Showing that is a normal operator
$begingroup$
Let $H$ is a Hilbert space
$I$ is unit operator, $T in B(H)$ and $lambda in mathbb C$
$T$ is normal operator $Rightarrow$ $T-lambda I$ is a normal operator too.
I could only write :
I must show that $(T-lambda I)(T-lambda I)^{ast}=(T-lambda I)^{ast}(T-lambda I)$
$TT^{ast}=T^{ast}T$
$I^{ast}=I$
$(T-lambda I)^{ast}=T^{ast}- bar{lambda}I$
(where $ast$ means adjoint and $bar{lambda}I$ means complex conjugate.
I cannot continue. I really stuck
Thanks for any help
functional-analysis operator-theory hilbert-spaces adjoint-operators
$endgroup$
add a comment |
$begingroup$
Let $H$ is a Hilbert space
$I$ is unit operator, $T in B(H)$ and $lambda in mathbb C$
$T$ is normal operator $Rightarrow$ $T-lambda I$ is a normal operator too.
I could only write :
I must show that $(T-lambda I)(T-lambda I)^{ast}=(T-lambda I)^{ast}(T-lambda I)$
$TT^{ast}=T^{ast}T$
$I^{ast}=I$
$(T-lambda I)^{ast}=T^{ast}- bar{lambda}I$
(where $ast$ means adjoint and $bar{lambda}I$ means complex conjugate.
I cannot continue. I really stuck
Thanks for any help
functional-analysis operator-theory hilbert-spaces adjoint-operators
$endgroup$
add a comment |
$begingroup$
Let $H$ is a Hilbert space
$I$ is unit operator, $T in B(H)$ and $lambda in mathbb C$
$T$ is normal operator $Rightarrow$ $T-lambda I$ is a normal operator too.
I could only write :
I must show that $(T-lambda I)(T-lambda I)^{ast}=(T-lambda I)^{ast}(T-lambda I)$
$TT^{ast}=T^{ast}T$
$I^{ast}=I$
$(T-lambda I)^{ast}=T^{ast}- bar{lambda}I$
(where $ast$ means adjoint and $bar{lambda}I$ means complex conjugate.
I cannot continue. I really stuck
Thanks for any help
functional-analysis operator-theory hilbert-spaces adjoint-operators
$endgroup$
Let $H$ is a Hilbert space
$I$ is unit operator, $T in B(H)$ and $lambda in mathbb C$
$T$ is normal operator $Rightarrow$ $T-lambda I$ is a normal operator too.
I could only write :
I must show that $(T-lambda I)(T-lambda I)^{ast}=(T-lambda I)^{ast}(T-lambda I)$
$TT^{ast}=T^{ast}T$
$I^{ast}=I$
$(T-lambda I)^{ast}=T^{ast}- bar{lambda}I$
(where $ast$ means adjoint and $bar{lambda}I$ means complex conjugate.
I cannot continue. I really stuck
Thanks for any help
functional-analysis operator-theory hilbert-spaces adjoint-operators
functional-analysis operator-theory hilbert-spaces adjoint-operators
asked Dec 19 '18 at 19:07
esrabasaresrabasar
420110
420110
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
Then
$$(T-lambda I)(T-lambda I)^*=(T-lambda I)(T^*-overlinelambda I)
=TT^*-lambda T^*-overlinelambda T+|lambda|^2I.$$
What about $(T-lambda I)^*(T-lambda I)$?
$endgroup$
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
add a comment |
$begingroup$
Note that
$(T - lambda I)^ast = T^ast - bar lambda I; tag 1$
then, using $TT^ast = T^ast T$ (i.e., the normality of $T$), one has
$(T - lambda I)(T - lambda I)^ast = (T - lambda I)(T^ast - bar lambda I) = TT^ast - bar lambda T - lambda T^ast + lambda bar lambda I$
$= T^ast T - bar lambda T - lambda T^ast + lambda bar lambda I = T^ast (T - lambda I) - bar lambda (T - lambda I)$
$= (T^ast - bar lambda I)(T - lambda I) = (T - lambda I)^ast (T - lambda I), tag 2$
which asserts that $T - lambda$ is normal.
$endgroup$
add a comment |
$begingroup$
You can also use the characterization that $A in B(H)$ is normal if and only if $|Ax| = |A^*x|, forall x in H$.
