Closed set in topological space generated by sets of the form [a, b).

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Let $tau$ be the topology on $mathbbR$ generated by $B = [a, b)subset mathbbR: -infty<a<b<infty$. Then the set $xin mathbbR: 4sin^2xleq 1cup bigfracpi2big$ is closed in $(mathbbR, tau)$.



My effort:



We know that $4sin^2xleq 1$ whenever $-frac12leq sin x leq frac12$, i.e. $x in big[-fracpi6, fracpi6big]cup big[-frac11pi6, frac13pi6big]cupcdots$. How to proceed further?










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    $begingroup$


    Let $tau$ be the topology on $mathbbR$ generated by $B = [a, b)subset mathbbR: -infty<a<b<infty$. Then the set $xin mathbbR: 4sin^2xleq 1cup bigfracpi2big$ is closed in $(mathbbR, tau)$.



    My effort:



    We know that $4sin^2xleq 1$ whenever $-frac12leq sin x leq frac12$, i.e. $x in big[-fracpi6, fracpi6big]cup big[-frac11pi6, frac13pi6big]cupcdots$. How to proceed further?










    share|cite|improve this question









    $endgroup$














      2












      2








      2





      $begingroup$


      Let $tau$ be the topology on $mathbbR$ generated by $B = [a, b)subset mathbbR: -infty<a<b<infty$. Then the set $xin mathbbR: 4sin^2xleq 1cup bigfracpi2big$ is closed in $(mathbbR, tau)$.



      My effort:



      We know that $4sin^2xleq 1$ whenever $-frac12leq sin x leq frac12$, i.e. $x in big[-fracpi6, fracpi6big]cup big[-frac11pi6, frac13pi6big]cupcdots$. How to proceed further?










      share|cite|improve this question









      $endgroup$




      Let $tau$ be the topology on $mathbbR$ generated by $B = [a, b)subset mathbbR: -infty<a<b<infty$. Then the set $xin mathbbR: 4sin^2xleq 1cup bigfracpi2big$ is closed in $(mathbbR, tau)$.



      My effort:



      We know that $4sin^2xleq 1$ whenever $-frac12leq sin x leq frac12$, i.e. $x in big[-fracpi6, fracpi6big]cup big[-frac11pi6, frac13pi6big]cupcdots$. How to proceed further?







      real-analysis general-topology






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      asked Feb 25 at 17:47









      Mittal GMittal G

      1,376516




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          2 Answers
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          $begingroup$

          Since $leftfracpi2right$ is closed in $(mathbbR,tau)$, all you need to do is to prove that$$cdotscupleft[-fracpi6,fracpi6right]cupleft[-frac11pi6, frac13pi6right]cupcdots$$is closed there. But its complement is an union of intervals of the type $(a,b)$, and these intervals are open in $(mathbbR,tau)$.






          share|cite|improve this answer









          $endgroup$












          • $begingroup$
            I know it! And…?
            $endgroup$
            – José Carlos Santos
            Feb 25 at 17:59










          • $begingroup$
            Sorry! I got it! $(a,b)$ is open in the above topology also.
            $endgroup$
            – Ajay Kumar Nair
            Feb 25 at 18:00










          • $begingroup$
            Indeed. That's important.
            $endgroup$
            – José Carlos Santos
            Feb 25 at 18:02


















          3












          $begingroup$

          One way to show that a set is closed in a topology is to show that its complement is open. In this particular topology, any open interval is open:



          $$(a, b) = cup_n in Bbb N [a-frac1n, b).$$



          And it's easy to see that your set is the complement of a union of open intervals, which we now know are open in $tau$, so we're done.






          share|cite|improve this answer









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            Your Answer





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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            6












            $begingroup$

            Since $leftfracpi2right$ is closed in $(mathbbR,tau)$, all you need to do is to prove that$$cdotscupleft[-fracpi6,fracpi6right]cupleft[-frac11pi6, frac13pi6right]cupcdots$$is closed there. But its complement is an union of intervals of the type $(a,b)$, and these intervals are open in $(mathbbR,tau)$.






            share|cite|improve this answer









            $endgroup$












            • $begingroup$
              I know it! And…?
              $endgroup$
              – José Carlos Santos
              Feb 25 at 17:59










            • $begingroup$
              Sorry! I got it! $(a,b)$ is open in the above topology also.
              $endgroup$
              – Ajay Kumar Nair
              Feb 25 at 18:00










            • $begingroup$
              Indeed. That's important.
              $endgroup$
              – José Carlos Santos
              Feb 25 at 18:02















            6












            $begingroup$

            Since $leftfracpi2right$ is closed in $(mathbbR,tau)$, all you need to do is to prove that$$cdotscupleft[-fracpi6,fracpi6right]cupleft[-frac11pi6, frac13pi6right]cupcdots$$is closed there. But its complement is an union of intervals of the type $(a,b)$, and these intervals are open in $(mathbbR,tau)$.






