$a=cc^*c$ for some $c$. $a in A$ a $C^*$ algebra. The Next CEO of Stack OverflowSome examples...
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$a=cc^*c$ for some $c$. $a in A$ a $C^*$ algebra.
The Next CEO of Stack OverflowSome examples in C* algebras and Banach * algebrasMap to multiplier algebra for C*-subalgebraIntegration with values in a $C^*$-algebraShow the algebra of all bouned operators is a $W^*$-algebraInfinite dimensional C*-algebra contains infinite dimensional commutitive subalgebraExistence of state on a C*-algebra satisfying $|tau(ab)|=|ab|$Nonunital C*-Algebra: Proper IdealsAn equivalent condition for a cyclic von Neumann algebraA question on positive elements bigger than $1_A$ in a unital C*-algebra $A$Increasing continuous function preserves inequality for elements of $C^*$-algebra
$begingroup$
Let $A$ be a $C^*$ algebra. Let $a in A$, then there exists $c in A$ such that $a=cc^*c$.
This fact is used from example (1) of Prop 4.25. How does one show this?
functional-analysis operator-algebras adjoint-operators
$endgroup$
add a comment |
$begingroup$
Let $A$ be a $C^*$ algebra. Let $a in A$, then there exists $c in A$ such that $a=cc^*c$.
This fact is used from example (1) of Prop 4.25. How does one show this?
functional-analysis operator-algebras adjoint-operators
$endgroup$
$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36
add a comment |
$begingroup$
Let $A$ be a $C^*$ algebra. Let $a in A$, then there exists $c in A$ such that $a=cc^*c$.
This fact is used from example (1) of Prop 4.25. How does one show this?
functional-analysis operator-algebras adjoint-operators
$endgroup$
Let $A$ be a $C^*$ algebra. Let $a in A$, then there exists $c in A$ such that $a=cc^*c$.
This fact is used from example (1) of Prop 4.25. How does one show this?
functional-analysis operator-algebras adjoint-operators
functional-analysis operator-algebras adjoint-operators
asked Mar 16 at 17:31
CL.CL.
2,3452925
2,3452925
$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36
add a comment |
$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36
$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
$a^*a ge 0$ so the function $t mapsto frac{t^{1/6}}{sqrt{t+frac1n}}$ is continuous on $sigma(a^*a) subseteq [0,infty]$ and satisfies $0mapsto 0$.
Thus it makes sense to define a sequence $(c_n)_n$ as $$c_n = aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6} in A$$
Denote $d_{mn} = left(a^*a + frac1mright)^{-1/2} - left(a^*a + frac1nright)^{-1/2}$ for $m,ninmathbb{N}$. We have
begin{align}
|c_m - c_n|^2 &= |a ,d_{mn}(a^*a)^{1/6}|^2 \
&= |(a^*a)^{1/6}d_{mn}(a^*a)d_{mn}(a^*a)^{1/6}|\
&= |{d_{mn}}^2(a^*a)^{4/3}|\
&= |{d_{mn}}(a^*a)^{2/3}|^2\
&= left|left(a^*a + frac1mright)^{-1/2}(a^*a)^{2/3} - left(a^*a + frac1nright)^{-1/2}(a^*a)^{2/3}right|^2\
&xrightarrow{m,ntoinfty} |(a^*a)^{1/6}-(a^*a)^{1/6}|^2 \
&= 0
end{align}
because $$f_n(t) = frac{t^{2/3}}{sqrt{t+frac1n}} xrightarrow{ntoinfty} t^{1/6}$$ uniformly on $sigma(a^*a)$.
Hence $(c_n)_n$ converges to an element $c in A$. We claim that $c$ is the desired element.
begin{align}
cc^*c &= lim_{ntoinfty} c_nc_n^*c_n \
&= lim_{ntoinfty} aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}(a^*a)^{1/6}left(a^*a + frac1nright)^{-1/2}(a^*a)left(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}\
&= lim_{ntoinfty} acdot (a^*a)^{3/2}left(a^*a + frac1nright)^{-3/2}\
&= a cdot 1\
&= a
end{align}
because $$g_n(t) = frac{t^{3/2}}{left(t+frac1nright)^{3/2}} xrightarrow{ntoinfty} 1$$ uniformly on $sigma(a^*a)$. The limit function $1$ does not satisfy $0 mapsto 0$ but we can adjoin a unit to the algebra $A$ so that it makes sense.
Therefore $a = cc^*c$.
This proof was mostly from Pedersen's $C^*$-algebras and their Automorphism Groups, Lemma $1.4.4.$ and $1.4.5.$
$endgroup$
add a comment |
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1 Answer
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1 Answer
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$begingroup$
$a^*a ge 0$ so the function $t mapsto frac{t^{1/6}}{sqrt{t+frac1n}}$ is continuous on $sigma(a^*a) subseteq [0,infty]$ and satisfies $0mapsto 0$.
