Is the set of sequences $(x_n)$ in $ell^1$ such that $|x_n|leqslantfrac1n$ for every $n$, compact in $ell^1$?












2












$begingroup$


I was thinking about proving a set $S$ is compact in $l^{1}$ space.



$S = {f in l^{1} : |f(k)| leq frac{1}{k} forall k }$



where ${f_{n}}_{n=1}^{infty}$ be functions such that $f_{n} : Bbb{N} rightarrow Bbb{R}^{+}$



I thought of applying that a set $A subset l^{1}$ is compact if and only if $A$ is closed, bounded and $Sup_{x in A} sum_{j geq n}|x_{j}| rightarrow 0$ as $n rightarrow infty$.



Discussing a bit let me think of taking $f(n) = frac{1}{n}$, but it would not belong to $l^{1}$ as the harmonic series would not converge. How to proceed with this question.



EDIT -



Also, how the result changes if we would consider $l^{infty}$ instead of $l^{1}$ space. I think in this case we can take for a fixed integer $N$, $v_{k} = frac{1}{k}$ for $1 leq k leq N$ and $v_{k} =0$ otherwise.



then here $||v|| = 1$ and since $N$ is arbitrary so its bounded always, so the set is compact if we change the space from $l^{1}$ to $l^{infty}$. Is this correct?.










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
    $endgroup$
    – Did
    Dec 30 '18 at 21:16












  • $begingroup$
    Nice!! the $1_{n leq N}$ is the characteristic function?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:27
















2












$begingroup$


I was thinking about proving a set $S$ is compact in $l^{1}$ space.



$S = {f in l^{1} : |f(k)| leq frac{1}{k} forall k }$



where ${f_{n}}_{n=1}^{infty}$ be functions such that $f_{n} : Bbb{N} rightarrow Bbb{R}^{+}$



I thought of applying that a set $A subset l^{1}$ is compact if and only if $A$ is closed, bounded and $Sup_{x in A} sum_{j geq n}|x_{j}| rightarrow 0$ as $n rightarrow infty$.



Discussing a bit let me think of taking $f(n) = frac{1}{n}$, but it would not belong to $l^{1}$ as the harmonic series would not converge. How to proceed with this question.



EDIT -



Also, how the result changes if we would consider $l^{infty}$ instead of $l^{1}$ space. I think in this case we can take for a fixed integer $N$, $v_{k} = frac{1}{k}$ for $1 leq k leq N$ and $v_{k} =0$ otherwise.



then here $||v|| = 1$ and since $N$ is arbitrary so its bounded always, so the set is compact if we change the space from $l^{1}$ to $l^{infty}$. Is this correct?.










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
    $endgroup$
    – Did
    Dec 30 '18 at 21:16












  • $begingroup$
    Nice!! the $1_{n leq N}$ is the characteristic function?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:27














2












2








2


1



$begingroup$


I was thinking about proving a set $S$ is compact in $l^{1}$ space.



$S = {f in l^{1} : |f(k)| leq frac{1}{k} forall k }$



where ${f_{n}}_{n=1}^{infty}$ be functions such that $f_{n} : Bbb{N} rightarrow Bbb{R}^{+}$



I thought of applying that a set $A subset l^{1}$ is compact if and only if $A$ is closed, bounded and $Sup_{x in A} sum_{j geq n}|x_{j}| rightarrow 0$ as $n rightarrow infty$.



Discussing a bit let me think of taking $f(n) = frac{1}{n}$, but it would not belong to $l^{1}$ as the harmonic series would not converge. How to proceed with this question.



EDIT -



Also, how the result changes if we would consider $l^{infty}$ instead of $l^{1}$ space. I think in this case we can take for a fixed integer $N$, $v_{k} = frac{1}{k}$ for $1 leq k leq N$ and $v_{k} =0$ otherwise.



then here $||v|| = 1$ and since $N$ is arbitrary so its bounded always, so the set is compact if we change the space from $l^{1}$ to $l^{infty}$. Is this correct?.










share|cite|improve this question











$endgroup$




I was thinking about proving a set $S$ is compact in $l^{1}$ space.



