Product of Mrówka space and one point compactification discrete space.
$begingroup$
I was reading an article and I have some troubles to understand it. First, the required definition to understand the problem:
Let $mathcal{U}subseteq {Asubseteqomega: |A|=aleph_0 }$. We say that $mathcal{U}$ is an almost disjoint family if for all $A,Binmathcal{U}$ such that $Aneq B$ we have that $|Acap B|<aleph_0$
The proof that I was reading is the next:
The key part of the proof is the fact that $A$ is a closed subset of $Xtimes Y$. But I can't see that $A$ is closed only by the construction of the topology of $Xtimes Y$. In fact, I think that we need a lot of cases to prove that fact because if we take $(a,b)in (Xtimes Y)setminus A$ then
$b=d^{*}$.
$a=r_alpha$ and $b=d_beta$ with $alphaneqbeta$. Here probably we have two subcases because $alpha<beta$ or $beta<alpha$.
$ainomega$ and $b=d_alpha$ for some $alpha<mathfrak{c}$
$ainomega$ and $b=d^*$.
Are they all cases? Or am I forgetting some? I don't know if my thoughts are correct. Can you help me to complete the proof? I really appreciate any help you can provide me.
general-topology proof-explanation
$endgroup$
add a comment |
$begingroup$
I was reading an article and I have some troubles to understand it. First, the required definition to understand the problem:
Let $mathcal{U}subseteq {Asubseteqomega: |A|=aleph_0 }$. We say that $mathcal{U}$ is an almost disjoint family if for all $A,Binmathcal{U}$ such that $Aneq B$ we have that $|Acap B|<aleph_0$
The proof that I was reading is the next:
The key part of the proof is the fact that $A$ is a closed subset of $Xtimes Y$. But I can't see that $A$ is closed only by the construction of the topology of $Xtimes Y$. In fact, I think that we need a lot of cases to prove that fact because if we take $(a,b)in (Xtimes Y)setminus A$ then
$b=d^{*}$.
$a=r_alpha$ and $b=d_beta$ with $alphaneqbeta$. Here probably we have two subcases because $alpha<beta$ or $beta<alpha$.
$ainomega$ and $b=d_alpha$ for some $alpha<mathfrak{c}$
$ainomega$ and $b=d^*$.
Are they all cases? Or am I forgetting some? I don't know if my thoughts are correct. Can you help me to complete the proof? I really appreciate any help you can provide me.
general-topology proof-explanation
$endgroup$
2
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
2
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
1
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
1
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago
add a comment |
$begingroup$
I was reading an article and I have some troubles to understand it. First, the required definition to understand the problem:
Let $mathcal{U}subseteq {Asubseteqomega: |A|=aleph_0 }$. We say that $mathcal{U}$ is an almost disjoint family if for all $A,Binmathcal{U}$ such that $Aneq B$ we have that $|Acap B|<aleph_0$
The proof that I was reading is the next:
The key part of the proof is the fact that $A$ is a closed subset of $Xtimes Y$. But I can't see that $A$ is closed only by the construction of the topology of $Xtimes Y$. In fact, I think that we need a lot of cases to prove that fact because if we take $(a,b)in (Xtimes Y)setminus A$ then
$b=d^{*}$.
$a=r_alpha$ and $b=d_beta$ with $alphaneqbeta$. Here probably we have two subcases because $alpha<beta$ or $beta<alpha$.
$ainomega$ and $b=d_alpha$ for some $alpha<mathfrak{c}$
$ainomega$ and $b=d^*$.
Are they all cases? Or am I forgetting some? I don't know if my thoughts are correct. Can you help me to complete the proof? I really appreciate any help you can provide me.
general-topology proof-explanation
$endgroup$
I was reading an article and I have some troubles to understand it. First, the required definition to understand the problem:
Let $mathcal{U}subseteq {Asubseteqomega: |A|=aleph_0 }$. We say that $mathcal{U}$ is an almost disjoint family if for all $A,Binmathcal{U}$ such that $Aneq B$ we have that $|Acap B|<aleph_0$
The proof that I was reading is the next:
The key part of the proof is the fact that $A$ is a closed subset of $Xtimes Y$. But I can't see that $A$ is closed only by the construction of the topology of $Xtimes Y$. In fact, I think that we need a lot of cases to prove that fact because if we take $(a,b)in (Xtimes Y)setminus A$ then
$b=d^{*}$.
$a=r_alpha$ and $b=d_beta$ with $alphaneqbeta$. Here probably we have two subcases because $alpha<beta$ or $beta<alpha$.
$ainomega$ and $b=d_alpha$ for some $alpha<mathfrak{c}$
$ainomega$ and $b=d^*$.
