Gelfand-Naimark Theorem of non-unital case
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Let $X$ be a non-compact, locally compact Hausdorff space. Then $C_0 (X)$, the space of complex valued continuous functions on X vanishing at infinity, is a non-unital $C^*$-algebra and $Omega (C_0 (X))$, the space of linear functionals $T$ on $C_0 (X)$ satisfying $T(fg)=T(f)T(g)$ for all $f,g in C_0 (X)$, is a locally compact Hausdorff space with respect to weak${}^*$ topology in $C_0 (X)^*$.
Let $x in X$. Define $hat{x}(f)=f(x)$ for all $f in C_0 (X)$.
Then $hat{x} in Omega (C_0 (X))$.
I know the map from $X$ to $Omega (C_0 (X))$, $x mapsto hat{x}$, is bijective and continuous. How is the proof that the map is open map?
general-topology operator-algebras
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Let $X$ be a non-compact, locally compact Hausdorff space. Then $C_0 (X)$, the space of complex valued continuous functions on X vanishing at infinity, is a non-unital $C^*$-algebra and $Omega (C_0 (X))$, the space of linear functionals $T$ on $C_0 (X)$ satisfying $T(fg)=T(f)T(g)$ for all $f,g in C_0 (X)$, is a locally compact Hausdorff space with respect to weak${}^*$ topology in $C_0 (X)^*$.
Let $x in X$. Define $hat{x}(f)=f(x)$ for all $f in C_0 (X)$.
Then $hat{x} in Omega (C_0 (X))$.
I know the map from $X$ to $Omega (C_0 (X))$, $x mapsto hat{x}$, is bijective and continuous. How is the proof that the map is open map?
general-topology operator-algebras
add a comment |
up vote
1
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favorite
up vote
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down vote
favorite
Let $X$ be a non-compact, locally compact Hausdorff space. Then $C_0 (X)$, the space of complex valued continuous functions on X vanishing at infinity, is a non-unital $C^*$-algebra and $Omega (C_0 (X))$, the space of linear functionals $T$ on $C_0 (X)$ satisfying $T(fg)=T(f)T(g)$ for all $f,g in C_0 (X)$, is a locally compact Hausdorff space with respect to weak${}^*$ topology in $C_0 (X)^*$.
Let $x in X$. Define $hat{x}(f)=f(x)$ for all $f in C_0 (X)$.
Then $hat{x} in Omega (C_0 (X))$.
I know the map from $X$ to $Omega (C_0 (X))$, $x mapsto hat{x}$, is bijective and continuous. How is the proof that the map is open map?
general-topology operator-algebras
Let $X$ be a non-compact, locally compact Hausdorff space. Then $C_0 (X)$, the space of complex valued continuous functions on X vanishing at infinity, is a non-unital $C^*$-algebra and $Omega (C_0 (X))$, the space of linear functionals $T$ on $C_0 (X)$ satisfying $T(fg)=T(f)T(g)$ for all $f,g in C_0 (X)$, is a locally compact Hausdorff space with respect to weak${}^*$ topology in $C_0 (X)^*$.
Let $x in X$. Define $hat{x}(f)=f(x)$ for all $f in C_0 (X)$.
Then $hat{x} in Omega (C_0 (X))$.
I know the map from $X$ to $Omega (C_0 (X))$, $x mapsto hat{x}$, is bijective and continuous. How is the proof that the map is open map?
general-topology operator-algebras
general-topology operator-algebras
edited Nov 19 at 16:54
Aweygan
13.3k21441
13.3k21441
asked Nov 19 at 15:19
Ichiko
423
423
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1 Answer
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Let us write $F:Xto Omega(C_0(X))$ for the map $F(x)=hat{x}$. Let $Usubseteq X$ be open and $xin U$. Let $V$ be open such that $xin Vsubseteq overline{V}subseteq U$ and $overline{V}$ is compact. By Urysohn's lemma, there exists a continuous function $f:Xto [0,1]$ such that $f(x)=1$ and $f(y)=0$ for all $yin Xsetminus V$. Since $overline{V}$ is compact, $fin C_0(X)$. We now see that ${hat{y}inOmega(C_0(X)):hat{y}(f)neq 0}$ is an open subset of $Omega(C_0(X))$ which contains $hat{x}$ and is contained in $F(U)$. Since $xin U$ was arbitrary, this means $F(U)$ is open, and so $F$ is an open map.
