Proving the product rule for polynomials without limits or derivative rulesApostol Calculus Vol.1 Exercise 9 , Chapter 1.5 (Prove property of polynomial function)If something holds for n+1 distinct values then it holds for all values.Proving a property of polynomialShow that $(1+a_1x+ldots+a_rx^r)^k=1+x+x^r+1q(x)$Generalization of the derivative to polynomial ringsinduction with remaindersProve that $R(n) =frac2nsum_k=0^n-1 (R(k))+c$ is $R(n) = n*c$Proving by induction that order of the product is equal to the product of the ordersExpressing Laurent polynomials using generators and relationsPolynomial Length upper boundProving the connection between the Binomial Theorem and the product rule for derivatives

Do US professors/group leaders only get a salary, but no group budget?

Do I need to be arrogant to get ahead?

Recruiter wants very extensive technical details about all of my previous work

Maths symbols and unicode-math input inside siunitx commands

Print last inputted byte

Would it be believable to defy demographics in a story?

What favor did Moody owe Dumbledore?

How is the partial sum of a geometric sequence calculated?

Why is there so much iron?

How can an organ that provides biological immortality be unable to regenerate?

Should I use acronyms in dialogues before telling the readers what it stands for in fiction?

Hausdorff dimension of the boundary of fibres of Lipschitz maps

If "dar" means "to give", what does "daros" mean?

Light propagating through a sound wave

Is it true that good novels will automatically sell themselves on Amazon (and so on) and there is no need for one to waste time promoting?

Should I be concerned about student access to a test bank?

What exactly term 'companion plants' means?

Deletion of copy-ctor & copy-assignment - public, private or protected?

Practical application of matrices and determinants

Brake pads destroying wheels

When did antialiasing start being available?

What can I do if I am asked to learn different programming languages very frequently?

What does "^L" mean in C?

HP P840 HDD RAID 5 many strange drive failures



Proving the product rule for polynomials without limits or derivative rules


Apostol Calculus Vol.1 Exercise 9 , Chapter 1.5 (Prove property of polynomial function)If something holds for n+1 distinct values then it holds for all values.Proving a property of polynomialShow that $(1+a_1x+ldots+a_rx^r)^k=1+x+x^r+1q(x)$Generalization of the derivative to polynomial ringsinduction with remaindersProve that $R(n) =frac2nsum_k=0^n-1 (R(k))+c$ is $R(n) = n*c$Proving by induction that order of the product is equal to the product of the ordersExpressing Laurent polynomials using generators and relationsPolynomial Length upper boundProving the connection between the Binomial Theorem and the product rule for derivatives













1












$begingroup$


As I said in the title, in the book where this exercise was proposed, derivatives or limits were not introduced yet. The exercise was to show that given the function $':mathbbC[x]rightarrow mathbbC[x]$ I should show that $(fg)'=f'g+fg'$



$f'(x)$ was defined as $sum_k=1^nka_kx^k-1$ where $f(x)=sum_k=0^na_kx^k$.



I have found out that an equivalent representation for $f'(x)$ is $sum_k=0^n-1(k+1)a_k+1x^k$



I have tried to prove the result myself step by step but there must be an error somewhere in the proof because in the end the result does not fit the bill.



$fg=sum_k=0^m+nc_kx^k$ with $c_k=sum_r+s=ka_rb_s$



$(fg)'=sum_j=0^m+n-1(j+1)c_j+1x^j$



$f=sum_r=0^na_rx^r$



$f'=sum_r=0^n-1(r+1)a_r+1x^r$



$g=sum_s=0^mb_sx^s$



$g'=sum_s=0^m-1(s+1)b_s+1x^s$



$f'g=sum_j=0^m+n-1c'_jx^j$, with $c'_j=sum_r+s=j(r+1)a_r+1b_s$



$fg'=sum_j=0^m+n-1c''_jx^j$, with $c''_j=sum_r+s=ja_r(s+1)b_s+1$



I have tried to prove by induction that for every $j$ we have



$c'_j+c''_j=(j+1)c_j+1$



but I failed to show that the coefficients of both polynomials are equal. Where is the mistake? Can somebody give me a hint please?