Are they all cases? Or am I forgetting some? I don't know if my thoughts are correct. Can you help me to complete the proof? I really appreciate any help you can provide me.
general-topology proof-explanation
general-topology proof-explanation
asked 21 hours ago
Carlos JiménezCarlos Jiménez
2,3161621
2,3161621
2
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
2
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
1
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
1
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago
add a comment |
2
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
2
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
1
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
1
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago
2
2
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
2
2
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
1
1
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
1
1
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Suppose that $(x,y) notin A$. If $y neq d^ast$, then $y=d_alpha$ for some $alpha < mathfrak{c}$ while then $x neq r_alpha$. But then taking the neighbourhoods $U_x={r_beta} cup r_beta$ (if $x=r_beta$ for some $betaneq alpha$) or $U_x ={x}$ (if $x in omega$) and $V_y={d_alpha}$ we have that $U_x times V_y$ also misses $A$, as $V_y$ only contains $y$ so the only way it could intersect $A$ is when $r_alpha in U_x$ which is clearly not the case by construction.
So the case that $y neq d^ast$ has been covered. So suppose $y=d^ast$ and we need a neighbourhood of $(x,y)$ that misses $A$. If $x in omega$ take ${x} times Y$ which clearly works as $A$ has no points with first coordinate in $omega$, and if $x=r_beta$ for some $beta$, then it's easy to see that $({r_beta}cup r_beta) times (Y setminus { d_beta })$ is basic open and misses $A$ (as the neighbourhood of $r_beta$ contains no other $r_alpha$ by definition, just $r_beta$ and some isolated points in $omega$).
Just a word of warning:
Lemma 2.1 is false, and $X$ itself is a counterexample: $mathcal{R}$ is closed and uncountable and discrete so not weakly Lindelöf. So not every closed subset of $X$ is weakly Lindelöf, so $X$ is separable ($omega$ is dense) but not quasi-Lindelöf.
True is: a separable space is weakly Lindelöf. This is rather trivial to prove. The reference is not from a "proper" journal, so be warned...
Theorem 3.37 in the referenced paper [5] is also false (ccc implies quasi-Lindelöf) by the same counterexample. Don't believe everything in every random journal... I think this is the source of this paper's lemma 2.1 as I did not find a theorem on separable spaces (but of course separable implies ccc).
Also: 3.36 in [5] (every weakly Lindelöf normal space is quasi-Lindelöf) has a ZFC counterexample: $C_p(X)$ where $X$ is the one-point Lindelöfication of an uncountable discrete space (which can be seen to be the same as the $Sigma$-product of uncountably copies of $mathbb{R}$). Then $C_p(X)$ is (collectionwise) normal and is ccc; (it even has a dense Lindelöf subspace,) hence is weakly Lindelöf, but has a closed subspace homeomorphic to $omega_1$ and hence is not quasi-Lindelöf (the cover by intial segments witnesses that $omega_1$ is not weakly Lindelöf). For more details see this blog post, an excellent source for examples as these...
$endgroup$
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
add a comment |
$begingroup$
Let $f:mathcal Rlongrightarrow Y$ be $f(r_alpha)=d_alpha$. $f$ is clearly continuous because $mathcal R$, as a subspace of $X$, is discrete. Now, we claim that the graph of $f={langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $mathcal Rtimes Y$, which actully follows from this elementary theorem f is Continuous if and only if its Graph is Closed in 𝑋×𝑌 (here we only need the Hausdorffness of $Y$). As $mathcal R$ is closed in $X$, $mathcal R times Y$ is a closed subset of $Xtimes Y$. Hence ${langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $Xtimes Y$.
$endgroup$
add a comment |
Your Answer
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3193159%2fproduct-of-mr%25c3%25b3wka-space-and-one-point-compactification-discrete-space%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Suppose that $(x,y) notin A$. If $y neq d^ast$, then $y=d_alpha$ for some $alpha < mathfrak{c}$ while then $x neq r_alpha$. But then taking the neighbourhoods $U_x={r_beta} cup r_beta$ (if $x=r_beta$ for some $betaneq alpha$) or $U_x ={x}$ (if $x in omega$) and $V_y={d_alpha}$ we have that $U_x times V_y$ also misses $A$, as $V_y$ only contains $y$ so the only way it could intersect $A$ is when $r_alpha in U_x$ which is clearly not the case by construction.
