Any two of the subsequences ${a_{2k}}, {a_{2k+1}}, {a_{3k}}$ converge do not imply ${a_k}$ converges?












4












$begingroup$


If ${a_k}$ is a sequence, I have shown if the three subsequences ${a_{2k}}, {a_{2k+1}}, {a_{3k}}$ all converge, then the sequence converges.



I am asked whether the sequence converges if only two of them converge. Any help will be appreciated.





I first show the limits are the same. Let us assume $lim a_{2k} = a, lim a_{2k+1}=b, lim a_{3k}=c$. Then $lim_{6k} = a = c$ and $lim_{6k +3} = b = c$. Now pick any $varepsilon > 0$, there are $N_1, N_2$ such that
begin{align*}
&|a_{2k} -a | < varepsilon, text{ if }2k ge N_1, \
&|a_{2k+1} -a | < varepsilon, text{ if }2k+1 ge N_2.
end{align*}

So if $n ge max(N_1, N_2)$, we get $|a_n -a| < varepsilon$.










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$endgroup$








  • 3




    $begingroup$
    It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
    $endgroup$
    – Theo Bendit
    Dec 14 '18 at 18:34










  • $begingroup$
    You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
    $endgroup$
    – Shubham Johri
    Dec 14 '18 at 18:44










  • $begingroup$
    the intuition is I guess that the each of the sequences intertwines the other two.
    $endgroup$
    – Soham
    Dec 14 '18 at 19:16
















4












$begingroup$


If ${a_k}$ is a sequence, I have shown if the three subsequences ${a_{2k}}, {a_{2k+1}}, {a_{3k}}$ all converge, then the sequence converges.



I am asked whether the sequence converges if only two of them converge. Any help will be appreciated.





I first show the limits are the same. Let us assume $lim a_{2k} = a, lim a_{2k+1}=b, lim a_{3k}=c$. Then $lim_{6k} = a = c$ and $lim_{6k +3} = b = c$. Now pick any $varepsilon > 0$, there are $N_1, N_2$ such that
begin{align*}
&|a_{2k} -a | < varepsilon, text{ if }2k ge N_1, \
&|a_{2k+1} -a | < varepsilon, text{ if }2k+1 ge N_2.
end{align*}

So if $n ge max(N_1, N_2)$, we get $|a_n -a| < varepsilon$.










share|cite|improve this question











$endgroup$








  • 3




    $begingroup$
    It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
    $endgroup$
    – Theo Bendit
    Dec 14 '18 at 18:34










  • $begingroup$
    You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
    $endgroup$
    – Shubham Johri
    Dec 14 '18 at 18:44










  • $begingroup$
    the intuition is I guess that the each of the sequences intertwines the other two.
    $endgroup$
    – Soham
    Dec 14 '18 at 19:16














4












4








4


1



$begingroup$


If ${a_k}$ is a sequence, I have shown if the three subsequences ${a_{2k}}, {a_{2k+1}}, {a_{3k}}$ all converge, then the sequence converges.



I am asked whether the sequence converges if only two of them converge. Any help will be appreciated.





I first show the limits are the same. Let us assume $lim a_{2k} = a, lim a_{2k+1}=b, lim a_{3k}=c$. Then $lim_{6k} = a = c$ and $lim_{6k +3} = b = c$. Now pick any $varepsilon > 0$, there are $N_1, N_2$ such that
begin{align*}
&|a_{2k} -a | < varepsilon, text{ if }2k ge N_1, \
&|a_{2k+1} -a | < varepsilon, text{ if }2k+1 ge N_2.
end{align*}

So if $n ge max(N_1, N_2)$, we get $|a_n -a| < varepsilon$.










share|cite|improve this question











$endgroup$




If ${a_k}$ is a sequence, I have shown if the three subsequences ${a_{2k}}, {a_{2k+1}}, {a_{3k}}$ all converge, then the sequence converges.



I am asked whether the sequence converges if only two of them converge. Any help will be appreciated.





