let $A=(a_{ij})$ the matrix of $ntimes n$ so that $a_{ij}=lambda_i lambda_j$. Determine the rank of the matrix $A$.
The matrix $A$ is $$ left(
begin{array}{ccccc}
lambda_1^2 & lambda_1lambda_2 & cdots & lambda_1lambda_n \
lambda_2lambda_1 & lambda_2^2 &cdots & lambda_2lambda_n \
vdots& vdots & ddots & vdots \
lambda_nlambda_1 & lambda_nlambda_2& cdots & lambda_n^2 \
end{array}
right)$$
matrices
$endgroup$
Let $lambda=(lambda_1, lambda_2,...,lambda_n)in Bbb R^n$ a vector no zero. Let $A=(a_{ij})$ the matrix of $ntimes n$ so that $a_{ij}=lambda_i lambda_j$. Determine the rank of the matrix $A$.
The matrix $A$ is $$ left(
begin{array}{ccccc}
lambda_1^2 & lambda_1lambda_2 & cdots & lambda_1lambda_n \
lambda_2lambda_1 & lambda_2^2 &cdots & lambda_2lambda_n \
vdots& vdots & ddots & vdots \
lambda_nlambda_1 & lambda_nlambda_2& cdots & lambda_n^2 \
end{array}
right)$$
matrices
matrices
edited Feb 20 '16 at 23:23
John B
12.2k51840
12.2k51840
asked Jan 10 '15 at 4:35
Roiner Segura CuberoRoiner Segura Cubero
1,9151730
1,9151730
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
Fact: $text{Rank}(AB) le text{min}{text{Rank}(A),text{Rank}(B)}$
And your matrix is
$$left[ begin{matrix}lambda_1 \ lambda_2 \ vdots \ lambda_nend{matrix}right]left[ begin{matrix}lambda_1 & lambda_2 & cdots & lambda_nend{matrix}right]$$
$endgroup$
add a comment |
$begingroup$
Hint. Every row of $A$ is a scalar multiple of the row vector $lambda$, and $Ane0$. So, what is the row rank of $A$? Now $A$ is a square matrix. Therefore its rank is equal to its row rank.
$endgroup$
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
add a comment |
$begingroup$
The rank is one because you have in the rows scalar multiples of the original $lambda$.
$endgroup$
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
add a comment |
$begingroup$
You may alternatively exploit that the rank of a matrix is preserved under a change of a basis, which is described by similarity transformations.
Choose an orthogonal transformation $S$, thus $Scirc S^T=mathbb 1$, such that
$$Sbig(underbrace{(1,dots,0)}_{quad =,e_1}big) :=: frac 1{|lambda|}(lambda_1, lambda_2,dots,lambda_n),.$$
Now consider $A$ and the matrix products
$$A:=:big(lambda_1,dots,lambda_nbig)
begin{pmatrix}lambda_1\ vdots\ lambda_nend{pmatrix}
,=, |lambda|,Se_1,big(|lambda|,Se_1big)^T
,=, |lambda|^2,S:e_1e_1^T,S^T \[3ex]
,=, |lambda|^2,S,
left(begin{smallmatrix} 1&0&dots&0\ 0&0&&vdots\ vdots&&ddots\
0&ldots&&0end{smallmatrix}right),S^{-1}qquad
$$
from which $A$'s rank can be read off.
$endgroup$
add a comment |
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Fact: $text{Rank}(AB) le text{min}{text{Rank}(A),text{Rank}(B)}$
And your matrix is
$$left[ begin{matrix}lambda_1 \ lambda_2 \ vdots \ lambda_nend{matrix}right]left[ begin{matrix}lambda_1 & lambda_2 & cdots & lambda_nend{matrix}right]$$
$endgroup$
add a comment |
$begingroup$
Fact: $text{Rank}(AB) le text{min}{text{Rank}(A),text{Rank}(B)}$
And your matrix is
$$left[ begin{matrix}lambda_1 \ lambda_2 \ vdots \ lambda_nend{matrix}right]left[ begin{matrix}lambda_1 & lambda_2 & cdots & lambda_nend{matrix}right]$$
$endgroup$
add a comment |
$begingroup$
Fact: $text{Rank}(AB) le text{min}{text{Rank}(A),text{Rank}(B)}$
And your matrix is
$$left[ begin{matrix}lambda_1 \ lambda_2 \ vdots \ lambda_nend{matrix}right]left[ begin{matrix}lambda_1 & lambda_2 & cdots & lambda_nend{matrix}right]$$
$endgroup$
Fact: $text{Rank}(AB) le text{min}{text{Rank}(A),text{Rank}(B)}$
And your matrix is
$$left[ begin{matrix}lambda_1 \ lambda_2 \ vdots \ lambda_nend{matrix}right]left[ begin{matrix}lambda_1 & lambda_2 & cdots & lambda_nend{matrix}right]$$
answered Jan 10 '15 at 4:46
David PetersonDavid Peterson
8,87821935
8,87821935
add a comment |
add a comment |
$begingroup$
Hint. Every row of $A$ is a scalar multiple of the row vector $lambda$, and $Ane0$. So, what is the row rank of $A$? Now $A$ is a square matrix. Therefore its rank is equal to its row rank.
$endgroup$
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
add a comment |
$begingroup$
Hint. Every row of $A$ is a scalar multiple of the row vector $lambda$, and $Ane0$. So, what is the row rank of $A$? Now $A$ is a square matrix. Therefore its rank is equal to its row rank.
$endgroup$
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
add a comment |
$begingroup$
Hint. Every row of $A$ is a scalar multiple of the row vector $lambda$, and $Ane0$. So, what is the row rank of $A$? Now $A$ is a square matrix. Therefore its rank is equal to its row rank.
