Is my understanding of a proof from textbook Introduction to Set Theory by Hrbacek and Jech correct?
$begingroup$
3.8 Therorem Let us assume the Generalized Continuum Hypothesis. If $aleph_alpha$ is a regular cardinal, then $$aleph_alpha^{aleph_beta}=begin{cases}
aleph_alpha&text{if }beta<alpha\aleph_{beta+1}&text{if }betagealphaend{cases}$$
My textbook presents the theorem and its proof as follows:
I would like to ask if my understanding of the below statement is correct. $$B=bigcup_{delta<omega_alpha}mathcal{P}(delta)text{ be the collection of all bounded subsets of } omega_alpha$$
Thank you for your help!
$delta<omega_alpha implies delta$ is bounded
If not, there exists $delta<omega_alpha$ such that $sup delta=omega_alpha$. Let $(beta_ximidxi<lambda)$ be an increasing enumeration of $delta$. Then $|lambda|=|delta|ledelta<omega_alpha$ and $lim_{xitolambda}beta_xi=sup delta=omega_alpha$. It follows that $aleph_alpha$ is singular, which contradicts the fact that $aleph_alpha$ is regular.
$X$ is a bounded subset of $omega_alpha$ $implies Xsubseteqdelta$ for some $delta<omega_alpha$
Since $X$ is a bounded subset of $omega_alpha$, $sup X<omega_alpha$. We have $Xsubseteq {gamma intext{ Ord} mid gamma le sup X}$. It is clear that ${gamma intext{ Ord} mid gamma le sup X}$ is a proper initial segment of $omega_alpha$ and thus an ordinal. Then ${gamma intext{ Ord} mid gamma le sup X} =delta$ for some $delta<omega_alpha$.
$B$ is the collection of all bounded subsets of $omega_alpha$ $implies B=bigcup_{delta<omega_alpha}mathcal{P}(delta)$
$Xin B implies$ $X$ is a bounded subsets of $omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$.
$Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $sup X le sup delta < omega_alpha$ [Since $delta$ is bounded] $implies$ $X$ is bounded.
elementary-set-theory cardinals ordinals
$endgroup$
add a comment |
$begingroup$
3.8 Therorem Let us assume the Generalized Continuum Hypothesis. If $aleph_alpha$ is a regular cardinal, then $$aleph_alpha^{aleph_beta}=begin{cases}
aleph_alpha&text{if }beta<alpha\aleph_{beta+1}&text{if }betagealphaend{cases}$$
My textbook presents the theorem and its proof as follows:
I would like to ask if my understanding of the below statement is correct. $$B=bigcup_{delta<omega_alpha}mathcal{P}(delta)text{ be the collection of all bounded subsets of } omega_alpha$$
Thank you for your help!
$delta<omega_alpha implies delta$ is bounded
If not, there exists $delta<omega_alpha$ such that $sup delta=omega_alpha$. Let $(beta_ximidxi<lambda)$ be an increasing enumeration of $delta$. Then $|lambda|=|delta|ledelta<omega_alpha$ and $lim_{xitolambda}beta_xi=sup delta=omega_alpha$. It follows that $aleph_alpha$ is singular, which contradicts the fact that $aleph_alpha$ is regular.
$X$ is a bounded subset of $omega_alpha$ $implies Xsubseteqdelta$ for some $delta<omega_alpha$
Since $X$ is a bounded subset of $omega_alpha$, $sup X<omega_alpha$. We have $Xsubseteq {gamma intext{ Ord} mid gamma le sup X}$. It is clear that ${gamma intext{ Ord} mid gamma le sup X}$ is a proper initial segment of $omega_alpha$ and thus an ordinal. Then ${gamma intext{ Ord} mid gamma le sup X} =delta$ for some $delta<omega_alpha$.
$B$ is the collection of all bounded subsets of $omega_alpha$ $implies B=bigcup_{delta<omega_alpha}mathcal{P}(delta)$
$Xin B implies$ $X$ is a bounded subsets of $omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$.
$Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $sup X le sup delta < omega_alpha$ [Since $delta$ is bounded] $implies$ $X$ is bounded.
elementary-set-theory cardinals ordinals
$endgroup$
add a comment |
$begingroup$
3.8 Therorem Let us assume the Generalized Continuum Hypothesis. If $aleph_alpha$ is a regular cardinal, then $$aleph_alpha^{aleph_beta}=begin{cases}
aleph_alpha&text{if }beta<alpha\aleph_{beta+1}&text{if }betagealphaend{cases}$$
My textbook presents the theorem and its proof as follows:
I would like to ask if my understanding of the below statement is correct. $$B=bigcup_{delta<omega_alpha}mathcal{P}(delta)text{ be the collection of all bounded subsets of } omega_alpha$$
Thank you for your help!
$delta<omega_alpha implies delta$ is bounded
If not, there exists $delta<omega_alpha$ such that $sup delta=omega_alpha$. Let $(beta_ximidxi<lambda)$ be an increasing enumeration of $delta$. Then $|lambda|=|delta|ledelta<omega_alpha$ and $lim_{xitolambda}beta_xi=sup delta=omega_alpha$. It follows that $aleph_alpha$ is singular, which contradicts the fact that $aleph_alpha$ is regular.
$X$ is a bounded subset of $omega_alpha$ $implies Xsubseteqdelta$ for some $delta<omega_alpha$
Since $X$ is a bounded subset of $omega_alpha$, $sup X<omega_alpha$. We have $Xsubseteq {gamma intext{ Ord} mid gamma le sup X}$. It is clear that ${gamma intext{ Ord} mid gamma le sup X}$ is a proper initial segment of $omega_alpha$ and thus an ordinal. Then ${gamma intext{ Ord} mid gamma le sup X} =delta$ for some $delta<omega_alpha$.
$B$ is the collection of all bounded subsets of $omega_alpha$ $implies B=bigcup_{delta<omega_alpha}mathcal{P}(delta)$
$Xin B implies$ $X$ is a bounded subsets of $omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$.
$Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $sup X le sup delta < omega_alpha$ [Since $delta$ is bounded] $implies$ $X$ is bounded.
elementary-set-theory cardinals ordinals
$endgroup$
3.8 Therorem Let us assume the Generalized Continuum Hypothesis. If $aleph_alpha$ is a regular cardinal, then $$aleph_alpha^{aleph_beta}=begin{cases}
aleph_alpha&text{if }beta<alpha\aleph_{beta+1}&text{if }betagealphaend{cases}$$
My textbook presents the theorem and its proof as follows:
I would like to ask if my understanding of the below statement is correct. $$B=bigcup_{delta<omega_alpha}mathcal{P}(delta)text{ be the collection of all bounded subsets of } omega_alpha$$
Thank you for your help!
$delta<omega_alpha implies delta$ is bounded
If not, there exists $delta<omega_alpha$ such that $sup delta=omega_alpha$. Let $(beta_ximidxi<lambda)$ be an increasing enumeration of $delta$. Then $|lambda|=|delta|ledelta<omega_alpha$ and $lim_{xitolambda}beta_xi=sup delta=omega_alpha$. It follows that $aleph_alpha$ is singular, which contradicts the fact that $aleph_alpha$ is regular.
$X$ is a bounded subset of $omega_alpha$ $implies Xsubseteqdelta$ for some $delta<omega_alpha$
Since $X$ is a bounded subset of $omega_alpha$, $sup X<omega_alpha$. We have $Xsubseteq {gamma intext{ Ord} mid gamma le sup X}$. It is clear that ${gamma intext{ Ord} mid gamma le sup X}$ is a proper initial segment of $omega_alpha$ and thus an ordinal. Then ${gamma intext{ Ord} mid gamma le sup X} =delta$ for some $delta<omega_alpha$.