The Problem is the induction step; I don't know how I can use the induction hypothesis to my advantage; I have made the induction over $j$ and I could Show it for $j=0$ and $j=1$ (just to be sure):



The claim is



$$(j+1)sum_r+s=j+1a_rb_s=sum_r+s=j(r+1)a_r+1b_s+sum_r+s=ja_r(s+1)b_s+1$$



I also know that $(j+1)sum_r+s=j+1a_rb_s=(j+1)sum_r=0^j+1a_rb_j+1-r$










share|cite|improve this question











$endgroup$











  • $begingroup$
    Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
    $endgroup$
    – Jair Taylor
    Mar 12 at 21:08















1












$begingroup$


As I said in the title, in the book where this exercise was proposed, derivatives or limits were not introduced yet. The exercise was to show that given the function $':mathbbC[x]rightarrow mathbbC[x]$ I should show that $(fg)'=f'g+fg'$



$f'(x)$ was defined as $sum_k=1^nka_kx^k-1$ where $f(x)=sum_k=0^na_kx^k$.



I have found out that an equivalent representation for $f'(x)$ is $sum_k=0^n-1(k+1)a_k+1x^k$



I have tried to prove the result myself step by step but there must be an error somewhere in the proof because in the end the result does not fit the bill.



$fg=sum_k=0^m+nc_kx^k$ with $c_k=sum_r+s=ka_rb_s$



$(fg)'=sum_j=0^m+n-1(j+1)c_j+1x^j$



$f=sum_r=0^na_rx^r$



$f'=sum_r=0^n-1(r+1)a_r+1x^r$



$g=sum_s=0^mb_sx^s$



$g'=sum_s=0^m-1(s+1)b_s+1x^s$



$f'g=sum_j=0^m+n-1c'_jx^j$, with $c'_j=sum_r+s=j(r+1)a_r+1b_s$



$fg'=sum_j=0^m+n-1c''_jx^j$, with $c''_j=sum_r+s=ja_r(s+1)b_s+1$



I have tried to prove by induction that for every $j$ we have



$c'_j+c''_j=(j+1)c_j+1$



but I failed to show that the coefficients of both polynomials are equal. Where is the mistake? Can somebody give me a hint please?



The Problem is the induction step; I don't know how I can use the induction hypothesis to my advantage; I have made the induction over $j$ and I could Show it for $j=0$ and $j=1$ (just to be sure):



The claim is



$$(j+1)sum_r+s=j+1a_rb_s=sum_r+s=j(r+1)a_r+1b_s+sum_r+s=ja_r(s+1)b_s+1$$



I also know that $(j+1)sum_r+s=j+1a_rb_s=(j+1)sum_r=0^j+1a_rb_j+1-r$










share|cite|improve this question











$endgroup$











  • $begingroup$
    Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
    $endgroup$
    – Jair Taylor
    Mar 12 at 21:08













1












1








1


1



$begingroup$


As I said in the title, in the book where this exercise was proposed, derivatives or limits were not introduced yet. The exercise was to show that given the function $':mathbbC[x]rightarrow mathbbC[x]$ I should show that $(fg)'=f'g+fg'$



$f'(x)$ was defined as $sum_k=1^nka_kx^k-1$ where $f(x)=sum_k=0^na_kx^k$.



I have found out that an equivalent representation for $f'(x)$ is $sum_k=0^n-1(k+1)a_k+1x^k$



I have tried to prove the result myself step by step but there must be an error somewhere in the proof because in the end the result does not fit the bill.



$fg=sum_k=0^m+nc_kx^k$ with $c_k=sum_r+s=ka_rb_s$



$(fg)'=sum_j=0^m+n-1(j+1)c_j+1x^j$



$f=sum_r=0^na_rx^r$



$f'=sum_r=0^n-1(r+1)a_r+1x^r$



$g=sum_s=0^mb_sx^s$



$g'=sum_s=0^m-1(s+1)b_s+1x^s$



$f'g=sum_j=0^m+n-1c'_jx^j$, with $c'_j=sum_r+s=j(r+1)a_r+1b_s$



$fg'=sum_j=0^m+n-1c''_jx^j$, with $c''_j=sum_r+s=ja_r(s+1)b_s+1$



I have tried to prove by induction that for every $j$ we have



$c'_j+c''_j=(j+1)c_j+1$



but I failed to show that the coefficients of both polynomials are equal. Where is the mistake? Can somebody give me a hint please?