So the case that $y neq d^ast$ has been covered. So suppose $y=d^ast$ and we need a neighbourhood of $(x,y)$ that misses $A$. If $x in omega$ take ${x} times Y$ which clearly works as $A$ has no points with first coordinate in $omega$, and if $x=r_beta$ for some $beta$, then it's easy to see that $({r_beta}cup r_beta) times (Y setminus { d_beta })$ is basic open and misses $A$ (as the neighbourhood of $r_beta$ contains no other $r_alpha$ by definition, just $r_beta$ and some isolated points in $omega$).
Just a word of warning:
Lemma 2.1 is false, and $X$ itself is a counterexample: $mathcal{R}$ is closed and uncountable and discrete so not weakly Lindelöf. So not every closed subset of $X$ is weakly Lindelöf, so $X$ is separable ($omega$ is dense) but not quasi-Lindelöf.
True is: a separable space is weakly Lindelöf. This is rather trivial to prove. The reference is not from a "proper" journal, so be warned...
Theorem 3.37 in the referenced paper [5] is also false (ccc implies quasi-Lindelöf) by the same counterexample. Don't believe everything in every random journal... I think this is the source of this paper's lemma 2.1 as I did not find a theorem on separable spaces (but of course separable implies ccc).
Also: 3.36 in [5] (every weakly Lindelöf normal space is quasi-Lindelöf) has a ZFC counterexample: $C_p(X)$ where $X$ is the one-point Lindelöfication of an uncountable discrete space (which can be seen to be the same as the $Sigma$-product of uncountably copies of $mathbb{R}$). Then $C_p(X)$ is (collectionwise) normal and is ccc; (it even has a dense Lindelöf subspace,) hence is weakly Lindelöf, but has a closed subspace homeomorphic to $omega_1$ and hence is not quasi-Lindelöf (the cover by intial segments witnesses that $omega_1$ is not weakly Lindelöf). For more details see this blog post, an excellent source for examples as these...
$endgroup$
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
add a comment |
$begingroup$
Suppose that $(x,y) notin A$. If $y neq d^ast$, then $y=d_alpha$ for some $alpha < mathfrak{c}$ while then $x neq r_alpha$. But then taking the neighbourhoods $U_x={r_beta} cup r_beta$ (if $x=r_beta$ for some $betaneq alpha$) or $U_x ={x}$ (if $x in omega$) and $V_y={d_alpha}$ we have that $U_x times V_y$ also misses $A$, as $V_y$ only contains $y$ so the only way it could intersect $A$ is when $r_alpha in U_x$ which is clearly not the case by construction.
So the case that $y neq d^ast$ has been covered. So suppose $y=d^ast$ and we need a neighbourhood of $(x,y)$ that misses $A$. If $x in omega$ take ${x} times Y$ which clearly works as $A$ has no points with first coordinate in $omega$, and if $x=r_beta$ for some $beta$, then it's easy to see that $({r_beta}cup r_beta) times (Y setminus { d_beta })$ is basic open and misses $A$ (as the neighbourhood of $r_beta$ contains no other $r_alpha$ by definition, just $r_beta$ and some isolated points in $omega$).
Just a word of warning:
Lemma 2.1 is false, and $X$ itself is a counterexample: $mathcal{R}$ is closed and uncountable and discrete so not weakly Lindelöf. So not every closed subset of $X$ is weakly Lindelöf, so $X$ is separable ($omega$ is dense) but not quasi-Lindelöf.
True is: a separable space is weakly Lindelöf. This is rather trivial to prove. The reference is not from a "proper" journal, so be warned...
Theorem 3.37 in the referenced paper [5] is also false (ccc implies quasi-Lindelöf) by the same counterexample. Don't believe everything in every random journal... I think this is the source of this paper's lemma 2.1 as I did not find a theorem on separable spaces (but of course separable implies ccc).
Also: 3.36 in [5] (every weakly Lindelöf normal space is quasi-Lindelöf) has a ZFC counterexample: $C_p(X)$ where $X$ is the one-point Lindelöfication of an uncountable discrete space (which can be seen to be the same as the $Sigma$-product of uncountably copies of $mathbb{R}$). Then $C_p(X)$ is (collectionwise) normal and is ccc; (it even has a dense Lindelöf subspace,) hence is weakly Lindelöf, but has a closed subspace homeomorphic to $omega_1$ and hence is not quasi-Lindelöf (the cover by intial segments witnesses that $omega_1$ is not weakly Lindelöf). For more details see this blog post, an excellent source for examples as these...
$endgroup$
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
add a comment |
$begingroup$
Suppose that $(x,y) notin A$. If $y neq d^ast$, then $y=d_alpha$ for some $alpha < mathfrak{c}$ while then $x neq r_alpha$. But then taking the neighbourhoods $U_x={r_beta} cup r_beta$ (if $x=r_beta$ for some $betaneq alpha$) or $U_x ={x}$ (if $x in omega$) and $V_y={d_alpha}$ we have that $U_x times V_y$ also misses $A$, as $V_y$ only contains $y$ so the only way it could intersect $A$ is when $r_alpha in U_x$ which is clearly not the case by construction.
