$a=cc^*c$ 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












1












$begingroup$



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?










share|cite|improve this question









$endgroup$












  • $begingroup$
    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
















1












$begingroup$



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?










share|cite|improve this question









$endgroup$












  • $begingroup$
    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














1












1








1





$begingroup$



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?










share|cite|improve this question









$endgroup$





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






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










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


















  • $begingroup$
    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










1 Answer
1






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1












$begingroup$

$a^*a ge 0$ 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 $1.4.5.$






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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 $1.4.5.$






    share|cite|improve this answer









    $endgroup$


















      1












      $begingroup$

      $a^*a ge 0$ 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 $1.4.5.$






      share|cite|improve this answer









      $endgroup$
















        1












        1








        1





        $begingroup$

        $a^*a ge 0$ 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 $1.4.5.$






        share|cite|improve this answer









        $endgroup$



        $a^*a ge 0$ 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 $1.4.5.$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 16 at 21:58









        mechanodroidmechanodroid

        29k62648




        29k62648






























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