Radius of convergence of $sumlimits_{n=1}^infty((frac{1}{4})^n+(frac{1}{3})^n)x^n$












1












$begingroup$


according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:



$a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$



$implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$



$left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$



But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?



Hints to a better proof are greatly appreciated! :)










share|cite|improve this question











$endgroup$

















    1












    $begingroup$


    according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:



    $a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$



    $implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$



    $left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$



    But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?



    Hints to a better proof are greatly appreciated! :)










    share|cite|improve this question











    $endgroup$















      1












      1








      1


      1



      $begingroup$


      according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:



      $a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$



      $implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$



      $left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$



      But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?



      Hints to a better proof are greatly appreciated! :)










      share|cite|improve this question











      $endgroup$




      according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:



      $a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$



      $implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$



      $left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$



      But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?



      Hints to a better proof are greatly appreciated! :)







      real-analysis sequences-and-series power-series






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 4 '18 at 22:18









      Did

      246k23221457




      246k23221457










      asked Dec 4 '18 at 22:00









      DDevelopsDDevelops

      533




      533






















          3 Answers
          3






          active

          oldest

          votes


















          2












          $begingroup$

          Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
          $$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$



          Hence, $R=3$.






          share|cite|improve this answer









          $endgroup$





















            1












            $begingroup$

            You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit



            $$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$



            exists then $rho$ is the radius of convergence of the power series. In this case we have that



            $$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              This is the ratio test, not the root test (and yes, it works smoothly in the present case).
              $endgroup$
              – Did
              Dec 4 '18 at 22:16





















            1












            $begingroup$

            We have that



            $$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$



            and therefore the radius of convergence is $3$.



            Moreover note that




            • for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$


            • for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$







            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Why the downvote, something wrong?
              $endgroup$
              – gimusi
              Dec 4 '18 at 22:21











            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%2f3026242%2fradius-of-convergence-of-sum-limits-n-1-infty-frac14n-frac13%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            3 Answers
            3






            active

            oldest

            votes








            3 Answers
            3






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            2












            $begingroup$

            Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
            $$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$



            Hence, $R=3$.






            share|cite|improve this answer









            $endgroup$


















              2












              $begingroup$

              Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
              $$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$



              Hence, $R=3$.






              share|cite|improve this answer









              $endgroup$
















                2












                2








                2





                $begingroup$

                Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
                $$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$



                Hence, $R=3$.






                share|cite|improve this answer









                $endgroup$



                Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
                $$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$



                Hence, $R=3$.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Dec 4 '18 at 22:11









                Tito EliatronTito Eliatron

                1,448622




                1,448622























                    1












                    $begingroup$

                    You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit



                    $$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$



                    exists then $rho$ is the radius of convergence of the power series. In this case we have that



                    $$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$






                    share|cite|improve this answer











                    $endgroup$













                    • $begingroup$
                      This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                      $endgroup$
                      – Did
                      Dec 4 '18 at 22:16


















                    1












                    $begingroup$

                    You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit



                    $$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$



                    exists then $rho$ is the radius of convergence of the power series. In this case we have that



                    $$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$






                    share|cite|improve this answer











                    $endgroup$













                    • $begingroup$
                      This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                      $endgroup$
                      – Did
                      Dec 4 '18 at 22:16
















                    1












                    1








                    1





                    $begingroup$

                    You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit



                    $$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$



                    exists then $rho$ is the radius of convergence of the power series. In this case we have that



                    $$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$






                    share|cite|improve this answer











                    $endgroup$



                    You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit



                    $$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$



                    exists then $rho$ is the radius of convergence of the power series. In this case we have that



                    $$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Dec 4 '18 at 22:16

























                    answered Dec 4 '18 at 22:16









                    MasacrosoMasacroso

                    13k41746




                    13k41746












                    • $begingroup$
                      This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                      $endgroup$
                      – Did
                      Dec 4 '18 at 22:16




















                    • $begingroup$
                      This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                      $endgroup$
                      – Did
                      Dec 4 '18 at 22:16


















                    $begingroup$
                    This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                    $endgroup$
                    – Did
                    Dec 4 '18 at 22:16






                    $begingroup$
                    This is the ratio test, not the root test (and yes, it works smoothly in the present case).
                    $endgroup$
                    – Did
                    Dec 4 '18 at 22:16













                    1












                    $begingroup$

                    We have that



                    $$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$



                    and therefore the radius of convergence is $3$.



                    Moreover note that




                    • for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$


                    • for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$







                    share|cite|improve this answer











                    $endgroup$













                    • $begingroup$
                      Why the downvote, something wrong?
                      $endgroup$
                      – gimusi
                      Dec 4 '18 at 22:21
















                    1












                    $begingroup$

                    We have that



                    $$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$



                    and therefore the radius of convergence is $3$.



                    Moreover note that




                    • for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$


                    • for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$







                    share|cite|improve this answer











                    $endgroup$













                    • $begingroup$
                      Why the downvote, something wrong?
                      $endgroup$
                      – gimusi
                      Dec 4 '18 at 22:21














                    1












                    1








                    1





                    $begingroup$

                    We have that



                    $$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$



                    and therefore the radius of convergence is $3$.



                    Moreover note that




                    • for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$


                    • for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$







                    share|cite|improve this answer











                    $endgroup$



                    We have that



                    $$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$



                    and therefore the radius of convergence is $3$.



                    Moreover note that




                    • for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$


                    • for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$








                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Dec 4 '18 at 22:18

























                    answered Dec 4 '18 at 22:11









                    gimusigimusi

                    1




                    1












                    • $begingroup$
                      Why the downvote, something wrong?
                      $endgroup$
                      – gimusi
                      Dec 4 '18 at 22:21


















                    • $begingroup$
                      Why the downvote, something wrong?
                      $endgroup$
                      – gimusi
                      Dec 4 '18 at 22:21
















                    $begingroup$
                    Why the downvote, something wrong?
                    $endgroup$
                    – gimusi
                    Dec 4 '18 at 22:21




                    $begingroup$
                    Why the downvote, something wrong?
                    $endgroup$
                    – gimusi
                    Dec 4 '18 at 22:21


















                    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%2f3026242%2fradius-of-convergence-of-sum-limits-n-1-infty-frac14n-frac13%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

                    Wiesbaden

                    Marschland

                    Dieringhausen