Eigenvalues of a matrix whose square is zero
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Let $A$ be a nonzero 3×3 matrix such that $A^2=0$. Then what is the number of non-zero eigenvalues of the matrix?
I am unable to figure out the eigenvalues of the above matrix.
P.S.: how would the answer change if it were given that $A^3=0$?
linear-algebra eigenvalues-eigenvectors
$endgroup$
add a comment |
$begingroup$
Let $A$ be a nonzero 3×3 matrix such that $A^2=0$. Then what is the number of non-zero eigenvalues of the matrix?
I am unable to figure out the eigenvalues of the above matrix.
P.S.: how would the answer change if it were given that $A^3=0$?
linear-algebra eigenvalues-eigenvectors
$endgroup$
$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago
add a comment |
$begingroup$
Let $A$ be a nonzero 3×3 matrix such that $A^2=0$. Then what is the number of non-zero eigenvalues of the matrix?
I am unable to figure out the eigenvalues of the above matrix.
P.S.: how would the answer change if it were given that $A^3=0$?
linear-algebra eigenvalues-eigenvectors
$endgroup$
Let $A$ be a nonzero 3×3 matrix such that $A^2=0$. Then what is the number of non-zero eigenvalues of the matrix?
I am unable to figure out the eigenvalues of the above matrix.
P.S.: how would the answer change if it were given that $A^3=0$?
linear-algebra eigenvalues-eigenvectors
linear-algebra eigenvalues-eigenvectors
edited 3 hours ago
J. W. Tanner
1,306114
1,306114
asked 5 hours ago
Jor_ElJor_El
356
356
$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago
add a comment |
$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago
$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago
$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
A square matrix $A$ is called nilpotent if there is a $p in mathbb{N}$ such that $A^p=0$. So let $A$ be a nilpotent matrix. Then we have by definition of an eigenvalue
$$Av=lambda v,$$
where $lambda$ is an eigenvalue of $A$ and $vneq 0$ is an eigenvector of $A$ to the corresponding eigenvalue. Because $A$ is nilpotent we also have
$$0=A^p v=lambda^p v$$
and because $v neq 0$ it follows $lambda^p=0$, i. e. $lambda=0$. So to your question: The number of non zero eigenvalues is in this case $0$.
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So does it mean that a nilpotent matrix has all eigen values equal to 0?
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– Jor_El
5 hours ago
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@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
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– Jan
5 hours ago
add a comment |
$begingroup$
Another approach is this one:
Since $A^2 = 0$, the polynomial $g(x) = x^2$ annihilates A (and this means that the Linear operator defined by $g(T)$ is the null operator). However, the minimal polynomial of A must divide every polynomial that annihilates $A$, so if $m(x)$ is such polynomial, $m$ must divide $g$.
Hence, $m(x) = x^2$, because $A ≠ 0$.
Thus, the characteristical polynomial of $A$ is $p(x) = x^2$, because $p(x)$ has the same roots of $m(x)$ (why?), and $p(x)$ annihilates $A$ (by Cayley-Hamilton theorem).
In conclusion, the characteristical polynomial of $A$ has only a single root, $0$, and since every eigenvalue of $A$ is a root of it's characteristical polynomial, we have that 0 is the only eigenvalue of $A$.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
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– Mike Earnest
1 hour ago
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More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
add a comment |
$begingroup$
Clearly, as the characteristic polynomial of the matrix $A$ is $x^n = 0$, and the eigenvalues are roots of the characteristic polynomial, there can not be any non-zero eigenvalue.
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
A square matrix $A$ is called nilpotent if there is a $p in mathbb{N}$ such that $A^p=0$. So let $A$ be a nilpotent matrix. Then we have by definition of an eigenvalue
$$Av=lambda v,$$
where $lambda$ is an eigenvalue of $A$ and $vneq 0$ is an eigenvector of $A$ to the corresponding eigenvalue. Because $A$ is nilpotent we also have
$$0=A^p v=lambda^p v$$
and because $v neq 0$ it follows $lambda^p=0$, i. e. $lambda=0$. So to your question: The number of non zero eigenvalues is in this case $0$.
$endgroup$
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
add a comment |
$begingroup$
A square matrix $A$ is called nilpotent if there is a $p in mathbb{N}$ such that $A^p=0$. So let $A$ be a nilpotent matrix. Then we have by definition of an eigenvalue
$$Av=lambda v,$$
where $lambda$ is an eigenvalue of $A$ and $vneq 0$ is an eigenvector of $A$ to the corresponding eigenvalue. Because $A$ is nilpotent we also have
$$0=A^p v=lambda^p v$$
and because $v neq 0$ it follows $lambda^p=0$, i. e. $lambda=0$. So to your question: The number of non zero eigenvalues is in this case $0$.
$endgroup$
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
add a comment |
$begingroup$
A square matrix $A$ is called nilpotent if there is a $p in mathbb{N}$ such that $A^p=0$. So let $A$ be a nilpotent matrix. Then we have by definition of an eigenvalue
$$Av=lambda v,$$
where $lambda$ is an eigenvalue of $A$ and $vneq 0$ is an eigenvector of $A$ to the corresponding eigenvalue. Because $A$ is nilpotent we also have
$$0=A^p v=lambda^p v$$
and because $v neq 0$ it follows $lambda^p=0$, i. e. $lambda=0$. So to your question: The number of non zero eigenvalues is in this case $0$.