For $x in H$ we have
begin{align}
|(T - lambda I)x|^2 &= langle Tx - lambda x, Tx - lambda xrangle \
&= langle Tx,Txrangle - 2operatorname{Re} langle Tx, lambda xrangle + langle lambda x, lambda xrangle\
&= |Tx|^2 - 2operatorname{Re} overline{lambda}langle Tx, xrangle + |lambda|^2|x|^2\
&= |T^*x|^2 - 2operatorname{Re} overline{lambda}langle x, T^*xrangle + left|overline{lambda}right|^2|x|^2\
&= langle overline{lambda} x,overline{lambda}xrangle - 2operatorname{Re} langle overline{lambda}x, T^*xrangle+ langle T^*x,T^*xrangle\
&= leftlangle overline{lambda}x - T^*x,overline{lambda}x - T^*xrightrangle\
&= left|left(overline{lambda} I - T^*right)xright|^2\
&= left|(T-lambda I)^*xright|^2
end{align}
so $T - lambda I$ is normal.
$endgroup$
add a comment |
$begingroup$
$T$ is normal iff $T$ commutes with its adjoint $T^*$. If $T$ is normal, then $T^*$ commutes with $(T-lambda I)$ and $overline{lambda}I$ commutes with $(T-lambda I)$; hence $T^*-overline{lambda}I=(T-lambda I)^*$ commutes with $T-lambda I$, which makes $T-lambda I$ normal.
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Then
$$(T-lambda I)(T-lambda I)^*=(T-lambda I)(T^*-overlinelambda I)
=TT^*-lambda T^*-overlinelambda T+|lambda|^2I.$$
What about $(T-lambda I)^*(T-lambda I)$?
$endgroup$
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
add a comment |
$begingroup$
Then
$$(T-lambda I)(T-lambda I)^*=(T-lambda I)(T^*-overlinelambda I)
=TT^*-lambda T^*-overlinelambda T+|lambda|^2I.$$
What about $(T-lambda I)^*(T-lambda I)$?
$endgroup$
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
add a comment |
$begingroup$
Then
$$(T-lambda I)(T-lambda I)^*=(T-lambda I)(T^*-overlinelambda I)
=TT^*-lambda T^*-overlinelambda T+|lambda|^2I.$$
What about $(T-lambda I)^*(T-lambda I)$?
$endgroup$
Then
$$(T-lambda I)(T-lambda I)^*=(T-lambda I)(T^*-overlinelambda I)
=TT^*-lambda T^*-overlinelambda T+|lambda|^2I.$$
What about $(T-lambda I)^*(T-lambda I)$?
answered Dec 19 '18 at 19:11
Lord Shark the UnknownLord Shark the Unknown
106k1161133
106k1161133
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
add a comment |
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
$begingroup$
Why can we use distribution? It is a composite operator. Could you please explain if it is possible?
$endgroup$
– user519955
Dec 19 '18 at 19:26
1
1
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
$begingroup$
@user519955 These are all linear operators.
$endgroup$
– Lord Shark the Unknown
Dec 20 '18 at 2:57
add a comment |
$begingroup$
Note that
$(T - lambda I)^ast = T^ast - bar lambda I; tag 1$
then, using $TT^ast = T^ast T$ (i.e., the normality of $T$), one has
$(T - lambda I)(T - lambda I)^ast = (T - lambda I)(T^ast - bar lambda I) = TT^ast - bar lambda T - lambda T^ast + lambda bar lambda I$
$= T^ast T - bar lambda T - lambda T^ast + lambda bar lambda I = T^ast (T - lambda I) - bar lambda (T - lambda I)$
$= (T^ast - bar lambda I)(T - lambda I) = (T - lambda I)^ast (T - lambda I), tag 2$
which asserts that $T - lambda$ is normal.
$endgroup$
add a comment |
$begingroup$
Note that
$(T - lambda I)^ast = T^ast - bar lambda I; tag 1$
then, using $TT^ast = T^ast T$ (i.e., the normality of $T$), one has
$(T - lambda I)(T - lambda I)^ast = (T - lambda I)(T^ast - bar lambda I) = TT^ast - bar lambda T - lambda T^ast + lambda bar lambda I$
$= T^ast T - bar lambda T - lambda T^ast + lambda bar lambda I = T^ast (T - lambda I) - bar lambda (T - lambda I)$
$= (T^ast - bar lambda I)(T - lambda I) = (T - lambda I)^ast (T - lambda I), tag 2$
which asserts that $T - lambda$ is normal.