I first show the limits are the same. Let us assume $lim a_{2k} = a, lim a_{2k+1}=b, lim a_{3k}=c$. Then $lim_{6k} = a = c$ and $lim_{6k +3} = b = c$. Now pick any $varepsilon > 0$, there are $N_1, N_2$ such that
begin{align*}
&|a_{2k} -a | < varepsilon, text{ if }2k ge N_1, \
&|a_{2k+1} -a | < varepsilon, text{ if }2k+1 ge N_2.
end{align*}

So if $n ge max(N_1, N_2)$, we get $|a_n -a| < varepsilon$.







sequences-and-series convergence






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edited Dec 14 '18 at 18:46







user43210

















asked Dec 14 '18 at 18:32









user43210user43210

406




406








  • 3




    $begingroup$
    It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
    $endgroup$
    – Theo Bendit
    Dec 14 '18 at 18:34










  • $begingroup$
    You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
    $endgroup$
    – Shubham Johri
    Dec 14 '18 at 18:44










  • $begingroup$
    the intuition is I guess that the each of the sequences intertwines the other two.
    $endgroup$
    – Soham
    Dec 14 '18 at 19:16














  • 3




    $begingroup$
    It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
    $endgroup$
    – Theo Bendit
    Dec 14 '18 at 18:34










  • $begingroup$
    You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
    $endgroup$
    – Shubham Johri
    Dec 14 '18 at 18:44










  • $begingroup$
    the intuition is I guess that the each of the sequences intertwines the other two.
    $endgroup$
    – Soham
    Dec 14 '18 at 19:16








3




3




$begingroup$
It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
$endgroup$
– Theo Bendit
Dec 14 '18 at 18:34




$begingroup$
It would be better if you included your proof that the three subsequences converging implies the sequence converges. Otherwise, this may be closed for lack of context.
$endgroup$
– Theo Bendit
Dec 14 '18 at 18:34












$begingroup$
You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
$endgroup$
– Shubham Johri
Dec 14 '18 at 18:44




$begingroup$
You can show by counter-example. $a_n=(-1)^n; a_{2k}to1,a_{2k+1}to-1$ but $a_k$ doesn't converge
$endgroup$
– Shubham Johri
Dec 14 '18 at 18:44












$begingroup$
the intuition is I guess that the each of the sequences intertwines the other two.
$endgroup$
– Soham
Dec 14 '18 at 19:16




$begingroup$
the intuition is I guess that the each of the sequences intertwines the other two.
$endgroup$
– Soham
Dec 14 '18 at 19:16










5 Answers
5






active

oldest

votes


















6












$begingroup$

The answer is no. I remember I learned the example from my analysis class.



Take $a_n = (-1)^n$, then $lim_{k to infty} a_{2k} =1$ and $lim_{k to infty} a_{2k-1} = -1$.



If $$a_k = begin{cases} 0, & text{ if } n=2^k, k = 0, 1, dots , \
1 & text{ otherwise. } end{cases}$$
Then $lim_{k to infty} a_{3k} = 1 $ and $lim_{k to infty} a_{2k+1} = 1$ but $lim_{k to infty} a_{2k}$ does not exist.



Finally, if
$$ a_n = begin{cases} 0 & text{ if $n$ is prime }, \ 1 & text{ if $n$ is not prime.} end{cases}$$
We can check $lim_{k to infty} a_{3k} = 1$ and $lim_{k to infty} a_{2k} =1 $ but $lim_{k to infty} a_{2k+1}$ does not exist.






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$endgroup$





















    2












    $begingroup$

    This is not true. Consider the sequence defined by $a_{2k}=0$ and $a_{2k+1}=1$.






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      Let $(a_n)$ defined by



      $$a_{6k}=1$$
      $$a_{6k+1}=a_{6k+5}=0$$
      $$a_{6k+2}=1$$
      $$a_{6k+3}=1$$
      $$a_{6k+4}=1$$
      then



      $(a_{3n})$ and $(a_{2n})$ converge but



      $(a_n)$ does not.






      share|cite|improve this answer









      $endgroup$





















        1












        $begingroup$

        Convergence of Two of the Subsequences



        Consider the following three sequences
        $$
        a_kleft{begin{array}{}0&text{if }kequiv0pmod2\1&text{if }kequiv1pmod2end{array}right.
        $$