So the case that $y neq d^ast$ has been covered. So suppose $y=d^ast$ and we need a neighbourhood of $(x,y)$ that misses $A$. If $x in omega$ take ${x} times Y$ which clearly works as $A$ has no points with first coordinate in $omega$, and if $x=r_beta$ for some $beta$, then it's easy to see that $({r_beta}cup r_beta) times (Y setminus { d_beta })$ is basic open and misses $A$ (as the neighbourhood of $r_beta$ contains no other $r_alpha$ by definition, just $r_beta$ and some isolated points in $omega$).
Just a word of warning:
Lemma 2.1 is false, and $X$ itself is a counterexample: $mathcal{R}$ is closed and uncountable and discrete so not weakly Lindelöf. So not every closed subset of $X$ is weakly Lindelöf, so $X$ is separable ($omega$ is dense) but not quasi-Lindelöf.
True is: a separable space is weakly Lindelöf. This is rather trivial to prove. The reference is not from a "proper" journal, so be warned...
Theorem 3.37 in the referenced paper [5] is also false (ccc implies quasi-Lindelöf) by the same counterexample. Don't believe everything in every random journal... I think this is the source of this paper's lemma 2.1 as I did not find a theorem on separable spaces (but of course separable implies ccc).
Also: 3.36 in [5] (every weakly Lindelöf normal space is quasi-Lindelöf) has a ZFC counterexample: $C_p(X)$ where $X$ is the one-point Lindelöfication of an uncountable discrete space (which can be seen to be the same as the $Sigma$-product of uncountably copies of $mathbb{R}$). Then $C_p(X)$ is (collectionwise) normal and is ccc; (it even has a dense Lindelöf subspace,) hence is weakly Lindelöf, but has a closed subspace homeomorphic to $omega_1$ and hence is not quasi-Lindelöf (the cover by intial segments witnesses that $omega_1$ is not weakly Lindelöf). For more details see this blog post, an excellent source for examples as these...
$endgroup$
Suppose that $(x,y) notin A$. If $y neq d^ast$, then $y=d_alpha$ for some $alpha < mathfrak{c}$ while then $x neq r_alpha$. But then taking the neighbourhoods $U_x={r_beta} cup r_beta$ (if $x=r_beta$ for some $betaneq alpha$) or $U_x ={x}$ (if $x in omega$) and $V_y={d_alpha}$ we have that $U_x times V_y$ also misses $A$, as $V_y$ only contains $y$ so the only way it could intersect $A$ is when $r_alpha in U_x$ which is clearly not the case by construction.
So the case that $y neq d^ast$ has been covered. So suppose $y=d^ast$ and we need a neighbourhood of $(x,y)$ that misses $A$. If $x in omega$ take ${x} times Y$ which clearly works as $A$ has no points with first coordinate in $omega$, and if $x=r_beta$ for some $beta$, then it's easy to see that $({r_beta}cup r_beta) times (Y setminus { d_beta })$ is basic open and misses $A$ (as the neighbourhood of $r_beta$ contains no other $r_alpha$ by definition, just $r_beta$ and some isolated points in $omega$).
Just a word of warning:
Lemma 2.1 is false, and $X$ itself is a counterexample: $mathcal{R}$ is closed and uncountable and discrete so not weakly Lindelöf. So not every closed subset of $X$ is weakly Lindelöf, so $X$ is separable ($omega$ is dense) but not quasi-Lindelöf.
True is: a separable space is weakly Lindelöf. This is rather trivial to prove. The reference is not from a "proper" journal, so be warned...
Theorem 3.37 in the referenced paper [5] is also false (ccc implies quasi-Lindelöf) by the same counterexample. Don't believe everything in every random journal... I think this is the source of this paper's lemma 2.1 as I did not find a theorem on separable spaces (but of course separable implies ccc).
Also: 3.36 in [5] (every weakly Lindelöf normal space is quasi-Lindelöf) has a ZFC counterexample: $C_p(X)$ where $X$ is the one-point Lindelöfication of an uncountable discrete space (which can be seen to be the same as the $Sigma$-product of uncountably copies of $mathbb{R}$). Then $C_p(X)$ is (collectionwise) normal and is ccc; (it even has a dense Lindelöf subspace,) hence is weakly Lindelöf, but has a closed subspace homeomorphic to $omega_1$ and hence is not quasi-Lindelöf (the cover by intial segments witnesses that $omega_1$ is not weakly Lindelöf). For more details see this blog post, an excellent source for examples as these...
edited 13 hours ago
answered 18 hours ago
Henno BrandsmaHenno Brandsma
117k350128
117k350128
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
add a comment |
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
1
1
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
$begingroup$
+1 for the warning!
