Associative property in a ring












0














Let $R$ a ring and we define $$text{seq}R={f=(a_0,a_1,dots,);|;a_iin R}.$$



On this set we define the following operation: let $f=(a_n)_{nge0}$ and $g=(b_n)_{nge0}$ $$fg=(c_0,c_1,dots,),$$ where $$c_k=sum_{i+j=k} a_ib_j.$$



My problem is to prove that this product is associative.



That is let $f=(a_n)_{nge0}, g=(b_n)_{nge 0}, h=(c_n)_{nge 0}$. I must prove that $(fg)h=f(gh)$.



Now
$$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots),$$
where $d_k=sum_{i+j=k} a_ib_j$. Then $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots,)=(e_0,e_1,dots),$$
where $$e_s=sum_{k+t=s}d_kc_t,$$
therefore $$e_s=sum_{k+t=s}bigg(sum_{i+j=k}a_ib_jbigg)c_t.$$



How can I proceed now to show that $(fg)h=f(gh)$?



Thanks!






proof-verification proof-writing proof-explanation







share|cite|improve this question





























    0














    Let $R$ a ring and we define $$text{seq}R={f=(a_0,a_1,dots,);|;a_iin R}.$$



    On this set we define the following operation: let $f=(a_n)_{nge0}$ and $g=(b_n)_{nge0}$ $$fg=(c_0,c_1,dots,),$$ where $$c_k=sum_{i+j=k} a_ib_j.$$



    My problem is to prove that this product is associative.



    That is let $f=(a_n)_{nge0}, g=(b_n)_{nge 0}, h=(c_n)_{nge 0}$. I must prove that $(fg)h=f(gh)$.



    Now
    $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots),$$
    where $d_k=sum_{i+j=k} a_ib_j$. Then $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots,)=(e_0,e_1,dots),$$
    where $$e_s=sum_{k+t=s}d_kc_t,$$
    therefore $$e_s=sum_{k+t=s}bigg(sum_{i+j=k}a_ib_jbigg)c_t.$$



    How can I proceed now to show that $(fg)h=f(gh)$?



    Thanks!






    proof-verification proof-writing proof-explanation







    share|cite|improve this question



























      0












      0








      0







      Let $R$ a ring and we define $$text{seq}R={f=(a_0,a_1,dots,);|;a_iin R}.$$



      On this set we define the following operation: let $f=(a_n)_{nge0}$ and $g=(b_n)_{nge0}$ $$fg=(c_0,c_1,dots,),$$ where $$c_k=sum_{i+j=k} a_ib_j.$$



      My problem is to prove that this product is associative.



      That is let $f=(a_n)_{nge0}, g=(b_n)_{nge 0}, h=(c_n)_{nge 0}$. I must prove that $(fg)h=f(gh)$.



      Now
      $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots),$$
      where $d_k=sum_{i+j=k} a_ib_j$. Then $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots,)=(e_0,e_1,dots),$$
      where $$e_s=sum_{k+t=s}d_kc_t,$$
      therefore $$e_s=sum_{k+t=s}bigg(sum_{i+j=k}a_ib_jbigg)c_t.$$



      How can I proceed now to show that $(fg)h=f(gh)$?



      Thanks!






      proof-verification proof-writing proof-explanation







      share|cite|improve this question















      Let $R$ a ring and we define $$text{seq}R={f=(a_0,a_1,dots,);|;a_iin R}.$$



      On this set we define the following operation: let $f=(a_n)_{nge0}$ and $g=(b_n)_{nge0}$ $$fg=(c_0,c_1,dots,),$$ where $$c_k=sum_{i+j=k} a_ib_j.$$



      My problem is to prove that this product is associative.



      That is let $f=(a_n)_{nge0}, g=(b_n)_{nge 0}, h=(c_n)_{nge 0}$. I must prove that $(fg)h=f(gh)$.



      Now
      $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots),$$
      where $d_k=sum_{i+j=k} a_ib_j$. Then $$(fg)h=(d_0,d_1,dots,)(c_0,c_1,dots,)=(e_0,e_1,dots),$$
      where $$e_s=sum_{k+t=s}d_kc_t,$$
      therefore $$e_s=sum_{k+t=s}bigg(sum_{i+j=k}a_ib_jbigg)c_t.$$



      How can I proceed now to show that $(fg)h=f(gh)$?