$B$ is the collection of all bounded subsets of $omega_alpha$ $implies B=bigcup_{delta<omega_alpha}mathcal{P}(delta)$
$Xin B implies$ $X$ is a bounded subsets of $omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$.
$Xinbigcup_{delta<omega_alpha}mathcal{P}(delta)$ $implies$ $Xinmathcal{P}(delta)$ for some $delta<omega_alpha$ $implies$ $Xsubseteqdelta$ for some $delta<omega_alpha$ $implies$ $sup X le sup delta < omega_alpha$ [Since $delta$ is bounded] $implies$ $X$ is bounded.
elementary-set-theory cardinals ordinals
elementary-set-theory cardinals ordinals
edited Dec 29 '18 at 5:15
Le Anh Dung
asked Dec 29 '18 at 5:02
Le Anh DungLe Anh Dung
1,4511621
1,4511621
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
- This doesn't really make any sense. Of course an ordinal is bounded in any ordinal that it is less than: $supdelta le delta < omega_alpha.$ And this has nothing to do with regularity.
- Yes, this is correct.
- Yes, this is correct.
You could have made part 3 a little slicker by showing in part 2 that the reverse implication holds as well.
$endgroup$
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
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
});
}
});
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%2f3055549%2fis-my-understanding-of-a-proof-from-textbook-introduction-to-set-theory-by-hrbac%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
- This doesn't really make any sense. Of course an ordinal is bounded in any ordinal that it is less than: $supdelta le delta < omega_alpha.$ And this has nothing to do with regularity.
- Yes, this is correct.
- Yes, this is correct.
You could have made part 3 a little slicker by showing in part 2 that the reverse implication holds as well.
$endgroup$
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
add a comment |
$begingroup$
- This doesn't really make any sense. Of course an ordinal is bounded in any ordinal that it is less than: $supdelta le delta < omega_alpha.$ And this has nothing to do with regularity.
- Yes, this is correct.
- Yes, this is correct.
You could have made part 3 a little slicker by showing in part 2 that the reverse implication holds as well.
$endgroup$
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
add a comment |
$begingroup$
- This doesn't really make any sense. Of course an ordinal is bounded in any ordinal that it is less than: $supdelta le delta < omega_alpha.$ And this has nothing to do with regularity.
- Yes, this is correct.
- Yes, this is correct.
You could have made part 3 a little slicker by showing in part 2 that the reverse implication holds as well.
$endgroup$
- This doesn't really make any sense. Of course an ordinal is bounded in any ordinal that it is less than: $supdelta le delta < omega_alpha.$ And this has nothing to do with regularity.
- Yes, this is correct.
- Yes, this is correct.
You could have made part 3 a little slicker by showing in part 2 that the reverse implication holds as well.
edited Dec 29 '18 at 7:46
answered Dec 29 '18 at 6:11
spaceisdarkgreenspaceisdarkgreen
33.5k21753
33.5k21753
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
add a comment |
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
If you don't know already, a good thing to think about next would be where in the proof regularity of $aleph_alpha$ is used.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 6:17
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
I have fixed 1. as follows: $forallgammaindelta:gamma<delta implies sup delta le delta < omega_alpha implies delta$ is bounded. I guess you have some implicit assumption when writing $color{blue}{supdelta = delta < omega_alpha}$. I have a counter-example: $5={0,1,2,3,4}$ and thus $sup 5= 4 neq 5$. I think the regularity of $aleph_alpha$ is used in the statement every $Xin S$ is a bounded subset of $omega_alpha$. Please have a check on my above reasoning. Thank you for your help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:39
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
@LeAnhDung Yes, that's all correct and I edited to fix the error. It's easy to forget that successor ordinals exist.
$endgroup$
– spaceisdarkgreen
Dec 29 '18 at 7:45
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
$begingroup$
Thank you for all of your dedicated help!
$endgroup$
– Le Anh Dung
Dec 29 '18 at 7:57
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%2f3055549%2fis-my-understanding-of-a-proof-from-textbook-introduction-to-set-theory-by-hrbac%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