Thus it makes sense to define a sequence $(c_n)_n$ as $$c_n = aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6} in A$$
Denote $d_{mn} = left(a^*a + frac1mright)^{-1/2} - left(a^*a + frac1nright)^{-1/2}$ for $m,ninmathbb{N}$. We have
begin{align}
|c_m - c_n|^2 &= |a ,d_{mn}(a^*a)^{1/6}|^2 \
&= |(a^*a)^{1/6}d_{mn}(a^*a)d_{mn}(a^*a)^{1/6}|\
&= |{d_{mn}}^2(a^*a)^{4/3}|\
&= |{d_{mn}}(a^*a)^{2/3}|^2\
&= left|left(a^*a + frac1mright)^{-1/2}(a^*a)^{2/3} - left(a^*a + frac1nright)^{-1/2}(a^*a)^{2/3}right|^2\
&xrightarrow{m,ntoinfty} |(a^*a)^{1/6}-(a^*a)^{1/6}|^2 \
&= 0
end{align}
because $$f_n(t) = frac{t^{2/3}}{sqrt{t+frac1n}} xrightarrow{ntoinfty} t^{1/6}$$ uniformly on $sigma(a^*a)$.
Hence $(c_n)_n$ converges to an element $c in A$. We claim that $c$ is the desired element.
begin{align}
cc^*c &= lim_{ntoinfty} c_nc_n^*c_n \
&= lim_{ntoinfty} aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}(a^*a)^{1/6}left(a^*a + frac1nright)^{-1/2}(a^*a)left(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}\
&= lim_{ntoinfty} acdot (a^*a)^{3/2}left(a^*a + frac1nright)^{-3/2}\
&= a cdot 1\
&= a
end{align}
because $$g_n(t) = frac{t^{3/2}}{left(t+frac1nright)^{3/2}} xrightarrow{ntoinfty} 1$$ uniformly on $sigma(a^*a)$. The limit function $1$ does not satisfy $0 mapsto 0$ but we can adjoin a unit to the algebra $A$ so that it makes sense.
Therefore $a = cc^*c$.
This proof was mostly from Pedersen's $C^*$-algebras and their Automorphism Groups, Lemma $1.4.4.$ and $1.4.5.$
$endgroup$
add a comment |
$begingroup$
$a^*a ge 0$ so the function $t mapsto frac{t^{1/6}}{sqrt{t+frac1n}}$ is continuous on $sigma(a^*a) subseteq [0,infty]$ and satisfies $0mapsto 0$.
Thus it makes sense to define a sequence $(c_n)_n$ as $$c_n = aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6} in A$$
Denote $d_{mn} = left(a^*a + frac1mright)^{-1/2} - left(a^*a + frac1nright)^{-1/2}$ for $m,ninmathbb{N}$. We have
begin{align}
|c_m - c_n|^2 &= |a ,d_{mn}(a^*a)^{1/6}|^2 \
&= |(a^*a)^{1/6}d_{mn}(a^*a)d_{mn}(a^*a)^{1/6}|\
&= |{d_{mn}}^2(a^*a)^{4/3}|\
&= |{d_{mn}}(a^*a)^{2/3}|^2\
&= left|left(a^*a + frac1mright)^{-1/2}(a^*a)^{2/3} - left(a^*a + frac1nright)^{-1/2}(a^*a)^{2/3}right|^2\
&xrightarrow{m,ntoinfty} |(a^*a)^{1/6}-(a^*a)^{1/6}|^2 \
&= 0
end{align}
because $$f_n(t) = frac{t^{2/3}}{sqrt{t+frac1n}} xrightarrow{ntoinfty} t^{1/6}$$ uniformly on $sigma(a^*a)$.
Hence $(c_n)_n$ converges to an element $c in A$. We claim that $c$ is the desired element.
begin{align}
cc^*c &= lim_{ntoinfty} c_nc_n^*c_n \
&= lim_{ntoinfty} aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}(a^*a)^{1/6}left(a^*a + frac1nright)^{-1/2}(a^*a)left(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}\
&= lim_{ntoinfty} acdot (a^*a)^{3/2}left(a^*a + frac1nright)^{-3/2}\
&= a cdot 1\
&= a
end{align}
because $$g_n(t) = frac{t^{3/2}}{left(t+frac1nright)^{3/2}} xrightarrow{ntoinfty} 1$$ uniformly on $sigma(a^*a)$. The limit function $1$ does not satisfy $0 mapsto 0$ but we can adjoin a unit to the algebra $A$ so that it makes sense.
Therefore $a = cc^*c$.
This proof was mostly from Pedersen's $C^*$-algebras and their Automorphism Groups, Lemma $1.4.4.$ and $1.4.5.$
$endgroup$
add a comment |
$begingroup$
$a^*a ge 0$ so the function $t mapsto frac{t^{1/6}}{sqrt{t+frac1n}}$ is continuous on $sigma(a^*a) subseteq [0,infty]$ and satisfies $0mapsto 0$.