        $limlimits_{ktoinfty}a_{2k}=0$ and $limlimits_{ktoinfty}a_{2k+1}=1$.
        $$
        b_kleft{begin{array}{}0&text{if }knotequiv1pmod6\1&text{if }kequiv1pmod6end{array}right.
        $$

        $limlimits_{ktoinfty}b_{2k}=0$ and $limlimits_{ktoinfty}b_{3k}=0$, but $limlimits_{ktoinfty}b_{6k+1}=1$.
        $$
        c_kleft{begin{array}{}0&text{if }knotequiv2pmod6\1&text{if }kequiv2pmod6end{array}right.
        $$

        $limlimits_{ktoinfty}c_{2k+1}=0$ and $limlimits_{ktoinfty}c_{3k}=0$, but $limlimits_{ktoinfty}c_{6k+2}=1$.



        For a sequence to converge, all subsequences must converge to the same limit.



        Thus, the convergence of two of the subsequences does not imply the convergence of the whole sequence.








        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          This should be the accepted answer imo, it shows the underlying issue clearly.
          $endgroup$
          – copper.hat
          Dec 14 '18 at 21:24



















        -1












        $begingroup$

        Let $a_{2k}to a, a_{2k+1}to bne a$. Now since $a_{6k}to a$ and $a_{6k+3}to bne a, a_{3k}$ is not convergent.



        Since $a_{2k}to a, forallvarepsilon>0, exists n_1inBbb N$ such that $|a_{2k}-a|<varepsilon, forall2kge n_1$



        Since $a_{2k+1}to b,exists n_2inBbb N$ such that $|a_{2k+1}-b|<varepsilon, forall2k+1ge n_2$



        Let us assume $a_n$ converges. This means $a_n$ is Cauchy, or $forall varepsilon_1>0,exists n_3inBbb N$ such that $|x_n-x_m|<varepsilon_1forall n,mge n_3$.



        Chose $n=m-1=2kgemax{n_1,n_2,n_3},varepsilon=varepsilon_1=frac14|a-b|>0$



        $|a_{2k}-a_{2k+1}|<frac14|a-b|$ from the convergence of $a_n$, but



        $a_{2k}in(a-varepsilon, a+varepsilon), a_{2k+1}in(b-varepsilon,b+varepsilon)implies |a_{2k}-a_{2k+1}|>varepsilon_1$



        which is a contradiction.






        share|cite|improve this answer











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          5 Answers
          5






          active

          oldest

          votes








          5 Answers
          5






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          6












          $begingroup$

          The answer is no. I remember I learned the example from my analysis class.



          Take $a_n = (-1)^n$, then $lim_{k to infty} a_{2k} =1$ and $lim_{k to infty} a_{2k-1} = -1$.



          If $$a_k = begin{cases} 0, & text{ if } n=2^k, k = 0, 1, dots , \
          1 & text{ otherwise. } end{cases}$$
          Then $lim_{k to infty} a_{3k} = 1 $ and $lim_{k to infty} a_{2k+1} = 1$ but $lim_{k to infty} a_{2k}$ does not exist.



          Finally, if
          $$ a_n = begin{cases} 0 & text{ if $n$ is prime }, \ 1 & text{ if $n$ is not prime.} end{cases}$$
          We can check $lim_{k to infty} a_{3k} = 1$ and $lim_{k to infty} a_{2k} =1 $ but $lim_{k to infty} a_{2k+1}$ does not exist.






          share|cite|improve this answer









          $endgroup$


















            6












            $begingroup$

            The answer is no. I remember I learned the example from my analysis class.



            Take $a_n = (-1)^n$, then $lim_{k to infty} a_{2k} =1$ and $lim_{k to infty} a_{2k-1} = -1$.



            If $$a_k = begin{cases} 0, & text{ if } n=2^k, k = 0, 1, dots , \
            1 & text{ otherwise. } end{cases}$$
            Then $lim_{k to infty} a_{3k} = 1 $ and $lim_{k to infty} a_{2k+1} = 1$ but $lim_{k to infty} a_{2k}$ does not exist.