$endgroup$
– YuiTo Cheng
17 hours ago
1
1
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
$begingroup$
@YuiToCheng Your proof of closedness was simpler (but I wanted to show the claim by the most elementary means),+1 for noting it anyway. These papers the OP quotes (or indirectly quotes) are full of errors...
$endgroup$
– Henno Brandsma
17 hours ago
1
1
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
$begingroup$
Just out of interest, are you Dan Ma?
$endgroup$
– YuiTo Cheng
13 hours ago
1
1
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
$begingroup$
@YuiToCheng no but he writes blog posts that are in my sphere of interest.
$endgroup$
– Henno Brandsma
13 hours ago
add a comment |
$begingroup$
Let $f:mathcal Rlongrightarrow Y$ be $f(r_alpha)=d_alpha$. $f$ is clearly continuous because $mathcal R$, as a subspace of $X$, is discrete. Now, we claim that the graph of $f={langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $mathcal Rtimes Y$, which actully follows from this elementary theorem f is Continuous if and only if its Graph is Closed in 𝑋×𝑌 (here we only need the Hausdorffness of $Y$). As $mathcal R$ is closed in $X$, $mathcal R times Y$ is a closed subset of $Xtimes Y$. Hence ${langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $Xtimes Y$.
$endgroup$
add a comment |
$begingroup$
Let $f:mathcal Rlongrightarrow Y$ be $f(r_alpha)=d_alpha$. $f$ is clearly continuous because $mathcal R$, as a subspace of $X$, is discrete. Now, we claim that the graph of $f={langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $mathcal Rtimes Y$, which actully follows from this elementary theorem f is Continuous if and only if its Graph is Closed in 𝑋×𝑌 (here we only need the Hausdorffness of $Y$). As $mathcal R$ is closed in $X$, $mathcal R times Y$ is a closed subset of $Xtimes Y$. Hence ${langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $Xtimes Y$.
$endgroup$
add a comment |
$begingroup$
Let $f:mathcal Rlongrightarrow Y$ be $f(r_alpha)=d_alpha$. $f$ is clearly continuous because $mathcal R$, as a subspace of $X$, is discrete. Now, we claim that the graph of $f={langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $mathcal Rtimes Y$, which actully follows from this elementary theorem f is Continuous if and only if its Graph is Closed in 𝑋×𝑌 (here we only need the Hausdorffness of $Y$). As $mathcal R$ is closed in $X$, $mathcal R times Y$ is a closed subset of $Xtimes Y$. Hence ${langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $Xtimes Y$.
$endgroup$
Let $f:mathcal Rlongrightarrow Y$ be $f(r_alpha)=d_alpha$. $f$ is clearly continuous because $mathcal R$, as a subspace of $X$, is discrete. Now, we claim that the graph of $f={langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $mathcal Rtimes Y$, which actully follows from this elementary theorem f is Continuous if and only if its Graph is Closed in 𝑋×𝑌 (here we only need the Hausdorffness of $Y$). As $mathcal R$ is closed in $X$, $mathcal R times Y$ is a closed subset of $Xtimes Y$. Hence ${langle r_alpha,d_alpharangle mid alpha<mathfrak{c}}$ is closed in $Xtimes Y$.
answered 18 hours ago
YuiTo ChengYuiTo Cheng
2,58841037
2,58841037
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3193159%2fproduct-of-mr%25c3%25b3wka-space-and-one-point-compactification-discrete-space%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
2
$begingroup$
Note that lemma 2.1 is false, see my answer.
$endgroup$
– Henno Brandsma
17 hours ago
2
$begingroup$
And thus the main result's example fails: the author has not given an example of a quasi-Lindelöf space $X$ (as $X$ is weakly Lindelöf, not quasi-Lindelöf). Of course $X times Y$ is not quasi-Lindelöf as it contains $X$ as a closed subset. So there is no example.
$endgroup$
– Henno Brandsma
16 hours ago
1
$begingroup$
It's still (IMHO) interesting whether there is a quasi-Lindelöf space $X$ whose product with a compact space is no longer quasi-Lindelöf. But this attempt at a counterexample fails.
$endgroup$
– Henno Brandsma
14 hours ago
1
$begingroup$
It is even more interesting MSE can help to spot flawed papers. Cheers!
$endgroup$
– YuiTo Cheng
14 hours ago