How do I determine the divergence/convergence of $sum_n frac{1}{log(log(n))}$?












0












$begingroup$


I am working through some problems in Durrett's probability book, and one of them involves a variant of the law of iterated logarithm.



I've managed to reduce the result to showing that



$$sum_n frac{1}{log log (n)}exp(-log log(n)) < infty$$



Using upperbounds for tail probabilities of standard normal. But, as I'm really not good with this stuff, I'm unsure how I am supposed to show this (if it's indeed true?).



Clearly without the exponential, the series would diverge since $log (n) leq n$. But, with the exponential it seems as though this could converge.



Could someone advise me how to complete this last step?










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    I am working through some problems in Durrett's probability book, and one of them involves a variant of the law of iterated logarithm.



    I've managed to reduce the result to showing that



    $$sum_n frac{1}{log log (n)}exp(-log log(n)) < infty$$



    Using upperbounds for tail probabilities of standard normal. But, as I'm really not good with this stuff, I'm unsure how I am supposed to show this (if it's indeed true?).



    Clearly without the exponential, the series would diverge since $log (n) leq n$. But, with the exponential it seems as though this could converge.



    Could someone advise me how to complete this last step?










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      I am working through some problems in Durrett's probability book, and one of them involves a variant of the law of iterated logarithm.



      I've managed to reduce the result to showing that



      $$sum_n frac{1}{log log (n)}exp(-log log(n)) < infty$$



      Using upperbounds for tail probabilities of standard normal. But, as I'm really not good with this stuff, I'm unsure how I am supposed to show this (if it's indeed true?).



      Clearly without the exponential, the series would diverge since $log (n) leq n$. But, with the exponential it seems as though this could converge.



      Could someone advise me how to complete this last step?










      share|cite|improve this question











      $endgroup$




      I am working through some problems in Durrett's probability book, and one of them involves a variant of the law of iterated logarithm.



      I've managed to reduce the result to showing that



      $$sum_n frac{1}{log log (n)}exp(-log log(n)) < infty$$



      Using upperbounds for tail probabilities of standard normal. But, as I'm really not good with this stuff, I'm unsure how I am supposed to show this (if it's indeed true?).



      Clearly without the exponential, the series would diverge since $log (n) leq n$. But, with the exponential it seems as though this could converge.



      Could someone advise me how to complete this last step?







      convergence






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 1 '18 at 6:15









      Chinnapparaj R

      5,4872928




      5,4872928










      asked Dec 1 '18 at 6:12









      XiaomiXiaomi

      1,057115




      1,057115






















          5 Answers
          5






          active

          oldest

          votes


















          4












          $begingroup$

          We can simplify
          $$exp(- log log n) = frac{1}{e^{log log n}} = frac{1}{log n}$$
          Since $(log log n) log n leq n$, this diverges.






          share|cite|improve this answer









          $endgroup$





















            2












            $begingroup$

            $sum_n frac{1}{n}$ is divergent. So is $sum_n frac{1}{log(log(n))}$



            But I feel $sum_n frac{exp(-log(log(n))}{log(log(n))}$ might still be convergent, you just need to find another way to prove it.






            share|cite|improve this answer









            $endgroup$





















              1












              $begingroup$

              $$
              frac 1{(log log n )exp(log log n)} = frac 1 {log log n cdot log n} geqslant frac 1{log ^2 n} geqslant frac 1{n^{1/2 times 2}} = frac 1n,
              $$

              so it still diverges.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                $endgroup$
                – Xiaomi
                Dec 1 '18 at 6:21












              • $begingroup$
                @Xiaomi Sorry, I suck at probability theory.
                $endgroup$
                – xbh
                Dec 1 '18 at 7:15



















              1












              $begingroup$

              Cauchy condensation shows
              $$sum_n frac{1}{log(log(n))} sim sum_n frac{2^n}{log(log(2^n))} = sum_n frac{2^n}{log(nlog(2))} = sum_n frac{2^n}{log(n) + log(log(2))} $$
              So, it is divergent.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                $endgroup$
                – Xiaomi
                Dec 1 '18 at 6:24










              • $begingroup$
                You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                $endgroup$
                – trancelocation
                Dec 1 '18 at 6:48





















              1












              $begingroup$

              Credits to user MoonKnight.



              $log n <n ; $



              And once more:



              $log (log n) lt log n lt n.$



              $dfrac {1}{n} lt dfrac{1}{log n} lt dfrac{1}{log (log n)}.$



              Comparison test.






              share|cite|improve this answer









              $endgroup$













                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%2f3021045%2fhow-do-i-determine-the-divergence-convergence-of-sum-n-frac1-log-logn%23new-answer', 'question_page');
                }
                );

                Post as a guest















                Required, but never shown

























                5 Answers
                5






                active

                oldest

                votes








                5 Answers
                5






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                4












                $begingroup$

                We can simplify
                $$exp(- log log n) = frac{1}{e^{log log n}} = frac{1}{log n}$$
                Since $(log log n) log n leq n$, this diverges.