Thus it makes sense to define a sequence $(c_n)_n$ as $$c_n = aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6} in A$$
Denote $d_{mn} = left(a^*a + frac1mright)^{-1/2} - left(a^*a + frac1nright)^{-1/2}$ for $m,ninmathbb{N}$. We have
begin{align}
|c_m - c_n|^2 &= |a ,d_{mn}(a^*a)^{1/6}|^2 \
&= |(a^*a)^{1/6}d_{mn}(a^*a)d_{mn}(a^*a)^{1/6}|\
&= |{d_{mn}}^2(a^*a)^{4/3}|\
&= |{d_{mn}}(a^*a)^{2/3}|^2\
&= left|left(a^*a + frac1mright)^{-1/2}(a^*a)^{2/3} - left(a^*a + frac1nright)^{-1/2}(a^*a)^{2/3}right|^2\
&xrightarrow{m,ntoinfty} |(a^*a)^{1/6}-(a^*a)^{1/6}|^2 \
&= 0
end{align}
because $$f_n(t) = frac{t^{2/3}}{sqrt{t+frac1n}} xrightarrow{ntoinfty} t^{1/6}$$ uniformly on $sigma(a^*a)$.
Hence $(c_n)_n$ converges to an element $c in A$. We claim that $c$ is the desired element.
begin{align}
cc^*c &= lim_{ntoinfty} c_nc_n^*c_n \
&= lim_{ntoinfty} aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}(a^*a)^{1/6}left(a^*a + frac1nright)^{-1/2}(a^*a)left(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}\
&= lim_{ntoinfty} acdot (a^*a)^{3/2}left(a^*a + frac1nright)^{-3/2}\
&= a cdot 1\
&= a
end{align}
because $$g_n(t) = frac{t^{3/2}}{left(t+frac1nright)^{3/2}} xrightarrow{ntoinfty} 1$$ uniformly on $sigma(a^*a)$. The limit function $1$ does not satisfy $0 mapsto 0$ but we can adjoin a unit to the algebra $A$ so that it makes sense.
Therefore $a = cc^*c$.
This proof was mostly from Pedersen's $C^*$-algebras and their Automorphism Groups, Lemma $1.4.4.$ and $1.4.5.$
$endgroup$
$a^*a ge 0$ so the function $t mapsto frac{t^{1/6}}{sqrt{t+frac1n}}$ is continuous on $sigma(a^*a) subseteq [0,infty]$ and satisfies $0mapsto 0$.
Thus it makes sense to define a sequence $(c_n)_n$ as $$c_n = aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6} in A$$
Denote $d_{mn} = left(a^*a + frac1mright)^{-1/2} - left(a^*a + frac1nright)^{-1/2}$ for $m,ninmathbb{N}$. We have
begin{align}
|c_m - c_n|^2 &= |a ,d_{mn}(a^*a)^{1/6}|^2 \
&= |(a^*a)^{1/6}d_{mn}(a^*a)d_{mn}(a^*a)^{1/6}|\
&= |{d_{mn}}^2(a^*a)^{4/3}|\
&= |{d_{mn}}(a^*a)^{2/3}|^2\
&= left|left(a^*a + frac1mright)^{-1/2}(a^*a)^{2/3} - left(a^*a + frac1nright)^{-1/2}(a^*a)^{2/3}right|^2\
&xrightarrow{m,ntoinfty} |(a^*a)^{1/6}-(a^*a)^{1/6}|^2 \
&= 0
end{align}
because $$f_n(t) = frac{t^{2/3}}{sqrt{t+frac1n}} xrightarrow{ntoinfty} t^{1/6}$$ uniformly on $sigma(a^*a)$.
Hence $(c_n)_n$ converges to an element $c in A$. We claim that $c$ is the desired element.
begin{align}
cc^*c &= lim_{ntoinfty} c_nc_n^*c_n \
&= lim_{ntoinfty} aleft(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}(a^*a)^{1/6}left(a^*a + frac1nright)^{-1/2}(a^*a)left(a^*a + frac1nright)^{-1/2}(a^*a)^{1/6}\
&= lim_{ntoinfty} acdot (a^*a)^{3/2}left(a^*a + frac1nright)^{-3/2}\
&= a cdot 1\
&= a
end{align}
because $$g_n(t) = frac{t^{3/2}}{left(t+frac1nright)^{3/2}} xrightarrow{ntoinfty} 1$$ uniformly on $sigma(a^*a)$. The limit function $1$ does not satisfy $0 mapsto 0$ but we can adjoin a unit to the algebra $A$ so that it makes sense.
Therefore $a = cc^*c$.
This proof was mostly from Pedersen's $C^*$-algebras and their Automorphism Groups, Lemma $1.4.4.$ and $1.4.5.$
answered Mar 16 at 21:58
mechanodroidmechanodroid
29k62648
29k62648
add a comment |
add a comment |
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$begingroup$
The construction of $c$ is sketched in the proof of Prop. 4.25. Where do you have problems?
$endgroup$
– MaoWao
Mar 16 at 18:17
$begingroup$
Ah, ok, I did not read the proof, and jumped to the example for some intuition.
$endgroup$
– CL.
Mar 16 at 18:36