Non-unique Hahn Banach extension for a functional defined on a closed subspace of $l^1$
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I'm trying to solve the following exercise, but I'm stuck on the possible conclusion.
Let $Y={ {x_k}_k in l^1: x_{2015} = 0 } subset l^1, quad T in Y'$. Characterize all the linear and bounded extensions $Phi in (l^1)'$ which preserves the dual norm.
I've tried so far to use the duality and write $Phi: l^1 rightarrow mathbb{R}$ as $Phi(x)= sum_{k=1}^{infty} x_k c_k$ where ${c_k } in l^{infty}$, but I don't know how to get equality of dual norms.
sequences-and-series functional-analysis duality-theorems
$endgroup$
add a comment |
$begingroup$
I'm trying to solve the following exercise, but I'm stuck on the possible conclusion.
Let $Y={ {x_k}_k in l^1: x_{2015} = 0 } subset l^1, quad T in Y'$. Characterize all the linear and bounded extensions $Phi in (l^1)'$ which preserves the dual norm.
I've tried so far to use the duality and write $Phi: l^1 rightarrow mathbb{R}$ as $Phi(x)= sum_{k=1}^{infty} x_k c_k$ where ${c_k } in l^{infty}$, but I don't know how to get equality of dual norms.
sequences-and-series functional-analysis duality-theorems
$endgroup$
$begingroup$
Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
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– SmileyCraft
Dec 19 '18 at 17:15
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I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
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So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
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You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43
add a comment |
$begingroup$
I'm trying to solve the following exercise, but I'm stuck on the possible conclusion.
Let $Y={ {x_k}_k in l^1: x_{2015} = 0 } subset l^1, quad T in Y'$. Characterize all the linear and bounded extensions $Phi in (l^1)'$ which preserves the dual norm.
I've tried so far to use the duality and write $Phi: l^1 rightarrow mathbb{R}$ as $Phi(x)= sum_{k=1}^{infty} x_k c_k$ where ${c_k } in l^{infty}$, but I don't know how to get equality of dual norms.
sequences-and-series functional-analysis duality-theorems
$endgroup$
I'm trying to solve the following exercise, but I'm stuck on the possible conclusion.
Let $Y={ {x_k}_k in l^1: x_{2015} = 0 } subset l^1, quad T in Y'$. Characterize all the linear and bounded extensions $Phi in (l^1)'$ which preserves the dual norm.
I've tried so far to use the duality and write $Phi: l^1 rightarrow mathbb{R}$ as $Phi(x)= sum_{k=1}^{infty} x_k c_k$ where ${c_k } in l^{infty}$, but I don't know how to get equality of dual norms.
sequences-and-series functional-analysis duality-theorems
sequences-and-series functional-analysis duality-theorems
asked Dec 19 '18 at 16:27
VoBVoB
734413
734413
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Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
$endgroup$
– SmileyCraft
Dec 19 '18 at 17:15
$begingroup$
I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
$begingroup$
So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
$begingroup$
You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43
add a comment |
$begingroup$
Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
$endgroup$
– SmileyCraft
Dec 19 '18 at 17:15
$begingroup$
I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
$begingroup$
So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
$begingroup$
You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43
$begingroup$
Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
$endgroup$
– SmileyCraft
Dec 19 '18 at 17:15
$begingroup$
Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
$endgroup$
– SmileyCraft
Dec 19 '18 at 17:15
$begingroup$
I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
$begingroup$
I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
$begingroup$
So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
$begingroup$
So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
$begingroup$
You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43
add a comment |
1 Answer
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Notice that $|T| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Indeed, if $T$ is bounded then in particular $(Te_n)_n$ is a bounded sequence in the field and for $x = sum_{substack{n=1\ nne 2015}}^infty x_ne_n in Y$ we have
$$|Tx| = left|sum_{substack{n=1\ nne 2015}}^infty x_n Te_nright| le sum_{substack{n=1\ nne 2015}}^infty |x_n||Te_n| le left(sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|right) |x|_1$$
so $|T| le sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Conversely, for all $n in mathbb{N}, n ne 2015$ we have
$$|T| ge frac{|Te_n|}{|e_n|_1} = |Te_n|$$
so $|T| ge sup_{ninmathbb{N}, nne 2015} |Te_n|$.
