A space is Hausdorff iff…
Here's an exercise from Marco Manetti's "Topologia" book (ex. 3.59 in the italian version) that i'm stuck on:
Prove that a topological space $X$ is $mathrm{T2}iff{x}=bigcaplimits_{Uin,mathcal{I}(x)}kern{-7pt}overline{U};;;forall xin X$
Partial proof:
$(Rightarrow)$ Let $xin X$, suppose that $yin X$ and $xneq y$. They have disjoint neighborhoods because $X$ is $mathrm{T2}$. So $exists A,Bsubset X$ open and disjoint, such that $xin A$ and $yin B$, and as they're disjoint $Asubset B^{kern{1pt}mathrm{c}}$ and $B^{kern{1pt}mathrm{c}}$ is a closed neighborhood of $x$, choosing such a $B_y$ for each $yneq x$ yields a family of closed neighborhoods of $x$, and $bigcaplimits_{yneq x}kern{-3pt}overline{B^{kern{1pt}mathrm{c}}_y}=bigcaplimits_{yneq x}kern{-3pt}B^{kern{1pt}mathrm{c}}_y={x}$.
$(Leftarrow)$ My initial idea was to prove that if in a space $X$ with the property on the right we assume that given a pair of points $x,yin X$ they have no $Ainmathcal{I}(x),Binmathcal{I}(y)$ open and disjoint we have a contradiction, but all my ideas seemed ineffective, how should i proceed?
general-topology separation-axioms
New contributor
add a comment |
Here's an exercise from Marco Manetti's "Topologia" book (ex. 3.59 in the italian version) that i'm stuck on:
Prove that a topological space $X$ is $mathrm{T2}iff{x}=bigcaplimits_{Uin,mathcal{I}(x)}kern{-7pt}overline{U};;;forall xin X$
Partial proof:
$(Rightarrow)$ Let $xin X$, suppose that $yin X$ and $xneq y$. They have disjoint neighborhoods because $X$ is $mathrm{T2}$. So $exists A,Bsubset X$ open and disjoint, such that $xin A$ and $yin B$, and as they're disjoint $Asubset B^{kern{1pt}mathrm{c}}$ and $B^{kern{1pt}mathrm{c}}$ is a closed neighborhood of $x$, choosing such a $B_y$ for each $yneq x$ yields a family of closed neighborhoods of $x$, and $bigcaplimits_{yneq x}kern{-3pt}overline{B^{kern{1pt}mathrm{c}}_y}=bigcaplimits_{yneq x}kern{-3pt}B^{kern{1pt}mathrm{c}}_y={x}$.
$(Leftarrow)$ My initial idea was to prove that if in a space $X$ with the property on the right we assume that given a pair of points $x,yin X$ they have no $Ainmathcal{I}(x),Binmathcal{I}(y)$ open and disjoint we have a contradiction, but all my ideas seemed ineffective, how should i proceed?
general-topology separation-axioms
New contributor
add a comment |
Here's an exercise from Marco Manetti's "Topologia" book (ex. 3.59 in the italian version) that i'm stuck on:
Prove that a topological space $X$ is $mathrm{T2}iff{x}=bigcaplimits_{Uin,mathcal{I}(x)}kern{-7pt}overline{U};;;forall xin X$
Partial proof:
$(Rightarrow)$ Let $xin X$, suppose that $yin X$ and $xneq y$. They have disjoint neighborhoods because $X$ is $mathrm{T2}$. So $exists A,Bsubset X$ open and disjoint, such that $xin A$ and $yin B$, and as they're disjoint $Asubset B^{kern{1pt}mathrm{c}}$ and $B^{kern{1pt}mathrm{c}}$ is a closed neighborhood of $x$, choosing such a $B_y$ for each $yneq x$ yields a family of closed neighborhoods of $x$, and $bigcaplimits_{yneq x}kern{-3pt}overline{B^{kern{1pt}mathrm{c}}_y}=bigcaplimits_{yneq x}kern{-3pt}B^{kern{1pt}mathrm{c}}_y={x}$.
$(Leftarrow)$ My initial idea was to prove that if in a space $X$ with the property on the right we assume that given a pair of points $x,yin X$ they have no $Ainmathcal{I}(x),Binmathcal{I}(y)$ open and disjoint we have a contradiction, but all my ideas seemed ineffective, how should i proceed?
general-topology separation-axioms
New contributor
Here's an exercise from Marco Manetti's "Topologia" book (ex. 3.59 in the italian version) that i'm stuck on:
Prove that a topological space $X$ is $mathrm{T2}iff{x}=bigcaplimits_{Uin,mathcal{I}(x)}kern{-7pt}overline{U};;;forall xin X$
Partial proof:
$(Rightarrow)$ Let $xin X$, suppose that $yin X$ and $xneq y$. They have disjoint neighborhoods because $X$ is $mathrm{T2}$. So $exists A,Bsubset X$ open and disjoint, such that $xin A$ and $yin B$, and as they're disjoint $Asubset B^{kern{1pt}mathrm{c}}$ and $B^{kern{1pt}mathrm{c}}$ is a closed neighborhood of $x$, choosing such a $B_y$ for each $yneq x$ yields a family of closed neighborhoods of $x$, and $bigcaplimits_{yneq x}kern{-3pt}overline{B^{kern{1pt}mathrm{c}}_y}=bigcaplimits_{yneq x}kern{-3pt}B^{kern{1pt}mathrm{c}}_y={x}$.