                share|cite|improve this answer









                $endgroup$


















                  4












                  $begingroup$

                  We can simplify
                  $$exp(- log log n) = frac{1}{e^{log log n}} = frac{1}{log n}$$
                  Since $(log log n) log n leq n$, this diverges.






                  share|cite|improve this answer









                  $endgroup$
















                    4












                    4








                    4





                    $begingroup$

                    We can simplify
                    $$exp(- log log n) = frac{1}{e^{log log n}} = frac{1}{log n}$$
                    Since $(log log n) log n leq n$, this diverges.






                    share|cite|improve this answer









                    $endgroup$



                    We can simplify
                    $$exp(- log log n) = frac{1}{e^{log log n}} = frac{1}{log n}$$
                    Since $(log log n) log n leq n$, this diverges.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 1 '18 at 6:17









                    plattyplatty

                    3,370320




                    3,370320























                        2












                        $begingroup$

                        $sum_n frac{1}{n}$ is divergent. So is $sum_n frac{1}{log(log(n))}$



                        But I feel $sum_n frac{exp(-log(log(n))}{log(log(n))}$ might still be convergent, you just need to find another way to prove it.






                        share|cite|improve this answer









                        $endgroup$


















                          2












                          $begingroup$

                          $sum_n frac{1}{n}$ is divergent. So is $sum_n frac{1}{log(log(n))}$



                          But I feel $sum_n frac{exp(-log(log(n))}{log(log(n))}$ might still be convergent, you just need to find another way to prove it.






                          share|cite|improve this answer









                          $endgroup$
















                            2












                            2








                            2





                            $begingroup$

                            $sum_n frac{1}{n}$ is divergent. So is $sum_n frac{1}{log(log(n))}$



                            But I feel $sum_n frac{exp(-log(log(n))}{log(log(n))}$ might still be convergent, you just need to find another way to prove it.






                            share|cite|improve this answer









                            $endgroup$



                            $sum_n frac{1}{n}$ is divergent. So is $sum_n frac{1}{log(log(n))}$



                            But I feel $sum_n frac{exp(-log(log(n))}{log(log(n))}$ might still be convergent, you just need to find another way to prove it.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 1 '18 at 6:19









                            MoonKnightMoonKnight

                            1,369611




                            1,369611























                                1












                                $begingroup$

                                $$
                                frac 1{(log log n )exp(log log n)} = frac 1 {log log n cdot log n} geqslant frac 1{log ^2 n} geqslant frac 1{n^{1/2 times 2}} = frac 1n,
                                $$

                                so it still diverges.






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:21












                                • $begingroup$
                                  @Xiaomi Sorry, I suck at probability theory.
                                  $endgroup$
                                  – xbh
                                  Dec 1 '18 at 7:15
















                                1












                                $begingroup$

                                $$
                                frac 1{(log log n )exp(log log n)} = frac 1 {log log n cdot log n} geqslant frac 1{log ^2 n} geqslant frac 1{n^{1/2 times 2}} = frac 1n,
                                $$

                                so it still diverges.






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:21












                                • $begingroup$
                                  @Xiaomi Sorry, I suck at probability theory.
                                  $endgroup$
                                  – xbh
                                  Dec 1 '18 at 7:15














                                1












                                1








                                1





                                $begingroup$

                                $$
                                frac 1{(log log n )exp(log log n)} = frac 1 {log log n cdot log n} geqslant frac 1{log ^2 n} geqslant frac 1{n^{1/2 times 2}} = frac 1n,
                                $$

                                so it still diverges.






                                share|cite|improve this answer









                                $endgroup$



                                $$
                                frac 1{(log log n )exp(log log n)} = frac 1 {log log n cdot log n} geqslant frac 1{log ^2 n} geqslant frac 1{n^{1/2 times 2}} = frac 1n,
                                $$

                                so it still diverges.







                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered Dec 1 '18 at 6:18









                                xbhxbh

                                6,1351522




                                6,1351522












                                • $begingroup$
                                  Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:21












                                • $begingroup$
                                  @Xiaomi Sorry, I suck at probability theory.
                                  $endgroup$
                                  – xbh
                                  Dec 1 '18 at 7:15


















                                • $begingroup$
                                  Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:21












                                • $begingroup$
                                  @Xiaomi Sorry, I suck at probability theory.
                                  $endgroup$
                                  – xbh
                                  Dec 1 '18 at 7:15
















                                $begingroup$
                                Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                $endgroup$
                                – Xiaomi
                                Dec 1 '18 at 6:21






                                $begingroup$
                                Oh, ok. Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$?
                                $endgroup$
                                – Xiaomi
                                Dec 1 '18 at 6:21














                                $begingroup$
                                @Xiaomi Sorry, I suck at probability theory.
                                $endgroup$
                                – xbh
                                Dec 1 '18 at 7:15




                                $begingroup$
                                @Xiaomi Sorry, I suck at probability theory.
                                $endgroup$
                                – xbh
                                Dec 1 '18 at 7:15











                                1












                                $begingroup$

                                Cauchy condensation shows
                                $$sum_n frac{1}{log(log(n))} sim sum_n frac{2^n}{log(log(2^n))} = sum_n frac{2^n}{log(nlog(2))} = sum_n frac{2^n}{log(n) + log(log(2))} $$
                                So, it is divergent.