$endgroup$
Hint. Every row of $A$ is a scalar multiple of the row vector $lambda$, and $Ane0$. So, what is the row rank of $A$? Now $A$ is a square matrix. Therefore its rank is equal to its row rank.
answered Jan 10 '15 at 4:40
user1551user1551
73.6k566129
73.6k566129
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
add a comment |
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
which is the rank of a vector?
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:46
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
$begingroup$
@RoinerSeguraCubero: a vector is a matrix $ntimes 1$ or $1times n$
$endgroup$
– janmarqz
Jan 10 '15 at 4:50
add a comment |
$begingroup$
The rank is one because you have in the rows scalar multiples of the original $lambda$.
$endgroup$
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
add a comment |
$begingroup$
The rank is one because you have in the rows scalar multiples of the original $lambda$.
$endgroup$
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
add a comment |
$begingroup$
The rank is one because you have in the rows scalar multiples of the original $lambda$.
$endgroup$
The rank is one because you have in the rows scalar multiples of the original $lambda$.
answered Jan 10 '15 at 4:40
janmarqzjanmarqz
6,22741630
6,22741630
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
add a comment |
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
I do not understand that the rank of the matrix is one
$endgroup$
– Roiner Segura Cubero
Jan 10 '15 at 4:44
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
$begingroup$
how do you define rank of a matrix?
$endgroup$
– janmarqz
Jan 10 '15 at 4:46
add a comment |
$begingroup$
You may alternatively exploit that the rank of a matrix is preserved under a change of a basis, which is described by similarity transformations.
Choose an orthogonal transformation $S$, thus $Scirc S^T=mathbb 1$, such that
$$Sbig(underbrace{(1,dots,0)}_{quad =,e_1}big) :=: frac 1{|lambda|}(lambda_1, lambda_2,dots,lambda_n),.$$
Now consider $A$ and the matrix products
$$A:=:big(lambda_1,dots,lambda_nbig)
begin{pmatrix}lambda_1\ vdots\ lambda_nend{pmatrix}
,=, |lambda|,Se_1,big(|lambda|,Se_1big)^T
,=, |lambda|^2,S:e_1e_1^T,S^T \[3ex]
,=, |lambda|^2,S,
left(begin{smallmatrix} 1&0&dots&0\ 0&0&&vdots\ vdots&&ddots\
0&ldots&&0end{smallmatrix}right),S^{-1}qquad
$$
from which $A$'s rank can be read off.
$endgroup$
add a comment |
$begingroup$
You may alternatively exploit that the rank of a matrix is preserved under a change of a basis, which is described by similarity transformations.
Choose an orthogonal transformation $S$, thus $Scirc S^T=mathbb 1$, such that
$$Sbig(underbrace{(1,dots,0)}_{quad =,e_1}big) :=: frac 1{|lambda|}(lambda_1, lambda_2,dots,lambda_n),.$$
Now consider $A$ and the matrix products
$$A:=:big(lambda_1,dots,lambda_nbig)
begin{pmatrix}lambda_1\ vdots\ lambda_nend{pmatrix}
,=, |lambda|,Se_1,big(|lambda|,Se_1big)^T
,=, |lambda|^2,S:e_1e_1^T,S^T \[3ex]
,=, |lambda|^2,S,
left(begin{smallmatrix} 1&0&dots&0\ 0&0&&vdots\ vdots&&ddots\
0&ldots&&0end{smallmatrix}right),S^{-1}qquad
$$
from which $A$'s rank can be read off.
$endgroup$
add a comment |
$begingroup$
You may alternatively exploit that the rank of a matrix is preserved under a change of a basis, which is described by similarity transformations.
Choose an orthogonal transformation $S$, thus $Scirc S^T=mathbb 1$, such that
$$Sbig(underbrace{(1,dots,0)}_{quad =,e_1}big) :=: frac 1{|lambda|}(lambda_1, lambda_2,dots,lambda_n),.$$
Now consider $A$ and the matrix products
$$A:=:big(lambda_1,dots,lambda_nbig)
begin{pmatrix}lambda_1\ vdots\ lambda_nend{pmatrix}
,=, |lambda|,Se_1,big(|lambda|,Se_1big)^T
,=, |lambda|^2,S:e_1e_1^T,S^T \[3ex]
,=, |lambda|^2,S,
left(begin{smallmatrix} 1&0&dots&0\ 0&0&&vdots\ vdots&&ddots\
0&ldots&&0end{smallmatrix}right),S^{-1}qquad
$$
from which $A$'s rank can be read off.
$endgroup$
You may alternatively exploit that the rank of a matrix is preserved under a change of a basis, which is described by similarity transformations.
Choose an orthogonal transformation $S$, thus $Scirc S^T=mathbb 1$, such that
$$Sbig(underbrace{(1,dots,0)}_{quad =,e_1}big) :=: frac 1{|lambda|}(lambda_1, lambda_2,dots,lambda_n),.$$
Now consider $A$ and the matrix products
$$A:=:big(lambda_1,dots,lambda_nbig)
begin{pmatrix}lambda_1\ vdots\ lambda_nend{pmatrix}
,=, |lambda|,Se_1,big(|lambda|,Se_1big)^T
,=, |lambda|^2,S:e_1e_1^T,S^T \[3ex]
,=, |lambda|^2,S,
left(begin{smallmatrix} 1&0&dots&0\ 0&0&&vdots\ vdots&&ddots\
0&ldots&&0end{smallmatrix}right),S^{-1}qquad
$$
from which $A$'s rank can be read off.
answered Jan 4 at 1:27
HannoHanno
2,284628
2,284628
add a comment |
add a comment |
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