Is my understanding of a proof from textbook Introduction to Set Theory by Hrbacek and Jech correct?












0












$begingroup$



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:




enter image description here



enter image description here




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!





  1. $delta<omega_alpha implies delta$ 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.





  1. $X$ 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 $delta<omega_alpha$.





  1. $B$ 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.













share|cite|improve this question











$endgroup$

















    0












    $begingroup$



    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:




    enter image description here



    enter image description here




    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!





    1. $delta<omega_alpha implies delta$ 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.





    1. $X$ 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 $delta<omega_alpha$.





    1. $B$ 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.













    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$



      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:




      enter image description here



      enter image description here




      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!





      1. $delta<omega_alpha implies delta$ 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.





      1. $X$ 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 $delta<omega_alpha$.





      1. $B$ 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.













      share|cite|improve this question











      $endgroup$





      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:




      enter image description here



      enter image description here




      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!





      1. $delta<omega_alpha implies delta$ 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.





      1. $X$ 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 $delta<omega_alpha$.





      1. $B$ 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






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      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






















          1 Answer
          1






          active

          oldest

          votes


















          1












          $begingroup$


          1. 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.

          2. Yes, this is correct.

          3. 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.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            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











          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
          });


          }
          });














          draft saved

          draft discarded


















          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









          1












          $begingroup$


          1. 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.

          2. Yes, this is correct.

          3. 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.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            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
















          1












          $begingroup$


          1. 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.

          2. Yes, this is correct.

          3. 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.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            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














          1












          1








          1





          $begingroup$


          1. 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.

          2. Yes, this is correct.

          3. 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.






          share|cite|improve this answer











          $endgroup$




          1. 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.

          2. Yes, this is correct.

          3. 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.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          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


















          • $begingroup$
            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


















          draft saved

          draft discarded




















































          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.




          draft saved


          draft discarded














          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





















































          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







          Popular posts from this blog

          How do I know what Microsoft account the skydrive app is syncing to?

          When does type information flow backwards in C++?

          Grease: Live!