      Thanks!






      proof-verification proof-writing proof-explanation




      proof-verification proof-writing proof-explanation






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 10 at 9:51

























      asked Dec 8 at 16:47









      Jack J.

      4361419




      4361419






















          1 Answer
          1






          active

          oldest

          votes


















          1














          Letting $a_n=b_n=c_n=0$ for $n<0$, you can rewrite the formula as
          $$
          e_s=sum_{k=0}^inftybiggl(sum_{i=0}^{infty}a_ib_{k-i}biggr)c_{s-k}=
          sum_{k=0}^{infty}sum_{i=0}^{infty}a_ib_{k-i}c_{s-k}
          $$
          and now you can switch the sums:
          $$
          e_s=sum_{i=0}^inftysum_{k=0}^{infty}a_ib_{k-i}c_{s-k}=
          sum_{i=0}^infty a_ibiggl(sum_{k=0}^{infty}b_{k-i}c_{s-k}biggr)
          $$






          share|cite|improve this answer





















          • Thanks for your ansewer. Why can we exchange the sums?
            – Jack J.
            Dec 10 at 21:19






          • 1




            @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
            – egreg
            Dec 10 at 21:36











          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%2f3031337%2fassociative-property-in-a-ring%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1














          Letting $a_n=b_n=c_n=0$ for $n<0$, you can rewrite the formula as
          $$
          e_s=sum_{k=0}^inftybiggl(sum_{i=0}^{infty}a_ib_{k-i}biggr)c_{s-k}=
          sum_{k=0}^{infty}sum_{i=0}^{infty}a_ib_{k-i}c_{s-k}
          $$
          and now you can switch the sums:
          $$
          e_s=sum_{i=0}^inftysum_{k=0}^{infty}a_ib_{k-i}c_{s-k}=
          sum_{i=0}^infty a_ibiggl(sum_{k=0}^{infty}b_{k-i}c_{s-k}biggr)
          $$






          share|cite|improve this answer





















          • Thanks for your ansewer. Why can we exchange the sums?
            – Jack J.
            Dec 10 at 21:19






          • 1




            @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
            – egreg
            Dec 10 at 21:36
















          1














          Letting $a_n=b_n=c_n=0$ for $n<0$, you can rewrite the formula as
          $$
          e_s=sum_{k=0}^inftybiggl(sum_{i=0}^{infty}a_ib_{k-i}biggr)c_{s-k}=
          sum_{k=0}^{infty}sum_{i=0}^{infty}a_ib_{k-i}c_{s-k}
          $$
          and now you can switch the sums:
          $$
          e_s=sum_{i=0}^inftysum_{k=0}^{infty}a_ib_{k-i}c_{s-k}=
          sum_{i=0}^infty a_ibiggl(sum_{k=0}^{infty}b_{k-i}c_{s-k}biggr)
          $$






          share|cite|improve this answer





















          • Thanks for your ansewer. Why can we exchange the sums?
            – Jack J.
            Dec 10 at 21:19






          • 1




            @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
            – egreg
            Dec 10 at 21:36














          1












          1








          1






          Letting $a_n=b_n=c_n=0$ for $n<0$, you can rewrite the formula as
          $$
          e_s=sum_{k=0}^inftybiggl(sum_{i=0}^{infty}a_ib_{k-i}biggr)c_{s-k}=
          sum_{k=0}^{infty}sum_{i=0}^{infty}a_ib_{k-i}c_{s-k}
          $$
          and now you can switch the sums:
          $$
          e_s=sum_{i=0}^inftysum_{k=0}^{infty}a_ib_{k-i}c_{s-k}=
          sum_{i=0}^infty a_ibiggl(sum_{k=0}^{infty}b_{k-i}c_{s-k}biggr)
          $$






          share|cite|improve this answer












          Letting $a_n=b_n=c_n=0$ for $n<0$, you can rewrite the formula as
          $$
          e_s=sum_{k=0}^inftybiggl(sum_{i=0}^{infty}a_ib_{k-i}biggr)c_{s-k}=
          sum_{k=0}^{infty}sum_{i=0}^{infty}a_ib_{k-i}c_{s-k}
          $$
          and now you can switch the sums:
          $$
          e_s=sum_{i=0}^inftysum_{k=0}^{infty}a_ib_{k-i}c_{s-k}=
          sum_{i=0}^infty a_ibiggl(sum_{k=0}^{infty}b_{k-i}c_{s-k}biggr)
          $$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 10 at 11:54









          egreg

          177k1484200




          177k1484200












          • Thanks for your ansewer. Why can we exchange the sums?
            – Jack J.
            Dec 10 at 21:19