The Problem is the induction step; I don't know how I can use the induction hypothesis to my advantage; I have made the induction over $j$ and I could Show it for $j=0$ and $j=1$ (just to be sure):



The claim is



$$(j+1)sum_r+s=j+1a_rb_s=sum_r+s=j(r+1)a_r+1b_s+sum_r+s=ja_r(s+1)b_s+1$$



I also know that $(j+1)sum_r+s=j+1a_rb_s=(j+1)sum_r=0^j+1a_rb_j+1-r$










share|cite|improve this question











$endgroup$




As I said in the title, in the book where this exercise was proposed, derivatives or limits were not introduced yet. The exercise was to show that given the function $':mathbbC[x]rightarrow mathbbC[x]$ I should show that $(fg)'=f'g+fg'$



$f'(x)$ was defined as $sum_k=1^nka_kx^k-1$ where $f(x)=sum_k=0^na_kx^k$.



I have found out that an equivalent representation for $f'(x)$ is $sum_k=0^n-1(k+1)a_k+1x^k$



I have tried to prove the result myself step by step but there must be an error somewhere in the proof because in the end the result does not fit the bill.



$fg=sum_k=0^m+nc_kx^k$ with $c_k=sum_r+s=ka_rb_s$



$(fg)'=sum_j=0^m+n-1(j+1)c_j+1x^j$



$f=sum_r=0^na_rx^r$



$f'=sum_r=0^n-1(r+1)a_r+1x^r$



$g=sum_s=0^mb_sx^s$



$g'=sum_s=0^m-1(s+1)b_s+1x^s$



$f'g=sum_j=0^m+n-1c'_jx^j$, with $c'_j=sum_r+s=j(r+1)a_r+1b_s$



$fg'=sum_j=0^m+n-1c''_jx^j$, with $c''_j=sum_r+s=ja_r(s+1)b_s+1$



I have tried to prove by induction that for every $j$ we have



$c'_j+c''_j=(j+1)c_j+1$



but I failed to show that the coefficients of both polynomials are equal. Where is the mistake? Can somebody give me a hint please?



The Problem is the induction step; I don't know how I can use the induction hypothesis to my advantage; I have made the induction over $j$ and I could Show it for $j=0$ and $j=1$ (just to be sure):



The claim is



$$(j+1)sum_r+s=j+1a_rb_s=sum_r+s=j(r+1)a_r+1b_s+sum_r+s=ja_r(s+1)b_s+1$$



I also know that $(j+1)sum_r+s=j+1a_rb_s=(j+1)sum_r=0^j+1a_rb_j+1-r$







polynomials complex-numbers induction






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Mar 12 at 21:09









J. W. Tanner

3,3351320




3,3351320










asked Mar 12 at 20:12









New2MathNew2Math

12312




12312











  • $begingroup$
    Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
    $endgroup$
    – Jair Taylor
    Mar 12 at 21:08
















  • $begingroup$
    Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
    $endgroup$
    – Jair Taylor
    Mar 12 at 21:08















$begingroup$
Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
$endgroup$
– Jair Taylor
Mar 12 at 21:08




$begingroup$
Try proving it first for monomials (the case $f(x) = x^r$, $g(x) = x^s$) and then use linearity to prove the general case.
$endgroup$
– Jair Taylor
Mar 12 at 21:08










1 Answer
1






active

oldest

votes


















3












$begingroup$

It is easier to not use coefficients at all - except that we initially check by straightforward computation
$$tag1 (f+g)'=f'+g',$$ $$tag2 (cf)'=cf',$$ $$tag3 (xf)'=f+f'.$$
$$tag 4c'=0$$
Let $Asubseteq Bbb C[x]$ be the set of polynomials $f$ with the property that $(fg)'=f'g+fg'$ for all polynomials $g$.