            share|cite|improve this answer









            $endgroup$












            • $begingroup$
              I know it! And…?
              $endgroup$
              – José Carlos Santos
              Feb 25 at 17:59










            • $begingroup$
              Sorry! I got it! $(a,b)$ is open in the above topology also.
              $endgroup$
              – Ajay Kumar Nair
              Feb 25 at 18:00










            • $begingroup$
              Indeed. That's important.
              $endgroup$
              – José Carlos Santos
              Feb 25 at 18:02













            6












            6








            6





            $begingroup$

            Since $leftfracpi2right$ is closed in $(mathbbR,tau)$, all you need to do is to prove that$$cdotscupleft[-fracpi6,fracpi6right]cupleft[-frac11pi6, frac13pi6right]cupcdots$$is closed there. But its complement is an union of intervals of the type $(a,b)$, and these intervals are open in $(mathbbR,tau)$.






            share|cite|improve this answer









            $endgroup$



            Since $leftfracpi2right$ is closed in $(mathbbR,tau)$, all you need to do is to prove that$$cdotscupleft[-fracpi6,fracpi6right]cupleft[-frac11pi6, frac13pi6right]cupcdots$$is closed there. But its complement is an union of intervals of the type $(a,b)$, and these intervals are open in $(mathbbR,tau)$.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Feb 25 at 17:55









            José Carlos SantosJosé Carlos Santos

            169k23132238




            169k23132238











            • $begingroup$
              I know it! And…?
              $endgroup$
              – José Carlos Santos
              Feb 25 at 17:59










            • $begingroup$
              Sorry! I got it! $(a,b)$ is open in the above topology also.
              $endgroup$
              – Ajay Kumar Nair
              Feb 25 at 18:00










            • $begingroup$
              Indeed. That's important.
              $endgroup$
              – José Carlos Santos
              Feb 25 at 18:02
















            • $begingroup$
              I know it! And…?
              $endgroup$
              – José Carlos Santos
              Feb 25 at 17:59










            • $begingroup$
              Sorry! I got it! $(a,b)$ is open in the above topology also.
              $endgroup$
              – Ajay Kumar Nair
              Feb 25 at 18:00










            • $begingroup$
              Indeed. That's important.
              $endgroup$
              – José Carlos Santos
              Feb 25 at 18:02















            $begingroup$
            I know it! And…?
            $endgroup$
            – José Carlos Santos
            Feb 25 at 17:59




            $begingroup$
            I know it! And…?
            $endgroup$
            – José Carlos Santos
            Feb 25 at 17:59












            $begingroup$
            Sorry! I got it! $(a,b)$ is open in the above topology also.
            $endgroup$
            – Ajay Kumar Nair
            Feb 25 at 18:00




            $begingroup$
            Sorry! I got it! $(a,b)$ is open in the above topology also.
            $endgroup$
            – Ajay Kumar Nair
            Feb 25 at 18:00












            $begingroup$
            Indeed. That's important.
            $endgroup$
            – José Carlos Santos
            Feb 25 at 18:02




            $begingroup$
            Indeed. That's important.
            $endgroup$
            – José Carlos Santos
            Feb 25 at 18:02











            3












            $begingroup$

            One way to show that a set is closed in a topology is to show that its complement is open. In this particular topology, any open interval is open:



            $$(a, b) = cup_n in Bbb N [a-frac1n, b).$$



            And it's easy to see that your set is the complement of a union of open intervals, which we now know are open in $tau$, so we're done.






            share|cite|improve this answer









            $endgroup$

















              3












              $begingroup$

              One way to show that a set is closed in a topology is to show that its complement is open. In this particular topology, any open interval is open:



              $$(a, b) = cup_n in Bbb N [a-frac1n, b).$$



              And it's easy to see that your set is the complement of a union of open intervals, which we now know are open in $tau$, so we're done.






              share|cite|improve this answer









              $endgroup$















                3












                3








                3





                $begingroup$

                One way to show that a set is closed in a topology is to show that its complement is open. In this particular topology, any open interval is open:



                $$(a, b) = cup_n in Bbb N [a-frac1n, b).$$



                And it's easy to see that your set is the complement of a union of open intervals, which we now know are open in $tau$, so we're done.






                share|cite|improve this answer









                $endgroup$



                One way to show that a set is closed in a topology is to show that its complement is open. In this particular topology, any open interval is open:



                $$(a, b) = cup_n in Bbb N [a-frac1n, b).$$



                And it's easy to see that your set is the complement of a union of open intervals, which we now know are open in $tau$, so we're done.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Feb 25 at 17:59









                Robert ShoreRobert Shore

                3,540324




                3,540324



























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