$S = {f in l^{1} : |f(k)| leq frac{1}{k} forall k }$



where ${f_{n}}_{n=1}^{infty}$ be functions such that $f_{n} : Bbb{N} rightarrow Bbb{R}^{+}$



I thought of applying that a set $A subset l^{1}$ is compact if and only if $A$ is closed, bounded and $Sup_{x in A} sum_{j geq n}|x_{j}| rightarrow 0$ as $n rightarrow infty$.



Discussing a bit let me think of taking $f(n) = frac{1}{n}$, but it would not belong to $l^{1}$ as the harmonic series would not converge. How to proceed with this question.



EDIT -



Also, how the result changes if we would consider $l^{infty}$ instead of $l^{1}$ space. I think in this case we can take for a fixed integer $N$, $v_{k} = frac{1}{k}$ for $1 leq k leq N$ and $v_{k} =0$ otherwise.



then here $||v|| = 1$ and since $N$ is arbitrary so its bounded always, so the set is compact if we change the space from $l^{1}$ to $l^{infty}$. Is this correct?.







real-analysis sequences-and-series compactness lp-spaces






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 30 '18 at 22:16







BAYMAX

















asked Dec 30 '18 at 21:10









BAYMAXBAYMAX

3,04421326




3,04421326








  • 2




    $begingroup$
    Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
    $endgroup$
    – Did
    Dec 30 '18 at 21:16












  • $begingroup$
    Nice!! the $1_{n leq N}$ is the characteristic function?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:27














  • 2




    $begingroup$
    Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
    $endgroup$
    – Did
    Dec 30 '18 at 21:16












  • $begingroup$
    Nice!! the $1_{n leq N}$ is the characteristic function?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:27








2




2




$begingroup$
Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
$endgroup$
– Did
Dec 30 '18 at 21:16






$begingroup$
Consider every $f_N$ defined by $f_N(n)=frac1nmathbf 1_{nleqslant N}$, and proceed as it seems you wanted to, then the proof is direct.
$endgroup$
– Did
Dec 30 '18 at 21:16














$begingroup$
Nice!! the $1_{n leq N}$ is the characteristic function?
$endgroup$
– BAYMAX
Dec 30 '18 at 21:27




$begingroup$
Nice!! the $1_{n leq N}$ is the characteristic function?
$endgroup$
– BAYMAX
Dec 30 '18 at 21:27










1 Answer
1






active

oldest

votes


















3












$begingroup$

The set $S$ is not bounded in $ell^1$: for a fixed integer $N$, let $v$ be the element of $S$ define as $v_k=1/k$ for $1leqslant kleqslant N$ and $v_k=0$ otherwise. Then
$leftlVert vrightrVert_1=sum_{k=1}^N1/k$ hence $sup_{vin S}leftlVert vrightrVert_1geqslant sum_{k=1}^N1/k$. As $N$ is arbitrary and the series $sum_{kgeqslant 1}1/k$ is infinite, we derive that $S$ is not bounded.



If we consider instead $ell^infty$, that is,
$$S = left{f in l^{infty} : |f(k)| leq frac{1}{k} forall k right},
$$

then $S$ is closed (since the functional $fmapsto f(k)$ is continuous for all $k$) and relatively compact: for a fixed $varepsilon$, choose $K$ such that $1/Kltvarepsilon$ and use relative compactness of $left[-1,1right]^k$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:28












  • $begingroup$
    I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:34










  • $begingroup$
    @BAYMAX Is $kle 1/k$?
    $endgroup$
    – David C. Ullrich
    Dec 31 '18 at 16:44










  • $begingroup$
    Nope, I realised that, so please see my edit in the question
    $endgroup$
    – BAYMAX
    Dec 31 '18 at 21:28