If $f_1,f_2in A$, then for every polynomial $g$,
$$beginalign((f_1+f_2)g)'&=(f_1g+f_2g)'\&=(f_1g)'+(f_2g)'&textby (1)\&=f_1g'+f_1'g+f_2g'+f_2'g\&=(f_1+f_2)g'+(f_1'+f_2')g\
&=(f_1+f_2)g'+(f_1+f_2)'g&textby (1)endalign $$

Thus,




$A$ is closed under addition.




Assume $f_1,f_2in A$. Then for any polynomial $g$,
$$beginalign
((f_1f_2)g)'&=(f_1(f_2g))'\
&=f_1'f_2g+f_1(f_2g)'\
&=f_1'f_2g+f_1(f_2'g+f_2g')\
&=(f_1'f_2+f_1f_2')g+f_1f_2g'.
endalign$$

We conclude that




$A$ is closed under multiplication.




From $(2)$ and $(4)$, we see that




$A$ contains all constants




and from $(3)$ (after verifying that in particular $x'=1$)




$xin A$.




Now observe that the four highlighted statements imply that $A=Bbb C[x]$.






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%2f3145611%2fproving-the-product-rule-for-polynomials-without-limits-or-derivative-rules%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









    3












    $begingroup$

    It is easier to not use coefficients at all - except that we initially check by straightforward computation
    $$tag1 (f+g)'=f'+g',$$ $$tag2 (cf)'=cf',$$ $$tag3 (xf)'=f+f'.$$
    $$tag 4c'=0$$
    Let $Asubseteq Bbb C[x]$ be the set of polynomials $f$ with the property that $(fg)'=f'g+fg'$ for all polynomials $g$.



    If $f_1,f_2in A$, then for every polynomial $g$,
    $$beginalign((f_1+f_2)g)'&=(f_1g+f_2g)'\&=(f_1g)'+(f_2g)'&textby (1)\&=f_1g'+f_1'g+f_2g'+f_2'g\&=(f_1+f_2)g'+(f_1'+f_2')g\
    &=(f_1+f_2)g'+(f_1+f_2)'g&textby (1)endalign $$

    Thus,




    $A$ is closed under addition.




    Assume $f_1,f_2in A$. Then for any polynomial $g$,
    $$beginalign
    ((f_1f_2)g)'&=(f_1(f_2g))'\
    &=f_1'f_2g+f_1(f_2g)'\
    &=f_1'f_2g+f_1(f_2'g+f_2g')\
    &=(f_1'f_2+f_1f_2')g+f_1f_2g'.
    endalign$$

    We conclude that




    $A$ is closed under multiplication.




    From $(2)$ and $(4)$, we see that




    $A$ contains all constants




    and from $(3)$ (after verifying that in particular $x'=1$)




    $xin A$.




    Now observe that the four highlighted statements imply that $A=Bbb C[x]$.






    share|cite|improve this answer









    $endgroup$

















      3












      $begingroup$

      It is easier to not use coefficients at all - except that we initially check by straightforward computation
      $$tag1 (f+g)'=f'+g',$$ $$tag2 (cf)'=cf',$$ $$tag3 (xf)'=f+f'.$$
      $$tag 4c'=0$$
      Let $Asubseteq Bbb C[x]$ be the set of polynomials $f$ with the property that $(fg)'=f'g+fg'$ for all polynomials $g$.



      If $f_1,f_2in A$, then for every polynomial $g$,
      $$beginalign((f_1+f_2)g)'&=(f_1g+f_2g)'\&=(f_1g)'+(f_2g)'&textby (1)\&=f_1g'+f_1'g+f_2g'+f_2'g\&=(f_1+f_2)g'+(f_1'+f_2')g\
      &=(f_1+f_2)g'+(f_1+f_2)'g&textby (1)endalign $$

      Thus,




      $A$ is closed under addition.




      Assume $f_1,f_2in A$. Then for any polynomial $g$,
      $$beginalign
      ((f_1f_2)g)'&=(f_1(f_2g))'\
      &=f_1'f_2g+f_1(f_2g)'\
      &=f_1'f_2g+f_1(f_2'g+f_2g')\
      &=(f_1'f_2+f_1f_2')g+f_1f_2g'.
      endalign$$

      We conclude that




      $A$ is closed under multiplication.