$endgroup$
A square matrix $A$ is called nilpotent if there is a $p in mathbb{N}$ such that $A^p=0$. So let $A$ be a nilpotent matrix. Then we have by definition of an eigenvalue
$$Av=lambda v,$$
where $lambda$ is an eigenvalue of $A$ and $vneq 0$ is an eigenvector of $A$ to the corresponding eigenvalue. Because $A$ is nilpotent we also have
$$0=A^p v=lambda^p v$$
and because $v neq 0$ it follows $lambda^p=0$, i. e. $lambda=0$. So to your question: The number of non zero eigenvalues is in this case $0$.
answered 5 hours ago
JanJan
331114
331114
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
add a comment |
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
So does it mean that a nilpotent matrix has all eigen values equal to 0?
$endgroup$
– Jor_El
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
$begingroup$
@Jor_El Yes - every eigenvalue of a nilpotent matrix is zero.
$endgroup$
– Jan
5 hours ago
add a comment |
$begingroup$
Another approach is this one:
Since $A^2 = 0$, the polynomial $g(x) = x^2$ annihilates A (and this means that the Linear operator defined by $g(T)$ is the null operator). However, the minimal polynomial of A must divide every polynomial that annihilates $A$, so if $m(x)$ is such polynomial, $m$ must divide $g$.
Hence, $m(x) = x^2$, because $A ≠ 0$.
Thus, the characteristical polynomial of $A$ is $p(x) = x^2$, because $p(x)$ has the same roots of $m(x)$ (why?), and $p(x)$ annihilates $A$ (by Cayley-Hamilton theorem).
In conclusion, the characteristical polynomial of $A$ has only a single root, $0$, and since every eigenvalue of $A$ is a root of it's characteristical polynomial, we have that 0 is the only eigenvalue of $A$.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
add a comment |
$begingroup$
Another approach is this one:
Since $A^2 = 0$, the polynomial $g(x) = x^2$ annihilates A (and this means that the Linear operator defined by $g(T)$ is the null operator). However, the minimal polynomial of A must divide every polynomial that annihilates $A$, so if $m(x)$ is such polynomial, $m$ must divide $g$.
Hence, $m(x) = x^2$, because $A ≠ 0$.
Thus, the characteristical polynomial of $A$ is $p(x) = x^2$, because $p(x)$ has the same roots of $m(x)$ (why?), and $p(x)$ annihilates $A$ (by Cayley-Hamilton theorem).
In conclusion, the characteristical polynomial of $A$ has only a single root, $0$, and since every eigenvalue of $A$ is a root of it's characteristical polynomial, we have that 0 is the only eigenvalue of $A$.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
add a comment |
$begingroup$
Another approach is this one:
Since $A^2 = 0$, the polynomial $g(x) = x^2$ annihilates A (and this means that the Linear operator defined by $g(T)$ is the null operator). However, the minimal polynomial of A must divide every polynomial that annihilates $A$, so if $m(x)$ is such polynomial, $m$ must divide $g$.
Hence, $m(x) = x^2$, because $A ≠ 0$.
Thus, the characteristical polynomial of $A$ is $p(x) = x^2$, because $p(x)$ has the same roots of $m(x)$ (why?), and $p(x)$ annihilates $A$ (by Cayley-Hamilton theorem).
In conclusion, the characteristical polynomial of $A$ has only a single root, $0$, and since every eigenvalue of $A$ is a root of it's characteristical polynomial, we have that 0 is the only eigenvalue of $A$.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
Another approach is this one:
Since $A^2 = 0$, the polynomial $g(x) = x^2$ annihilates A (and this means that the Linear operator defined by $g(T)$ is the null operator). However, the minimal polynomial of A must divide every polynomial that annihilates $A$, so if $m(x)$ is such polynomial, $m$ must divide $g$.
Hence, $m(x) = x^2$, because $A ≠ 0$.
Thus, the characteristical polynomial of $A$ is $p(x) = x^2$, because $p(x)$ has the same roots of $m(x)$ (why?), and $p(x)$ annihilates $A$ (by Cayley-Hamilton theorem).
In conclusion, the characteristical polynomial of $A$ has only a single root, $0$, and since every eigenvalue of $A$ is a root of it's characteristical polynomial, we have that 0 is the only eigenvalue of $A$.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 3 hours ago
Bruno TassoneBruno Tassone
213
213
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Bruno Tassone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
add a comment |
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
One small note, the characteristic polynomial of $A$ would be $x^3$. The degree of the char poly of a matrix is equal to its dimension.
$endgroup$
– Mike Earnest
1 hour ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
$begingroup$
More generally, if some linear operator $B$ has $Bv = lambda v$ with $vne 0$, and $P$ is a polynomial, then it is an easy calculation that $P(B)v = P(lambda)v$. Therefore for any polynomial with $P(B) = 0$, every eigenvalue $lambda$ of $B$ must be a root of $P$.
$endgroup$
– Paul Sinclair
26 mins ago
add a comment |
$begingroup$
Clearly, as the characteristic polynomial of the matrix $A$ is $x^n = 0$, and the eigenvalues are roots of the characteristic polynomial, there can not be any non-zero eigenvalue.
$endgroup$
add a comment |
$begingroup$
Clearly, as the characteristic polynomial of the matrix $A$ is $x^n = 0$, and the eigenvalues are roots of the characteristic polynomial, there can not be any non-zero eigenvalue.
$endgroup$
add a comment |
$begingroup$
Clearly, as the characteristic polynomial of the matrix $A$ is $x^n = 0$, and the eigenvalues are roots of the characteristic polynomial, there can not be any non-zero eigenvalue.
$endgroup$
Clearly, as the characteristic polynomial of the matrix $A$ is $x^n = 0$, and the eigenvalues are roots of the characteristic polynomial, there can not be any non-zero eigenvalue.
answered 5 hours ago
ab123ab123
1,781423
1,781423
add a comment |
add a comment |
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$begingroup$
Hint: Eigenvalues are roots of the characteristic polynomial
$endgroup$
– ab123
5 hours ago