$endgroup$
add a comment |
$begingroup$
Note that
$(T - lambda I)^ast = T^ast - bar lambda I; tag 1$
then, using $TT^ast = T^ast T$ (i.e., the normality of $T$), one has
$(T - lambda I)(T - lambda I)^ast = (T - lambda I)(T^ast - bar lambda I) = TT^ast - bar lambda T - lambda T^ast + lambda bar lambda I$
$= T^ast T - bar lambda T - lambda T^ast + lambda bar lambda I = T^ast (T - lambda I) - bar lambda (T - lambda I)$
$= (T^ast - bar lambda I)(T - lambda I) = (T - lambda I)^ast (T - lambda I), tag 2$
which asserts that $T - lambda$ is normal.
$endgroup$
Note that
$(T - lambda I)^ast = T^ast - bar lambda I; tag 1$
then, using $TT^ast = T^ast T$ (i.e., the normality of $T$), one has
$(T - lambda I)(T - lambda I)^ast = (T - lambda I)(T^ast - bar lambda I) = TT^ast - bar lambda T - lambda T^ast + lambda bar lambda I$
$= T^ast T - bar lambda T - lambda T^ast + lambda bar lambda I = T^ast (T - lambda I) - bar lambda (T - lambda I)$
$= (T^ast - bar lambda I)(T - lambda I) = (T - lambda I)^ast (T - lambda I), tag 2$
which asserts that $T - lambda$ is normal.
answered Dec 19 '18 at 20:16
Robert LewisRobert Lewis
48.3k23167
48.3k23167
add a comment |
add a comment |
$begingroup$
You can also use the characterization that $A in B(H)$ is normal if and only if $|Ax| = |A^*x|, forall x in H$.
For $x in H$ we have
begin{align}
|(T - lambda I)x|^2 &= langle Tx - lambda x, Tx - lambda xrangle \
&= langle Tx,Txrangle - 2operatorname{Re} langle Tx, lambda xrangle + langle lambda x, lambda xrangle\
&= |Tx|^2 - 2operatorname{Re} overline{lambda}langle Tx, xrangle + |lambda|^2|x|^2\
&= |T^*x|^2 - 2operatorname{Re} overline{lambda}langle x, T^*xrangle + left|overline{lambda}right|^2|x|^2\
&= langle overline{lambda} x,overline{lambda}xrangle - 2operatorname{Re} langle overline{lambda}x, T^*xrangle+ langle T^*x,T^*xrangle\
&= leftlangle overline{lambda}x - T^*x,overline{lambda}x - T^*xrightrangle\
&= left|left(overline{lambda} I - T^*right)xright|^2\
&= left|(T-lambda I)^*xright|^2
end{align}
so $T - lambda I$ is normal.
$endgroup$
add a comment |
$begingroup$
You can also use the characterization that $A in B(H)$ is normal if and only if $|Ax| = |A^*x|, forall x in H$.
For $x in H$ we have
begin{align}
|(T - lambda I)x|^2 &= langle Tx - lambda x, Tx - lambda xrangle \
&= langle Tx,Txrangle - 2operatorname{Re} langle Tx, lambda xrangle + langle lambda x, lambda xrangle\
&= |Tx|^2 - 2operatorname{Re} overline{lambda}langle Tx, xrangle + |lambda|^2|x|^2\
&= |T^*x|^2 - 2operatorname{Re} overline{lambda}langle x, T^*xrangle + left|overline{lambda}right|^2|x|^2\
&= langle overline{lambda} x,overline{lambda}xrangle - 2operatorname{Re} langle overline{lambda}x, T^*xrangle+ langle T^*x,T^*xrangle\
&= leftlangle overline{lambda}x - T^*x,overline{lambda}x - T^*xrightrangle\
&= left|left(overline{lambda} I - T^*right)xright|^2\
&= left|(T-lambda I)^*xright|^2
end{align}
so $T - lambda I$ is normal.
