Question on Hilbert Space
$begingroup$
$S$ is a non-empty subset of a Hilbert Space $H$, show that the set of all linear combinations of vectors in $S$ is dense in $HLeftrightarrow S^{bot}={0}$
I have been able to show $Rightarrow$ direction : Call the set of all linear combinations of vectors in $S$ as $S_1$ Say, $vin S^{bot}, exists$ a sequence $v_n$ in $S_1$ such that $v_nrightarrow v$ as $nrightarrowinfty$, $v_nbot v $
$forall ninmathbf{N}$ $therefore lim_{nrightarrowinfty}|v_n-v|^2=lim_{nrightarrowinfty} |v_n|^2+|v|^2=2|v|^2=0 Rightarrow v=0$
I am struggling with the other implication, We have $S^{botbot}=H$, need to show closure of $S_1$ is $S^{botbot}=H$
functional-analysis hilbert-spaces
$endgroup$
add a comment |
$begingroup$
$S$ is a non-empty subset of a Hilbert Space $H$, show that the set of all linear combinations of vectors in $S$ is dense in $HLeftrightarrow S^{bot}={0}$
I have been able to show $Rightarrow$ direction : Call the set of all linear combinations of vectors in $S$ as $S_1$ Say, $vin S^{bot}, exists$ a sequence $v_n$ in $S_1$ such that $v_nrightarrow v$ as $nrightarrowinfty$, $v_nbot v $
$forall ninmathbf{N}$ $therefore lim_{nrightarrowinfty}|v_n-v|^2=lim_{nrightarrowinfty} |v_n|^2+|v|^2=2|v|^2=0 Rightarrow v=0$
I am struggling with the other implication, We have $S^{botbot}=H$, need to show closure of $S_1$ is $S^{botbot}=H$
functional-analysis hilbert-spaces
$endgroup$
add a comment |
$begingroup$
$S$ is a non-empty subset of a Hilbert Space $H$, show that the set of all linear combinations of vectors in $S$ is dense in $HLeftrightarrow S^{bot}={0}$
I have been able to show $Rightarrow$ direction : Call the set of all linear combinations of vectors in $S$ as $S_1$ Say, $vin S^{bot}, exists$ a sequence $v_n$ in $S_1$ such that $v_nrightarrow v$ as $nrightarrowinfty$, $v_nbot v $
$forall ninmathbf{N}$ $therefore lim_{nrightarrowinfty}|v_n-v|^2=lim_{nrightarrowinfty} |v_n|^2+|v|^2=2|v|^2=0 Rightarrow v=0$
I am struggling with the other implication, We have $S^{botbot}=H$, need to show closure of $S_1$ is $S^{botbot}=H$
functional-analysis hilbert-spaces
$endgroup$
$S$ is a non-empty subset of a Hilbert Space $H$, show that the set of all linear combinations of vectors in $S$ is dense in $HLeftrightarrow S^{bot}={0}$
I have been able to show $Rightarrow$ direction : Call the set of all linear combinations of vectors in $S$ as $S_1$ Say, $vin S^{bot}, exists$ a sequence $v_n$ in $S_1$ such that $v_nrightarrow v$ as $nrightarrowinfty$, $v_nbot v $
$forall ninmathbf{N}$ $therefore lim_{nrightarrowinfty}|v_n-v|^2=lim_{nrightarrowinfty} |v_n|^2+|v|^2=2|v|^2=0 Rightarrow v=0$
I am struggling with the other implication, We have $S^{botbot}=H$, need to show closure of $S_1$ is $S^{botbot}=H$
functional-analysis hilbert-spaces
functional-analysis hilbert-spaces
edited Jan 2 at 18:53
mechanodroid
28.3k62548
28.3k62548
asked Jan 2 at 16:18
PSGPSG
3849
3849
add a comment |
add a comment |
2 Answers
2
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$begingroup$
Notice that $left(overline{operatorname{span}S}right)^perp = S^perp$. The Riesz projection theorem implies $$left(overline{operatorname{span}S}right)oplus left(overline{operatorname{span}S}right)^perp = H$$
so $$text{linear span of } S text{ is dense in } H iffoverline{operatorname{span}S} = H iff left(overline{operatorname{span}S}right)^perp = {0} iff S^perp = {0}$$
$endgroup$
add a comment |
$begingroup$
The adherence $bar S_1$ of $S_1$ is a vector subspace, if $S_1$ is not dense, there exists $x$ which is not in $bar{S_1}$. The linear form defined on $bar{S_1}oplusmathbb{C}x$ by $f(bar{S_1})=0, f(x)=1$ can be extended to $H$ by $g$ since it is continuous (Hahn Banach). Use te Riesz representation theorem to write $g=langle x_0,.rangle$. This implies that $x_0$ is in the orthogonal of $bar{S_1}$.