            Finally, if
            $$ a_n = begin{cases} 0 & text{ if $n$ is prime }, \ 1 & text{ if $n$ is not prime.} end{cases}$$
            We can check $lim_{k to infty} a_{3k} = 1$ and $lim_{k to infty} a_{2k} =1 $ but $lim_{k to infty} a_{2k+1}$ does not exist.






            share|cite|improve this answer









            $endgroup$
















              6












              6








              6





              $begingroup$

              The answer is no. I remember I learned the example from my analysis class.



              Take $a_n = (-1)^n$, then $lim_{k to infty} a_{2k} =1$ and $lim_{k to infty} a_{2k-1} = -1$.



              If $$a_k = begin{cases} 0, & text{ if } n=2^k, k = 0, 1, dots , \
              1 & text{ otherwise. } end{cases}$$
              Then $lim_{k to infty} a_{3k} = 1 $ and $lim_{k to infty} a_{2k+1} = 1$ but $lim_{k to infty} a_{2k}$ does not exist.



              Finally, if
              $$ a_n = begin{cases} 0 & text{ if $n$ is prime }, \ 1 & text{ if $n$ is not prime.} end{cases}$$
              We can check $lim_{k to infty} a_{3k} = 1$ and $lim_{k to infty} a_{2k} =1 $ but $lim_{k to infty} a_{2k+1}$ does not exist.






              share|cite|improve this answer









              $endgroup$



              The answer is no. I remember I learned the example from my analysis class.



              Take $a_n = (-1)^n$, then $lim_{k to infty} a_{2k} =1$ and $lim_{k to infty} a_{2k-1} = -1$.



              If $$a_k = begin{cases} 0, & text{ if } n=2^k, k = 0, 1, dots , \
              1 & text{ otherwise. } end{cases}$$
              Then $lim_{k to infty} a_{3k} = 1 $ and $lim_{k to infty} a_{2k+1} = 1$ but $lim_{k to infty} a_{2k}$ does not exist.



              Finally, if
              $$ a_n = begin{cases} 0 & text{ if $n$ is prime }, \ 1 & text{ if $n$ is not prime.} end{cases}$$
              We can check $lim_{k to infty} a_{3k} = 1$ and $lim_{k to infty} a_{2k} =1 $ but $lim_{k to infty} a_{2k+1}$ does not exist.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Dec 14 '18 at 18:36









              user1101010user1101010

              7391730




              7391730























                  2












                  $begingroup$

                  This is not true. Consider the sequence defined by $a_{2k}=0$ and $a_{2k+1}=1$.






                  share|cite|improve this answer









                  $endgroup$


















                    2












                    $begingroup$

                    This is not true. Consider the sequence defined by $a_{2k}=0$ and $a_{2k+1}=1$.






                    share|cite|improve this answer









                    $endgroup$
















                      2












                      2








                      2





                      $begingroup$

                      This is not true. Consider the sequence defined by $a_{2k}=0$ and $a_{2k+1}=1$.






                      share|cite|improve this answer









                      $endgroup$



                      This is not true. Consider the sequence defined by $a_{2k}=0$ and $a_{2k+1}=1$.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 14 '18 at 18:36









                      GenericMathGuyGenericMathGuy

                      1312




                      1312























                          1












                          $begingroup$

                          Let $(a_n)$ defined by



                          $$a_{6k}=1$$
                          $$a_{6k+1}=a_{6k+5}=0$$
                          $$a_{6k+2}=1$$
                          $$a_{6k+3}=1$$
                          $$a_{6k+4}=1$$
                          then



                          $(a_{3n})$ and $(a_{2n})$ converge but



                          $(a_n)$ does not.






                          share|cite|improve this answer









                          $endgroup$


















                            1












                            $begingroup$

                            Let $(a_n)$ defined by



                            $$a_{6k}=1$$
                            $$a_{6k+1}=a_{6k+5}=0$$
                            $$a_{6k+2}=1$$
                            $$a_{6k+3}=1$$
                            $$a_{6k+4}=1$$
                            then



                            $(a_{3n})$ and $(a_{2n})$ converge but



                            $(a_n)$ does not.