$endgroup$
add a comment |
$begingroup$
You can also use the characterization that $A in B(H)$ is normal if and only if $|Ax| = |A^*x|, forall x in H$.
For $x in H$ we have
begin{align}
|(T - lambda I)x|^2 &= langle Tx - lambda x, Tx - lambda xrangle \
&= langle Tx,Txrangle - 2operatorname{Re} langle Tx, lambda xrangle + langle lambda x, lambda xrangle\
&= |Tx|^2 - 2operatorname{Re} overline{lambda}langle Tx, xrangle + |lambda|^2|x|^2\
&= |T^*x|^2 - 2operatorname{Re} overline{lambda}langle x, T^*xrangle + left|overline{lambda}right|^2|x|^2\
&= langle overline{lambda} x,overline{lambda}xrangle - 2operatorname{Re} langle overline{lambda}x, T^*xrangle+ langle T^*x,T^*xrangle\
&= leftlangle overline{lambda}x - T^*x,overline{lambda}x - T^*xrightrangle\
&= left|left(overline{lambda} I - T^*right)xright|^2\
&= left|(T-lambda I)^*xright|^2
end{align}
so $T - lambda I$ is normal.
$endgroup$
You can also use the characterization that $A in B(H)$ is normal if and only if $|Ax| = |A^*x|, forall x in H$.
For $x in H$ we have
begin{align}
|(T - lambda I)x|^2 &= langle Tx - lambda x, Tx - lambda xrangle \
&= langle Tx,Txrangle - 2operatorname{Re} langle Tx, lambda xrangle + langle lambda x, lambda xrangle\
&= |Tx|^2 - 2operatorname{Re} overline{lambda}langle Tx, xrangle + |lambda|^2|x|^2\
&= |T^*x|^2 - 2operatorname{Re} overline{lambda}langle x, T^*xrangle + left|overline{lambda}right|^2|x|^2\
&= langle overline{lambda} x,overline{lambda}xrangle - 2operatorname{Re} langle overline{lambda}x, T^*xrangle+ langle T^*x,T^*xrangle\
&= leftlangle overline{lambda}x - T^*x,overline{lambda}x - T^*xrightrangle\
&= left|left(overline{lambda} I - T^*right)xright|^2\
&= left|(T-lambda I)^*xright|^2
end{align}
so $T - lambda I$ is normal.
answered Dec 19 '18 at 20:30
mechanodroidmechanodroid
28.9k62548
28.9k62548
add a comment |
add a comment |
$begingroup$
$T$ is normal iff $T$ commutes with its adjoint $T^*$. If $T$ is normal, then $T^*$ commutes with $(T-lambda I)$ and $overline{lambda}I$ commutes with $(T-lambda I)$; hence $T^*-overline{lambda}I=(T-lambda I)^*$ commutes with $T-lambda I$, which makes $T-lambda I$ normal.
$endgroup$
add a comment |
$begingroup$
$T$ is normal iff $T$ commutes with its adjoint $T^*$. If $T$ is normal, then $T^*$ commutes with $(T-lambda I)$ and $overline{lambda}I$ commutes with $(T-lambda I)$; hence $T^*-overline{lambda}I=(T-lambda I)^*$ commutes with $T-lambda I$, which makes $T-lambda I$ normal.
$endgroup$
add a comment |
$begingroup$
$T$ is normal iff $T$ commutes with its adjoint $T^*$. If $T$ is normal, then $T^*$ commutes with $(T-lambda I)$ and $overline{lambda}I$ commutes with $(T-lambda I)$; hence $T^*-overline{lambda}I=(T-lambda I)^*$ commutes with $T-lambda I$, which makes $T-lambda I$ normal.
$endgroup$
$T$ is normal iff $T$ commutes with its adjoint $T^*$. If $T$ is normal, then $T^*$ commutes with $(T-lambda I)$ and $overline{lambda}I$ commutes with $(T-lambda I)$; hence $T^*-overline{lambda}I=(T-lambda I)^*$ commutes with $T-lambda I$, which makes $T-lambda I$ normal.
answered Dec 20 '18 at 2:46
DisintegratingByPartsDisintegratingByParts
59.8k42681
59.8k42681
add a comment |
add a comment |
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