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






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      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
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          $begingroup$


          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





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          1 Answer
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          1 Answer
          1






          active

          oldest

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          active

          oldest

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          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


















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