$(Leftarrow)$ My initial idea was to prove that if in a space $X$ with the property on the right we assume that given a pair of points $x,yin X$ they have no $Ainmathcal{I}(x),Binmathcal{I}(y)$ open and disjoint we have a contradiction, but all my ideas seemed ineffective, how should i proceed?
general-topology separation-axioms
general-topology separation-axioms
New contributor
New contributor
edited 2 hours ago
New contributor
asked 4 hours ago
Ladooscuro
234
234
New contributor
New contributor
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
Suppose that $X$ is $T_2$. If $x,yin X$, there are $Uinmathcal{I}(x)$ and $Vinmathcal{I}(Y)$ such that $Ucap V=emptyset$. Let $O$ be an open set such that $yin O$ and $Osubset V$. Then $O^complement$ is a closed set that containes $U$. Therefore, $overline Usubset O^complement$ and therefore $ynotinoverline U$. Sinse this occurs for every $yin X$, $bigcap_{Uinmathcal{I}(x)}overline{U}={x}$.
Now, suppose that $X$ is not $T_2$. Take $x,yin X$ such that, for any $Uinmathcal{I}(x)$ and any $Vinmathcal{I}(y)$, $Ucap Vneqemptyset$. It's not hard to prove that $yinbigcap_{Uinmathcal{I}(x)}overline{U}$.
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
add a comment |
Starting the proof with “let $x,yin X$” is not the right way.
Suppose $X$ is Hausdorff and let $xin X$. Suppose $yin X$ and $yne x$. Then there exist $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$ with $Ucap V=emptyset$; in particular $ynotinbar{U}$. Thus $ynotinbigcap_{Uinmathcal{I}(x)}bar{U}$.
Suppose $X$ is not Hausdorff and let $x,yin X$, with $xne y$, such that $Ucap Vneemptyset$, for every $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$. In particular, $yinbar{U}$, for every $Uinmathcal{I}(x)$.
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
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
});
}
});
Ladooscuro is a new contributor. Be nice, and check out our Code of Conduct.
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%2f3054317%2fa-space-is-hausdorff-iff%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
Suppose that $X$ is $T_2$. If $x,yin X$, there are $Uinmathcal{I}(x)$ and $Vinmathcal{I}(Y)$ such that $Ucap V=emptyset$. Let $O$ be an open set such that $yin O$ and $Osubset V$. Then $O^complement$ is a closed set that containes $U$. Therefore, $overline Usubset O^complement$ and therefore $ynotinoverline U$. Sinse this occurs for every $yin X$, $bigcap_{Uinmathcal{I}(x)}overline{U}={x}$.
Now, suppose that $X$ is not $T_2$. Take $x,yin X$ such that, for any $Uinmathcal{I}(x)$ and any $Vinmathcal{I}(y)$, $Ucap Vneqemptyset$. It's not hard to prove that $yinbigcap_{Uinmathcal{I}(x)}overline{U}$.
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
add a comment |
Suppose that $X$ is $T_2$. If $x,yin X$, there are $Uinmathcal{I}(x)$ and $Vinmathcal{I}(Y)$ such that $Ucap V=emptyset$. Let $O$ be an open set such that $yin O$ and $Osubset V$. Then $O^complement$ is a closed set that containes $U$. Therefore, $overline Usubset O^complement$ and therefore $ynotinoverline U$. Sinse this occurs for every $yin X$, $bigcap_{Uinmathcal{I}(x)}overline{U}={x}$.
Now, suppose that $X$ is not $T_2$. Take $x,yin X$ such that, for any $Uinmathcal{I}(x)$ and any $Vinmathcal{I}(y)$, $Ucap Vneqemptyset$. It's not hard to prove that $yinbigcap_{Uinmathcal{I}(x)}overline{U}$.
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
add a comment |
Suppose that $X$ is $T_2$. If $x,yin X$, there are $Uinmathcal{I}(x)$ and $Vinmathcal{I}(Y)$ such that $Ucap V=emptyset$. Let $O$ be an open set such that $yin O$ and $Osubset V$. Then $O^complement$ is a closed set that containes $U$. Therefore, $overline Usubset O^complement$ and therefore $ynotinoverline U$. Sinse this occurs for every $yin X$, $bigcap_{Uinmathcal{I}(x)}overline{U}={x}$.