                            share|cite|improve this answer









                            $endgroup$
















                              1












                              1








                              1





                              $begingroup$

                              Let $(a_n)$ defined by



                              $$a_{6k}=1$$
                              $$a_{6k+1}=a_{6k+5}=0$$
                              $$a_{6k+2}=1$$
                              $$a_{6k+3}=1$$
                              $$a_{6k+4}=1$$
                              then



                              $(a_{3n})$ and $(a_{2n})$ converge but



                              $(a_n)$ does not.






                              share|cite|improve this answer









                              $endgroup$



                              Let $(a_n)$ defined by



                              $$a_{6k}=1$$
                              $$a_{6k+1}=a_{6k+5}=0$$
                              $$a_{6k+2}=1$$
                              $$a_{6k+3}=1$$
                              $$a_{6k+4}=1$$
                              then



                              $(a_{3n})$ and $(a_{2n})$ converge but



                              $(a_n)$ does not.







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 14 '18 at 18:47









                              hamam_Abdallahhamam_Abdallah

                              38.1k21634




                              38.1k21634























                                  1












                                  $begingroup$

                                  Convergence of Two of the Subsequences



                                  Consider the following three sequences
                                  $$
                                  a_kleft{begin{array}{}0&text{if }kequiv0pmod2\1&text{if }kequiv1pmod2end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}a_{2k}=0$ and $limlimits_{ktoinfty}a_{2k+1}=1$.
                                  $$
                                  b_kleft{begin{array}{}0&text{if }knotequiv1pmod6\1&text{if }kequiv1pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}b_{2k}=0$ and $limlimits_{ktoinfty}b_{3k}=0$, but $limlimits_{ktoinfty}b_{6k+1}=1$.
                                  $$
                                  c_kleft{begin{array}{}0&text{if }knotequiv2pmod6\1&text{if }kequiv2pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}c_{2k+1}=0$ and $limlimits_{ktoinfty}c_{3k}=0$, but $limlimits_{ktoinfty}c_{6k+2}=1$.



                                  For a sequence to converge, all subsequences must converge to the same limit.



                                  Thus, the convergence of two of the subsequences does not imply the convergence of the whole sequence.








                                  share|cite|improve this answer











                                  $endgroup$













                                  • $begingroup$
                                    This should be the accepted answer imo, it shows the underlying issue clearly.
                                    $endgroup$
                                    – copper.hat
                                    Dec 14 '18 at 21:24
















                                  1












                                  $begingroup$

                                  Convergence of Two of the Subsequences



                                  Consider the following three sequences
                                  $$
                                  a_kleft{begin{array}{}0&text{if }kequiv0pmod2\1&text{if }kequiv1pmod2end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}a_{2k}=0$ and $limlimits_{ktoinfty}a_{2k+1}=1$.
                                  $$
                                  b_kleft{begin{array}{}0&text{if }knotequiv1pmod6\1&text{if }kequiv1pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}b_{2k}=0$ and $limlimits_{ktoinfty}b_{3k}=0$, but $limlimits_{ktoinfty}b_{6k+1}=1$.
                                  $$
                                  c_kleft{begin{array}{}0&text{if }knotequiv2pmod6\1&text{if }kequiv2pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}c_{2k+1}=0$ and $limlimits_{ktoinfty}c_{3k}=0$, but $limlimits_{ktoinfty}c_{6k+2}=1$.



                                  For a sequence to converge, all subsequences must converge to the same limit.



                                  Thus, the convergence of two of the subsequences does not imply the convergence of the whole sequence.








                                  share|cite|improve this answer











                                  $endgroup$













                                  • $begingroup$
                                    This should be the accepted answer imo, it shows the underlying issue clearly.
                                    $endgroup$
                                    – copper.hat
                                    Dec 14 '18 at 21:24














                                  1












                                  1








                                  1





                                  $begingroup$

                                  Convergence of Two of the Subsequences



                                  Consider the following three sequences
                                  $$
                                  a_kleft{begin{array}{}0&text{if }kequiv0pmod2\1&text{if }kequiv1pmod2end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}a_{2k}=0$ and $limlimits_{ktoinfty}a_{2k+1}=1$.
                                  $$
                                  b_kleft{begin{array}{}0&text{if }knotequiv1pmod6\1&text{if }kequiv1pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}b_{2k}=0$ and $limlimits_{ktoinfty}b_{3k}=0$, but $limlimits_{ktoinfty}b_{6k+1}=1$.
                                  $$
                                  c_kleft{begin{array}{}0&text{if }knotequiv2pmod6\1&text{if }kequiv2pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}c_{2k+1}=0$ and $limlimits_{ktoinfty}c_{3k}=0$, but $limlimits_{ktoinfty}c_{6k+2}=1$.