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
1
down vote
accepted
Let us write $F:Xto Omega(C_0(X))$ for the map $F(x)=hat{x}$. Let $Usubseteq X$ be open and $xin U$. Let $V$ be open such that $xin Vsubseteq overline{V}subseteq U$ and $overline{V}$ is compact. By Urysohn's lemma, there exists a continuous function $f:Xto [0,1]$ such that $f(x)=1$ and $f(y)=0$ for all $yin Xsetminus V$. Since $overline{V}$ is compact, $fin C_0(X)$. We now see that ${hat{y}inOmega(C_0(X)):hat{y}(f)neq 0}$ is an open subset of $Omega(C_0(X))$ which contains $hat{x}$ and is contained in $F(U)$. Since $xin U$ was arbitrary, this means $F(U)$ is open, and so $F$ is an open map.
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
add a comment |
up vote
1
down vote
accepted
Let us write $F:Xto Omega(C_0(X))$ for the map $F(x)=hat{x}$. Let $Usubseteq X$ be open and $xin U$. Let $V$ be open such that $xin Vsubseteq overline{V}subseteq U$ and $overline{V}$ is compact. By Urysohn's lemma, there exists a continuous function $f:Xto [0,1]$ such that $f(x)=1$ and $f(y)=0$ for all $yin Xsetminus V$. Since $overline{V}$ is compact, $fin C_0(X)$. We now see that ${hat{y}inOmega(C_0(X)):hat{y}(f)neq 0}$ is an open subset of $Omega(C_0(X))$ which contains $hat{x}$ and is contained in $F(U)$. Since $xin U$ was arbitrary, this means $F(U)$ is open, and so $F$ is an open map.
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
add a comment |
up vote
1
down vote
accepted
up vote
1
down vote
accepted
Let us write $F:Xto Omega(C_0(X))$ for the map $F(x)=hat{x}$. Let $Usubseteq X$ be open and $xin U$. Let $V$ be open such that $xin Vsubseteq overline{V}subseteq U$ and $overline{V}$ is compact. By Urysohn's lemma, there exists a continuous function $f:Xto [0,1]$ such that $f(x)=1$ and $f(y)=0$ for all $yin Xsetminus V$. Since $overline{V}$ is compact, $fin C_0(X)$. We now see that ${hat{y}inOmega(C_0(X)):hat{y}(f)neq 0}$ is an open subset of $Omega(C_0(X))$ which contains $hat{x}$ and is contained in $F(U)$. Since $xin U$ was arbitrary, this means $F(U)$ is open, and so $F$ is an open map.
Let us write $F:Xto Omega(C_0(X))$ for the map $F(x)=hat{x}$. Let $Usubseteq X$ be open and $xin U$. Let $V$ be open such that $xin Vsubseteq overline{V}subseteq U$ and $overline{V}$ is compact. By Urysohn's lemma, there exists a continuous function $f:Xto [0,1]$ such that $f(x)=1$ and $f(y)=0$ for all $yin Xsetminus V$. Since $overline{V}$ is compact, $fin C_0(X)$. We now see that ${hat{y}inOmega(C_0(X)):hat{y}(f)neq 0}$ is an open subset of $Omega(C_0(X))$ which contains $hat{x}$ and is contained in $F(U)$. Since $xin U$ was arbitrary, this means $F(U)$ is open, and so $F$ is an open map.
edited Nov 19 at 20:00
answered Nov 19 at 17:32
Eric Wofsey
177k12202328
177k12202328
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
add a comment |
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Thank you. But does ${ hat{y} in Omega (C_0 (X)) colon |hat{y}(f)|<1 }$ really contain $hat{x}$ and is it really contained in $F(U)$? I think $hat{x}(f)=1$.
– Ichiko
Nov 19 at 19:59
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
Oops, it should be $>0$ instead of $<1$. I'll fix that.
– Eric Wofsey
Nov 19 at 20:00
add a comment |
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