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:24










                                • $begingroup$
                                  You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                  $endgroup$
                                  – trancelocation
                                  Dec 1 '18 at 6:48


















                                1












                                $begingroup$

                                Cauchy condensation shows
                                $$sum_n frac{1}{log(log(n))} sim sum_n frac{2^n}{log(log(2^n))} = sum_n frac{2^n}{log(nlog(2))} = sum_n frac{2^n}{log(n) + log(log(2))} $$
                                So, it is divergent.






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:24










                                • $begingroup$
                                  You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                  $endgroup$
                                  – trancelocation
                                  Dec 1 '18 at 6:48
















                                1












                                1








                                1





                                $begingroup$

                                Cauchy condensation shows
                                $$sum_n frac{1}{log(log(n))} sim sum_n frac{2^n}{log(log(2^n))} = sum_n frac{2^n}{log(nlog(2))} = sum_n frac{2^n}{log(n) + log(log(2))} $$
                                So, it is divergent.






                                share|cite|improve this answer









                                $endgroup$



                                Cauchy condensation shows
                                $$sum_n frac{1}{log(log(n))} sim sum_n frac{2^n}{log(log(2^n))} = sum_n frac{2^n}{log(nlog(2))} = sum_n frac{2^n}{log(n) + log(log(2))} $$
                                So, it is divergent.







                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered Dec 1 '18 at 6:20









                                trancelocationtrancelocation

                                11.3k1724




                                11.3k1724












                                • $begingroup$
                                  Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:24










                                • $begingroup$
                                  You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                  $endgroup$
                                  – trancelocation
                                  Dec 1 '18 at 6:48




















                                • $begingroup$
                                  Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                  $endgroup$
                                  – Xiaomi
                                  Dec 1 '18 at 6:24










                                • $begingroup$
                                  You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                  $endgroup$
                                  – trancelocation
                                  Dec 1 '18 at 6:48


















                                $begingroup$
                                Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                $endgroup$
                                – Xiaomi
                                Dec 1 '18 at 6:24




                                $begingroup$
                                Thanks a lot. Do you have any advice on how to show $sum_n P(Z>sqrt{2log log n}(1+epsilon)) < infty$ for $Z sim N(0,1)$? The common probability inequalities do not seem tight enough to show this..
                                $endgroup$
                                – Xiaomi
                                Dec 1 '18 at 6:24












                                $begingroup$
                                You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                $endgroup$
                                – trancelocation
                                Dec 1 '18 at 6:48






                                $begingroup$
                                You may use the integral for $N(0,1)$ for each member of the series and estimate it: $$P(Z>sqrt{2log log n}(1+epsilon)) sim int_{sqrt{2log log n}(1+epsilon)}^{infty} e^{frac{-x^2}{2}}dx$$
                                $endgroup$
                                – trancelocation
                                Dec 1 '18 at 6:48













                                1












                                $begingroup$

                                Credits to user MoonKnight.



                                $log n <n ; $



                                And once more:



                                $log (log n) lt log n lt n.$



                                $dfrac {1}{n} lt dfrac{1}{log n} lt dfrac{1}{log (log n)}.$



                                Comparison test.






                                share|cite|improve this answer









                                $endgroup$


















                                  1












                                  $begingroup$

                                  Credits to user MoonKnight.



                                  $log n <n ; $



                                  And once more:



                                  $log (log n) lt log n lt n.$



                                  $dfrac {1}{n} lt dfrac{1}{log n} lt dfrac{1}{log (log n)}.$



                                  Comparison test.






                                  share|cite|improve this answer









                                  $endgroup$
















                                    1












                                    1








                                    1





                                    $begingroup$

                                    Credits to user MoonKnight.



                                    $log n <n ; $



                                    And once more:



                                    $log (log n) lt log n lt n.$



                                    $dfrac {1}{n} lt dfrac{1}{log n} lt dfrac{1}{log (log n)}.$



                                    Comparison test.






                                    share|cite|improve this answer









                                    $endgroup$



                                    Credits to user MoonKnight.



                                    $log n <n ; $



                                    And once more:



                                    $log (log n) lt log n lt n.$



                                    $dfrac {1}{n} lt dfrac{1}{log n} lt dfrac{1}{log (log n)}.$



                                    Comparison test.







                                    share|cite|improve this answer












                                    share|cite|improve this answer



                                    share|cite|improve this answer










                                    answered Dec 1 '18 at 7:15









                                    Peter SzilasPeter Szilas

                                    11.3k2822




                                    11.3k2822






























                                        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%2f3021045%2fhow-do-i-determine-the-divergence-convergence-of-sum-n-frac1-log-logn%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 to change which sound is reproduced for terminal bell?

                                        Title Spacing in Bjornstrup Chapter, Removing Chapter Number From Contents

                                        Can I use Tabulator js library in my java Spring + Thymeleaf project?