Now, suppose that $X$ is not $T_2$. Take $x,yin X$ such that, for any $Uinmathcal{I}(x)$ and any $Vinmathcal{I}(y)$, $Ucap Vneqemptyset$. It's not hard to prove that $yinbigcap_{Uinmathcal{I}(x)}overline{U}$.
Suppose that $X$ is $T_2$. If $x,yin X$, there are $Uinmathcal{I}(x)$ and $Vinmathcal{I}(Y)$ such that $Ucap V=emptyset$. Let $O$ be an open set such that $yin O$ and $Osubset V$. Then $O^complement$ is a closed set that containes $U$. Therefore, $overline Usubset O^complement$ and therefore $ynotinoverline U$. Sinse this occurs for every $yin X$, $bigcap_{Uinmathcal{I}(x)}overline{U}={x}$.
Now, suppose that $X$ is not $T_2$. Take $x,yin X$ such that, for any $Uinmathcal{I}(x)$ and any $Vinmathcal{I}(y)$, $Ucap Vneqemptyset$. It's not hard to prove that $yinbigcap_{Uinmathcal{I}(x)}overline{U}$.
answered 4 hours ago
José Carlos Santos
149k22117219
149k22117219
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
add a comment |
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Thanks for the suggestion!
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Then the proof follows from lemma 3.21 of the book: "x is in the closure of a subset B iff each neighborhood of x has non-empty intersection with B"
– Ladooscuro
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
Indeed it does.
– José Carlos Santos
3 hours ago
add a comment |
Starting the proof with “let $x,yin X$” is not the right way.
Suppose $X$ is Hausdorff and let $xin X$. Suppose $yin X$ and $yne x$. Then there exist $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$ with $Ucap V=emptyset$; in particular $ynotinbar{U}$. Thus $ynotinbigcap_{Uinmathcal{I}(x)}bar{U}$.
Suppose $X$ is not Hausdorff and let $x,yin X$, with $xne y$, such that $Ucap Vneemptyset$, for every $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$. In particular, $yinbar{U}$, for every $Uinmathcal{I}(x)$.
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
add a comment |
Starting the proof with “let $x,yin X$” is not the right way.
Suppose $X$ is Hausdorff and let $xin X$. Suppose $yin X$ and $yne x$. Then there exist $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$ with $Ucap V=emptyset$; in particular $ynotinbar{U}$. Thus $ynotinbigcap_{Uinmathcal{I}(x)}bar{U}$.
Suppose $X$ is not Hausdorff and let $x,yin X$, with $xne y$, such that $Ucap Vneemptyset$, for every $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$. In particular, $yinbar{U}$, for every $Uinmathcal{I}(x)$.
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
add a comment |
Starting the proof with “let $x,yin X$” is not the right way.
Suppose $X$ is Hausdorff and let $xin X$. Suppose $yin X$ and $yne x$. Then there exist $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$ with $Ucap V=emptyset$; in particular $ynotinbar{U}$. Thus $ynotinbigcap_{Uinmathcal{I}(x)}bar{U}$.
Suppose $X$ is not Hausdorff and let $x,yin X$, with $xne y$, such that $Ucap Vneemptyset$, for every $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$. In particular, $yinbar{U}$, for every $Uinmathcal{I}(x)$.
Starting the proof with “let $x,yin X$” is not the right way.
Suppose $X$ is Hausdorff and let $xin X$. Suppose $yin X$ and $yne x$. Then there exist $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$ with $Ucap V=emptyset$; in particular $ynotinbar{U}$. Thus $ynotinbigcap_{Uinmathcal{I}(x)}bar{U}$.
Suppose $X$ is not Hausdorff and let $x,yin X$, with $xne y$, such that $Ucap Vneemptyset$, for every $Uinmathcal{I}(x)$ and $Vinmathcal{I}(y)$. In particular, $yinbar{U}$, for every $Uinmathcal{I}(x)$.
answered 3 hours ago
egreg
177k1484200
177k1484200
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
add a comment |
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
Care to explain why that's not the right way? except for not assuming x neq y
– Ladooscuro
3 hours ago
1
1
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
@Ladooscuro Well, that's a very important point. Also you want to show something about any $xin X$. So it's better to start with $x$ and introduce the auxiliary $y$ later.
– egreg
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
Thanks, i'll keep it in mind
– Ladooscuro
2 hours ago
add a comment |
Ladooscuro is a new contributor. Be nice, and check out our Code of Conduct.
Ladooscuro is a new contributor. Be nice, and check out our Code of Conduct.
Ladooscuro is a new contributor. Be nice, and check out our Code of Conduct.
Ladooscuro is a new contributor. Be nice, and check out our Code of Conduct.
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.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- 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.
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%2f3054317%2fa-space-is-hausdorff-iff%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