                                  For a sequence to converge, all subsequences must converge to the same limit.



                                  Thus, the convergence of two of the subsequences does not imply the convergence of the whole sequence.








                                  share|cite|improve this answer











                                  $endgroup$



                                  Convergence of Two of the Subsequences



                                  Consider the following three sequences
                                  $$
                                  a_kleft{begin{array}{}0&text{if }kequiv0pmod2\1&text{if }kequiv1pmod2end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}a_{2k}=0$ and $limlimits_{ktoinfty}a_{2k+1}=1$.
                                  $$
                                  b_kleft{begin{array}{}0&text{if }knotequiv1pmod6\1&text{if }kequiv1pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}b_{2k}=0$ and $limlimits_{ktoinfty}b_{3k}=0$, but $limlimits_{ktoinfty}b_{6k+1}=1$.
                                  $$
                                  c_kleft{begin{array}{}0&text{if }knotequiv2pmod6\1&text{if }kequiv2pmod6end{array}right.
                                  $$

                                  $limlimits_{ktoinfty}c_{2k+1}=0$ and $limlimits_{ktoinfty}c_{3k}=0$, but $limlimits_{ktoinfty}c_{6k+2}=1$.



                                  For a sequence to converge, all subsequences must converge to the same limit.



                                  Thus, the convergence of two of the subsequences does not imply the convergence of the whole sequence.









                                  share|cite|improve this answer














                                  share|cite|improve this answer



                                  share|cite|improve this answer








                                  edited Dec 14 '18 at 19:58

























                                  answered Dec 14 '18 at 19:13









                                  robjohnrobjohn

                                  267k27308632




                                  267k27308632












                                  • $begingroup$
                                    This should be the accepted answer imo, it shows the underlying issue clearly.
                                    $endgroup$
                                    – copper.hat
                                    Dec 14 '18 at 21:24


















                                  • $begingroup$
                                    This should be the accepted answer imo, it shows the underlying issue clearly.
                                    $endgroup$
                                    – copper.hat
                                    Dec 14 '18 at 21:24
















                                  $begingroup$
                                  This should be the accepted answer imo, it shows the underlying issue clearly.
                                  $endgroup$
                                  – copper.hat
                                  Dec 14 '18 at 21:24




                                  $begingroup$
                                  This should be the accepted answer imo, it shows the underlying issue clearly.
                                  $endgroup$
                                  – copper.hat
                                  Dec 14 '18 at 21:24











                                  -1












                                  $begingroup$

                                  Let $a_{2k}to a, a_{2k+1}to bne a$. Now since $a_{6k}to a$ and $a_{6k+3}to bne a, a_{3k}$ is not convergent.



                                  Since $a_{2k}to a, forallvarepsilon>0, exists n_1inBbb N$ such that $|a_{2k}-a|<varepsilon, forall2kge n_1$



                                  Since $a_{2k+1}to b,exists n_2inBbb N$ such that $|a_{2k+1}-b|<varepsilon, forall2k+1ge n_2$



                                  Let us assume $a_n$ converges. This means $a_n$ is Cauchy, or $forall varepsilon_1>0,exists n_3inBbb N$ such that $|x_n-x_m|<varepsilon_1forall n,mge n_3$.



                                  Chose $n=m-1=2kgemax{n_1,n_2,n_3},varepsilon=varepsilon_1=frac14|a-b|>0$



                                  $|a_{2k}-a_{2k+1}|<frac14|a-b|$ from the convergence of $a_n$, but



                                  $a_{2k}in(a-varepsilon, a+varepsilon), a_{2k+1}in(b-varepsilon,b+varepsilon)implies |a_{2k}-a_{2k+1}|>varepsilon_1$



                                  which is a contradiction.