          • 1




            @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
            – egreg
            Dec 10 at 21:36


















          • Thanks for your ansewer. Why can we exchange the sums?
            – Jack J.
            Dec 10 at 21:19






          • 1




            @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
            – egreg
            Dec 10 at 21:36
















          Thanks for your ansewer. Why can we exchange the sums?
          – Jack J.
          Dec 10 at 21:19




          Thanks for your ansewer. Why can we exchange the sums?
          – Jack J.
          Dec 10 at 21:19




          1




          1




          @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
          – egreg
          Dec 10 at 21:36




          @JackJ. That's the part left as an exercise (hint: they're actually finite sums).
          – egreg
          Dec 10 at 21:36


















          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.





          Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


          Please pay close attention to the following guidance:


          • 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.


          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%2f3031337%2fassociative-property-in-a-ring%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

          Bressuire

          Image
          .mw-parser-output .infobox{border:1px solid #aaa;background-color:#f9f9f9;color:black;margin:.5em 0 .5em 1em;padding:.2em;float:right;clear:right;width:22em;text-align:left;font-size:88%;line-height:1.6em}.mw-parser-output .infobox td,.mw-parser-output .infobox th{vertical-align:top;padding:0 .2em}.mw-parser-output .infobox caption{font-size:larger}.mw-parser-output .infobox.bordered{border-collapse:collapse}.mw-parser-output .infobox.bordered td,.mw-parser-output .infobox.bordered th{border:1px solid #aaa}.mw-parser-output .infobox.bordered .borderless td,.mw-parser-output .infobox.bordered .borderless th{border:0}.mw-parser-output .infobox-showbutton .mw-collapsible-text{color:inherit}.mw-parser-output .infobox.bordered .mergedtoprow td,.mw-parser-output .infobox.bordered .mergedtoprow th{border:0;border-top:1px solid #aaa;border-right:1px solid #aaa}.mw-parser-output .infobox.bordered .mergedrow td,.mw-parser-output .infobox.bordered .mergedrow th{border:0;border-right:1px solid
          Read more

          Cabo Verde

          Image
            Nota: Cabo-verdiano redireciona para este artigo. Para a língua falada em Cabo Verde, veja crioulo cabo-verdiano. Para outros significados, veja Cabo Verde (desambiguação). Coordenadas: 14° 55' N 23° 31' O Cabo Verde República de Cabo Verde Bandeira Brasão de Armas Lema: Unidade, Trabalho, Progresso Hino nacional: Cântico da Liberdade Gentílico: cabo-verdiano [ 1 ] Capital Praia 14° 55' N 23° 31' O Cidade mais populosa Praia Língua oficial Português Governo República unitária semipresidencialista [ 2 ]  - Presidente Jorge Carlos Fonseca  - Primeiro-ministro Ulisses Correia e Silva História    - Descoberta 1460   - Independência 00 • de Portugal 5 de Julho de 1975   - Multipartidarismo 13 de Janeiro de 1990  Área    - Total 4 033 km² (146.º)  - Água (%) 0,08 População    - Estimativa para 2017 560 899 [ 3 ] hab. (173.º)
          Read more

          Gyllenstierna

          Image
          .mw-parser-output .infobox{border:1px solid #aaa;background-color:#f9f9f9;color:black;margin:.5em 0 .5em 1em;padding:.2em;float:right;clear:right;width:22em;text-align:left;font-size:88%;line-height:1.6em}.mw-parser-output .infobox td,.mw-parser-output .infobox th{vertical-align:top;padding:0 .2em}.mw-parser-output .infobox caption{font-size:larger}.mw-parser-output .infobox.bordered{border-collapse:collapse}.mw-parser-output .infobox.bordered td,.mw-parser-output .infobox.bordered th{border:1px solid #aaa}.mw-parser-output .infobox.bordered .borderless td,.mw-parser-output .infobox.bordered .borderless th{border:0}.mw-parser-output .infobox-showbutton .mw-collapsible-text{color:inherit}.mw-parser-output .infobox.bordered .mergedtoprow td,.mw-parser-output .infobox.bordered .mergedtoprow th{border:0;border-top:1px solid #aaa;border-right:1px solid #aaa}.mw-parser-output .infobox.bordered .mergedrow td,.mw-parser-output .infobox.bordered .mergedrow th{border:0;border-right:1px solid
          Read more