  • $begingroup$
    Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
    $endgroup$
    – Davide Giraudo
    Dec 31 '18 at 21:56












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1 Answer
1






active

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

The set $S$ is not bounded in $ell^1$: for a fixed integer $N$, let $v$ be the element of $S$ define as $v_k=1/k$ for $1leqslant kleqslant N$ and $v_k=0$ otherwise. Then
$leftlVert vrightrVert_1=sum_{k=1}^N1/k$ hence $sup_{vin S}leftlVert vrightrVert_1geqslant sum_{k=1}^N1/k$. As $N$ is arbitrary and the series $sum_{kgeqslant 1}1/k$ is infinite, we derive that $S$ is not bounded.



If we consider instead $ell^infty$, that is,
$$S = left{f in l^{infty} : |f(k)| leq frac{1}{k} forall k right},
$$

then $S$ is closed (since the functional $fmapsto f(k)$ is continuous for all $k$) and relatively compact: for a fixed $varepsilon$, choose $K$ such that $1/Kltvarepsilon$ and use relative compactness of $left[-1,1right]^k$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:28












  • $begingroup$
    I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:34










  • $begingroup$
    @BAYMAX Is $kle 1/k$?
    $endgroup$
    – David C. Ullrich
    Dec 31 '18 at 16:44










  • $begingroup$
    Nope, I realised that, so please see my edit in the question
    $endgroup$
    – BAYMAX
    Dec 31 '18 at 21:28










  • $begingroup$
    Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
    $endgroup$
    – Davide Giraudo
    Dec 31 '18 at 21:56
















3












$begingroup$

The set $S$ is not bounded in $ell^1$: for a fixed integer $N$, let $v$ be the element of $S$ define as $v_k=1/k$ for $1leqslant kleqslant N$ and $v_k=0$ otherwise. Then
$leftlVert vrightrVert_1=sum_{k=1}^N1/k$ hence $sup_{vin S}leftlVert vrightrVert_1geqslant sum_{k=1}^N1/k$. As $N$ is arbitrary and the series $sum_{kgeqslant 1}1/k$ is infinite, we derive that $S$ is not bounded.



If we consider instead $ell^infty$, that is,
$$S = left{f in l^{infty} : |f(k)| leq frac{1}{k} forall k right},
$$

then $S$ is closed (since the functional $fmapsto f(k)$ is continuous for all $k$) and relatively compact: for a fixed $varepsilon$, choose $K$ such that $1/Kltvarepsilon$ and use relative compactness of $left[-1,1right]^k$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:28












  • $begingroup$
    I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:34










  • $begingroup$
    @BAYMAX Is $kle 1/k$?
    $endgroup$
    – David C. Ullrich
    Dec 31 '18 at 16:44










  • $begingroup$
    Nope, I realised that, so please see my edit in the question
    $endgroup$
    – BAYMAX
    Dec 31 '18 at 21:28










  • $begingroup$
    Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
    $endgroup$
    – Davide Giraudo
    Dec 31 '18 at 21:56














3












3








3





$begingroup$

The set $S$ is not bounded in $ell^1$: for a fixed integer $N$, let $v$ be the element of $S$ define as $v_k=1/k$ for $1leqslant kleqslant N$ and $v_k=0$ otherwise. Then
$leftlVert vrightrVert_1=sum_{k=1}^N1/k$ hence $sup_{vin S}leftlVert vrightrVert_1geqslant sum_{k=1}^N1/k$. As $N$ is arbitrary and the series $sum_{kgeqslant 1}1/k$ is infinite, we derive that $S$ is not bounded.