                                  share|cite|improve this answer











                                  $endgroup$


















                                    -1












                                    $begingroup$

                                    Let $a_{2k}to a, a_{2k+1}to bne a$. Now since $a_{6k}to a$ and $a_{6k+3}to bne a, a_{3k}$ is not convergent.



                                    Since $a_{2k}to a, forallvarepsilon>0, exists n_1inBbb N$ such that $|a_{2k}-a|<varepsilon, forall2kge n_1$



                                    Since $a_{2k+1}to b,exists n_2inBbb N$ such that $|a_{2k+1}-b|<varepsilon, forall2k+1ge n_2$



                                    Let us assume $a_n$ converges. This means $a_n$ is Cauchy, or $forall varepsilon_1>0,exists n_3inBbb N$ such that $|x_n-x_m|<varepsilon_1forall n,mge n_3$.



                                    Chose $n=m-1=2kgemax{n_1,n_2,n_3},varepsilon=varepsilon_1=frac14|a-b|>0$



                                    $|a_{2k}-a_{2k+1}|<frac14|a-b|$ from the convergence of $a_n$, but



                                    $a_{2k}in(a-varepsilon, a+varepsilon), a_{2k+1}in(b-varepsilon,b+varepsilon)implies |a_{2k}-a_{2k+1}|>varepsilon_1$



                                    which is a contradiction.






                                    share|cite|improve this answer











                                    $endgroup$
















                                      -1












                                      -1








                                      -1





                                      $begingroup$

                                      Let $a_{2k}to a, a_{2k+1}to bne a$. Now since $a_{6k}to a$ and $a_{6k+3}to bne a, a_{3k}$ is not convergent.



                                      Since $a_{2k}to a, forallvarepsilon>0, exists n_1inBbb N$ such that $|a_{2k}-a|<varepsilon, forall2kge n_1$



                                      Since $a_{2k+1}to b,exists n_2inBbb N$ such that $|a_{2k+1}-b|<varepsilon, forall2k+1ge n_2$



                                      Let us assume $a_n$ converges. This means $a_n$ is Cauchy, or $forall varepsilon_1>0,exists n_3inBbb N$ such that $|x_n-x_m|<varepsilon_1forall n,mge n_3$.



                                      Chose $n=m-1=2kgemax{n_1,n_2,n_3},varepsilon=varepsilon_1=frac14|a-b|>0$



                                      $|a_{2k}-a_{2k+1}|<frac14|a-b|$ from the convergence of $a_n$, but



                                      $a_{2k}in(a-varepsilon, a+varepsilon), a_{2k+1}in(b-varepsilon,b+varepsilon)implies |a_{2k}-a_{2k+1}|>varepsilon_1$



                                      which is a contradiction.






                                      share|cite|improve this answer











                                      $endgroup$



                                      Let $a_{2k}to a, a_{2k+1}to bne a$. Now since $a_{6k}to a$ and $a_{6k+3}to bne a, a_{3k}$ is not convergent.



                                      Since $a_{2k}to a, forallvarepsilon>0, exists n_1inBbb N$ such that $|a_{2k}-a|<varepsilon, forall2kge n_1$



                                      Since $a_{2k+1}to b,exists n_2inBbb N$ such that $|a_{2k+1}-b|<varepsilon, forall2k+1ge n_2$



                                      Let us assume $a_n$ converges. This means $a_n$ is Cauchy, or $forall varepsilon_1>0,exists n_3inBbb N$ such that $|x_n-x_m|<varepsilon_1forall n,mge n_3$.



                                      Chose $n=m-1=2kgemax{n_1,n_2,n_3},varepsilon=varepsilon_1=frac14|a-b|>0$



                                      $|a_{2k}-a_{2k+1}|<frac14|a-b|$ from the convergence of $a_n$, but



                                      $a_{2k}in(a-varepsilon, a+varepsilon), a_{2k+1}in(b-varepsilon,b+varepsilon)implies |a_{2k}-a_{2k+1}|>varepsilon_1$



                                      which is a contradiction.







                                      share|cite|improve this answer














                                      share|cite|improve this answer



                                      share|cite|improve this answer








                                      edited Dec 14 '18 at 19:18

























                                      answered Dec 14 '18 at 18:50









                                      Shubham JohriShubham Johri

                                      5,172717




                                      5,172717






























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