Similarly (or using the $(ell^1)' leftrightarrow ell^infty$ duality) we get that $|Phi| = sup_{ninmathbb{N}} |Phi e_n|$.
Now, since $Phi$ extends $T$, for every $n in mathbb{N}, n ne 2015$ it follows that $Te_n = Phi e_n$.
Now we have
$$sup_{substack{ninmathbb{N}\ nne 2015}} |Phi e_n| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n| = |T| = |Phi| = sup_{ninmathbb{N}} |Phi e_n|$$
so $|Phi e_{2015}| le |T|$.
We also see that $Phi e_{n} = Te_n, forall n ne 2015$ and $|Phi e_{2015}| le |T|$ is a sufficient condition that $Phi in (ell^1)'$ is a Hahn-Banach extension of $T$.
$endgroup$
add a comment |
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1 Answer
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1 Answer
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active
oldest
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votes
$begingroup$
Notice that $|T| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Indeed, if $T$ is bounded then in particular $(Te_n)_n$ is a bounded sequence in the field and for $x = sum_{substack{n=1\ nne 2015}}^infty x_ne_n in Y$ we have
$$|Tx| = left|sum_{substack{n=1\ nne 2015}}^infty x_n Te_nright| le sum_{substack{n=1\ nne 2015}}^infty |x_n||Te_n| le left(sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|right) |x|_1$$
so $|T| le sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Conversely, for all $n in mathbb{N}, n ne 2015$ we have
$$|T| ge frac{|Te_n|}{|e_n|_1} = |Te_n|$$
so $|T| ge sup_{ninmathbb{N}, nne 2015} |Te_n|$.
Similarly (or using the $(ell^1)' leftrightarrow ell^infty$ duality) we get that $|Phi| = sup_{ninmathbb{N}} |Phi e_n|$.
Now, since $Phi$ extends $T$, for every $n in mathbb{N}, n ne 2015$ it follows that $Te_n = Phi e_n$.
Now we have
$$sup_{substack{ninmathbb{N}\ nne 2015}} |Phi e_n| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n| = |T| = |Phi| = sup_{ninmathbb{N}} |Phi e_n|$$
so $|Phi e_{2015}| le |T|$.
We also see that $Phi e_{n} = Te_n, forall n ne 2015$ and $|Phi e_{2015}| le |T|$ is a sufficient condition that $Phi in (ell^1)'$ is a Hahn-Banach extension of $T$.
$endgroup$
add a comment |
$begingroup$
Notice that $|T| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Indeed, if $T$ is bounded then in particular $(Te_n)_n$ is a bounded sequence in the field and for $x = sum_{substack{n=1\ nne 2015}}^infty x_ne_n in Y$ we have
$$|Tx| = left|sum_{substack{n=1\ nne 2015}}^infty x_n Te_nright| le sum_{substack{n=1\ nne 2015}}^infty |x_n||Te_n| le left(sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|right) |x|_1$$
so $|T| le sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Conversely, for all $n in mathbb{N}, n ne 2015$ we have
$$|T| ge frac{|Te_n|}{|e_n|_1} = |Te_n|$$
so $|T| ge sup_{ninmathbb{N}, nne 2015} |Te_n|$.
Similarly (or using the $(ell^1)' leftrightarrow ell^infty$ duality) we get that $|Phi| = sup_{ninmathbb{N}} |Phi e_n|$.
Now, since $Phi$ extends $T$, for every $n in mathbb{N}, n ne 2015$ it follows that $Te_n = Phi e_n$.
Now we have
$$sup_{substack{ninmathbb{N}\ nne 2015}} |Phi e_n| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n| = |T| = |Phi| = sup_{ninmathbb{N}} |Phi e_n|$$
so $|Phi e_{2015}| le |T|$.
We also see that $Phi e_{n} = Te_n, forall n ne 2015$ and $|Phi e_{2015}| le |T|$ is a sufficient condition that $Phi in (ell^1)'$ is a Hahn-Banach extension of $T$.