If we consider instead $ell^infty$, that is,
$$S = left{f in l^{infty} : |f(k)| leq frac{1}{k} forall k right},
$$

then $S$ is closed (since the functional $fmapsto f(k)$ is continuous for all $k$) and relatively compact: for a fixed $varepsilon$, choose $K$ such that $1/Kltvarepsilon$ and use relative compactness of $left[-1,1right]^k$.






share|cite|improve this answer











$endgroup$



The set $S$ is not bounded in $ell^1$: for a fixed integer $N$, let $v$ be the element of $S$ define as $v_k=1/k$ for $1leqslant kleqslant N$ and $v_k=0$ otherwise. Then
$leftlVert vrightrVert_1=sum_{k=1}^N1/k$ hence $sup_{vin S}leftlVert vrightrVert_1geqslant sum_{k=1}^N1/k$. As $N$ is arbitrary and the series $sum_{kgeqslant 1}1/k$ is infinite, we derive that $S$ is not bounded.



If we consider instead $ell^infty$, that is,
$$S = left{f in l^{infty} : |f(k)| leq frac{1}{k} forall k right},
$$

then $S$ is closed (since the functional $fmapsto f(k)$ is continuous for all $k$) and relatively compact: for a fixed $varepsilon$, choose $K$ such that $1/Kltvarepsilon$ and use relative compactness of $left[-1,1right]^k$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 31 '18 at 21:55

























answered Dec 30 '18 at 21:16









Davide GiraudoDavide Giraudo

128k17157269




128k17157269












  • $begingroup$
    nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:28












  • $begingroup$
    I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:34










  • $begingroup$
    @BAYMAX Is $kle 1/k$?
    $endgroup$
    – David C. Ullrich
    Dec 31 '18 at 16:44










  • $begingroup$
    Nope, I realised that, so please see my edit in the question
    $endgroup$
    – BAYMAX
    Dec 31 '18 at 21:28










  • $begingroup$
    Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
    $endgroup$
    – Davide Giraudo
    Dec 31 '18 at 21:56


















  • $begingroup$
    nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:28












  • $begingroup$
    I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
    $endgroup$
    – BAYMAX
    Dec 30 '18 at 21:34










  • $begingroup$
    @BAYMAX Is $kle 1/k$?
    $endgroup$
    – David C. Ullrich
    Dec 31 '18 at 16:44










  • $begingroup$
    Nope, I realised that, so please see my edit in the question
    $endgroup$
    – BAYMAX
    Dec 31 '18 at 21:28










  • $begingroup$
    Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
    $endgroup$
    – Davide Giraudo
    Dec 31 '18 at 21:56
















$begingroup$
nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
$endgroup$
– BAYMAX
Dec 30 '18 at 21:28






$begingroup$
nice solution! I was thinking how would the answer change if we consider $l^{infty}$ instead of $l^{1}$ ? I htink it is still unbounded in $l^{infty}$ too, so its not compact in both $l^{1}$ and $l^{infty}$?
$endgroup$
– BAYMAX
Dec 30 '18 at 21:28














$begingroup$
I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
$endgroup$
– BAYMAX
Dec 30 '18 at 21:34




$begingroup$
I think for the case of $l^{infty}$, for a fixed integer $N$, we could take $v_{k} = k$ for $1 leq k leq N$ then $||v||_{infty} = N$ but since $N$ is arbitrary so its unbounded.
$endgroup$
– BAYMAX
Dec 30 '18 at 21:34












$begingroup$
@BAYMAX Is $kle 1/k$?
$endgroup$
– David C. Ullrich
Dec 31 '18 at 16:44




$begingroup$
@BAYMAX Is $kle 1/k$?
$endgroup$
– David C. Ullrich
Dec 31 '18 at 16:44












$begingroup$
Nope, I realised that, so please see my edit in the question
$endgroup$
– BAYMAX
Dec 31 '18 at 21:28




$begingroup$
Nope, I realised that, so please see my edit in the question
$endgroup$
– BAYMAX
Dec 31 '18 at 21:28












$begingroup$
Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
$endgroup$
– Davide Giraudo
Dec 31 '18 at 21:56




$begingroup$
Usually, it is better to ask a separate question. For this time I addressed a sketch of prove in the edit.
$endgroup$
– Davide Giraudo
Dec 31 '18 at 21:56


















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