$endgroup$
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
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active
oldest
votes
$begingroup$
Notice that $left(overline{operatorname{span}S}right)^perp = S^perp$. The Riesz projection theorem implies $$left(overline{operatorname{span}S}right)oplus left(overline{operatorname{span}S}right)^perp = H$$
so $$text{linear span of } S text{ is dense in } H iffoverline{operatorname{span}S} = H iff left(overline{operatorname{span}S}right)^perp = {0} iff S^perp = {0}$$
$endgroup$
add a comment |
$begingroup$
Notice that $left(overline{operatorname{span}S}right)^perp = S^perp$. The Riesz projection theorem implies $$left(overline{operatorname{span}S}right)oplus left(overline{operatorname{span}S}right)^perp = H$$
so $$text{linear span of } S text{ is dense in } H iffoverline{operatorname{span}S} = H iff left(overline{operatorname{span}S}right)^perp = {0} iff S^perp = {0}$$
$endgroup$
add a comment |
$begingroup$
Notice that $left(overline{operatorname{span}S}right)^perp = S^perp$. The Riesz projection theorem implies $$left(overline{operatorname{span}S}right)oplus left(overline{operatorname{span}S}right)^perp = H$$
so $$text{linear span of } S text{ is dense in } H iffoverline{operatorname{span}S} = H iff left(overline{operatorname{span}S}right)^perp = {0} iff S^perp = {0}$$
$endgroup$
Notice that $left(overline{operatorname{span}S}right)^perp = S^perp$. The Riesz projection theorem implies $$left(overline{operatorname{span}S}right)oplus left(overline{operatorname{span}S}right)^perp = H$$
so $$text{linear span of } S text{ is dense in } H iffoverline{operatorname{span}S} = H iff left(overline{operatorname{span}S}right)^perp = {0} iff S^perp = {0}$$
answered Jan 2 at 18:52
mechanodroidmechanodroid
28.3k62548
28.3k62548
add a comment |
add a comment |
$begingroup$
The adherence $bar S_1$ of $S_1$ is a vector subspace, if $S_1$ is not dense, there exists $x$ which is not in $bar{S_1}$. The linear form defined on $bar{S_1}oplusmathbb{C}x$ by $f(bar{S_1})=0, f(x)=1$ can be extended to $H$ by $g$ since it is continuous (Hahn Banach). Use te Riesz representation theorem to write $g=langle x_0,.rangle$. This implies that $x_0$ is in the orthogonal of $bar{S_1}$.
$endgroup$
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
add a comment |
$begingroup$
The adherence $bar S_1$ of $S_1$ is a vector subspace, if $S_1$ is not dense, there exists $x$ which is not in $bar{S_1}$. The linear form defined on $bar{S_1}oplusmathbb{C}x$ by $f(bar{S_1})=0, f(x)=1$ can be extended to $H$ by $g$ since it is continuous (Hahn Banach). Use te Riesz representation theorem to write $g=langle x_0,.rangle$. This implies that $x_0$ is in the orthogonal of $bar{S_1}$.
$endgroup$
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
add a comment |
$begingroup$
The adherence $bar S_1$ of $S_1$ is a vector subspace, if $S_1$ is not dense, there exists $x$ which is not in $bar{S_1}$. The linear form defined on $bar{S_1}oplusmathbb{C}x$ by $f(bar{S_1})=0, f(x)=1$ can be extended to $H$ by $g$ since it is continuous (Hahn Banach). Use te Riesz representation theorem to write $g=langle x_0,.rangle$. This implies that $x_0$ is in the orthogonal of $bar{S_1}$.
$endgroup$
The adherence $bar S_1$ of $S_1$ is a vector subspace, if $S_1$ is not dense, there exists $x$ which is not in $bar{S_1}$. The linear form defined on $bar{S_1}oplusmathbb{C}x$ by $f(bar{S_1})=0, f(x)=1$ can be extended to $H$ by $g$ since it is continuous (Hahn Banach). Use te Riesz representation theorem to write $g=langle x_0,.rangle$. This implies that $x_0$ is in the orthogonal of $bar{S_1}$.
answered Jan 2 at 16:34
Tsemo AristideTsemo Aristide
59k11445
59k11445
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
add a comment |
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
Thanks for the answer, actually I am not supposed to use Hahn banach for this question, I got an idea from your solution, please hear me out. If $bar{S_1} neq H$, then $exists$ non-zero $vin bar {S_1} ^{bot} $, but $S_1 subseteq bar {S_1}Rightarrow S_1^{bot}supseteq bar{S_1}^{bot} $ Now I want to say that $S^{bot}=0Rightarrow S_1 ^{bot}=0$, so $bar{S_1}^{bot} =0$ , is this correct : $S^{bot}=0Rightarrow S_1 ^{bot}=0$
$endgroup$
– PSG
Jan 2 at 16:50
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
how you know the existence of $v$ ?
$endgroup$
– Tsemo Aristide
Jan 2 at 16:52
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
A theorem says that : If $M$ is a proper closed linear subspace of a Hilbert space $H$, then $exists$ a non zero vector $z_0$ in $H$ such that $z_0bot M$
$endgroup$
– PSG
Jan 2 at 16:54
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
$begingroup$
that's true I use Hahn Banach and Riez to show that theorem, you can show it also by considering a point which realize $d(x,bar{S_1})$.
$endgroup$
– Tsemo Aristide
Jan 2 at 16:56
add a comment |
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