$endgroup$
add a comment |
$begingroup$
Notice that $|T| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Indeed, if $T$ is bounded then in particular $(Te_n)_n$ is a bounded sequence in the field and for $x = sum_{substack{n=1\ nne 2015}}^infty x_ne_n in Y$ we have
$$|Tx| = left|sum_{substack{n=1\ nne 2015}}^infty x_n Te_nright| le sum_{substack{n=1\ nne 2015}}^infty |x_n||Te_n| le left(sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|right) |x|_1$$
so $|T| le sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Conversely, for all $n in mathbb{N}, n ne 2015$ we have
$$|T| ge frac{|Te_n|}{|e_n|_1} = |Te_n|$$
so $|T| ge sup_{ninmathbb{N}, nne 2015} |Te_n|$.
Similarly (or using the $(ell^1)' leftrightarrow ell^infty$ duality) we get that $|Phi| = sup_{ninmathbb{N}} |Phi e_n|$.
Now, since $Phi$ extends $T$, for every $n in mathbb{N}, n ne 2015$ it follows that $Te_n = Phi e_n$.
Now we have
$$sup_{substack{ninmathbb{N}\ nne 2015}} |Phi e_n| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n| = |T| = |Phi| = sup_{ninmathbb{N}} |Phi e_n|$$
so $|Phi e_{2015}| le |T|$.
We also see that $Phi e_{n} = Te_n, forall n ne 2015$ and $|Phi e_{2015}| le |T|$ is a sufficient condition that $Phi in (ell^1)'$ is a Hahn-Banach extension of $T$.
$endgroup$
Notice that $|T| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Indeed, if $T$ is bounded then in particular $(Te_n)_n$ is a bounded sequence in the field and for $x = sum_{substack{n=1\ nne 2015}}^infty x_ne_n in Y$ we have
$$|Tx| = left|sum_{substack{n=1\ nne 2015}}^infty x_n Te_nright| le sum_{substack{n=1\ nne 2015}}^infty |x_n||Te_n| le left(sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|right) |x|_1$$
so $|T| le sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n|$. Conversely, for all $n in mathbb{N}, n ne 2015$ we have
$$|T| ge frac{|Te_n|}{|e_n|_1} = |Te_n|$$
so $|T| ge sup_{ninmathbb{N}, nne 2015} |Te_n|$.
Similarly (or using the $(ell^1)' leftrightarrow ell^infty$ duality) we get that $|Phi| = sup_{ninmathbb{N}} |Phi e_n|$.
Now, since $Phi$ extends $T$, for every $n in mathbb{N}, n ne 2015$ it follows that $Te_n = Phi e_n$.
Now we have
$$sup_{substack{ninmathbb{N}\ nne 2015}} |Phi e_n| = sup_{substack{ninmathbb{N}\ nne 2015}} |Te_n| = |T| = |Phi| = sup_{ninmathbb{N}} |Phi e_n|$$
so $|Phi e_{2015}| le |T|$.
We also see that $Phi e_{n} = Te_n, forall n ne 2015$ and $|Phi e_{2015}| le |T|$ is a sufficient condition that $Phi in (ell^1)'$ is a Hahn-Banach extension of $T$.
answered Dec 19 '18 at 20:53
mechanodroidmechanodroid
27.6k62447
27.6k62447
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$begingroup$
Maybe first try a finite dimensional case: $Y={(x,0):xinmathbb{R}}subsetmathbb{R}^2$ where the norm on $mathbb{R}^2$ and $Y$ are $|(x,y)|_1=|x|+|y|$. Try to solve this first.
$endgroup$
– SmileyCraft
Dec 19 '18 at 17:15
$begingroup$
I found that $Phi(e_2) < T(e_1)$, where $e_i, quad i=1,2$ are the -th element of the canonical base
$endgroup$
– VoB
Dec 19 '18 at 17:43
$begingroup$
So, applying the same argument to the above exercise I would get that $Phi(e_{2015}) leq sum_{k ne 2015} T(e_k)$. Could it be right? Is this the right characterization
$endgroup$
– VoB
Dec 19 '18 at 17:45
$begingroup$
You're getting somewhere. Maybe also try $mathbb{R}^3$ and you will see the correct pattern.
$endgroup$
– SmileyCraft
Dec 19 '18 at 18:22
$begingroup$
Well, it seems I have to put $Phi(e_{2015}) leq sum_{k ne 2015} |T(e_k)| $
$endgroup$
– VoB
Dec 19 '18 at 18:43