      From $(2)$ and $(4)$, we see that




      $A$ contains all constants




      and from $(3)$ (after verifying that in particular $x'=1$)




      $xin A$.




      Now observe that the four highlighted statements imply that $A=Bbb C[x]$.






      share|cite|improve this answer









      $endgroup$















        3












        3








        3





        $begingroup$

        It is easier to not use coefficients at all - except that we initially check by straightforward computation
        $$tag1 (f+g)'=f'+g',$$ $$tag2 (cf)'=cf',$$ $$tag3 (xf)'=f+f'.$$
        $$tag 4c'=0$$
        Let $Asubseteq Bbb C[x]$ be the set of polynomials $f$ with the property that $(fg)'=f'g+fg'$ for all polynomials $g$.



        If $f_1,f_2in A$, then for every polynomial $g$,
        $$beginalign((f_1+f_2)g)'&=(f_1g+f_2g)'\&=(f_1g)'+(f_2g)'&textby (1)\&=f_1g'+f_1'g+f_2g'+f_2'g\&=(f_1+f_2)g'+(f_1'+f_2')g\
        &=(f_1+f_2)g'+(f_1+f_2)'g&textby (1)endalign $$

        Thus,




        $A$ is closed under addition.




        Assume $f_1,f_2in A$. Then for any polynomial $g$,
        $$beginalign
        ((f_1f_2)g)'&=(f_1(f_2g))'\
        &=f_1'f_2g+f_1(f_2g)'\
        &=f_1'f_2g+f_1(f_2'g+f_2g')\
        &=(f_1'f_2+f_1f_2')g+f_1f_2g'.
        endalign$$

        We conclude that




        $A$ is closed under multiplication.




        From $(2)$ and $(4)$, we see that




        $A$ contains all constants




        and from $(3)$ (after verifying that in particular $x'=1$)




        $xin A$.




        Now observe that the four highlighted statements imply that $A=Bbb C[x]$.






        share|cite|improve this answer









        $endgroup$



        It is easier to not use coefficients at all - except that we initially check by straightforward computation
        $$tag1 (f+g)'=f'+g',$$ $$tag2 (cf)'=cf',$$ $$tag3 (xf)'=f+f'.$$
        $$tag 4c'=0$$
        Let $Asubseteq Bbb C[x]$ be the set of polynomials $f$ with the property that $(fg)'=f'g+fg'$ for all polynomials $g$.



        If $f_1,f_2in A$, then for every polynomial $g$,
        $$beginalign((f_1+f_2)g)'&=(f_1g+f_2g)'\&=(f_1g)'+(f_2g)'&textby (1)\&=f_1g'+f_1'g+f_2g'+f_2'g\&=(f_1+f_2)g'+(f_1'+f_2')g\
        &=(f_1+f_2)g'+(f_1+f_2)'g&textby (1)endalign $$

        Thus,




        $A$ is closed under addition.




        Assume $f_1,f_2in A$. Then for any polynomial $g$,
        $$beginalign
        ((f_1f_2)g)'&=(f_1(f_2g))'\
        &=f_1'f_2g+f_1(f_2g)'\
        &=f_1'f_2g+f_1(f_2'g+f_2g')\
        &=(f_1'f_2+f_1f_2')g+f_1f_2g'.
        endalign$$

        We conclude that




        $A$ is closed under multiplication.




        From $(2)$ and $(4)$, we see that




        $A$ contains all constants




        and from $(3)$ (after verifying that in particular $x'=1$)




        $xin A$.




        Now observe that the four highlighted statements imply that $A=Bbb C[x]$.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 12 at 22:02









        Hagen von EitzenHagen von Eitzen

        283k23272507




        283k23272507



























            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%2f3145611%2fproving-the-product-rule-for-polynomials-without-limits-or-derivative-rules%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

            Moe incest case Sentencing See also References Navigation menu"'Australian Josef Fritzl' fathered four children by daughter""Small town recoils in horror at 'Australian Fritzl' incest case""Victorian rape allegations echo Fritzl case - Just In (Australian Broadcasting Corporation)""Incest father jailed for 22 years""'Australian Fritzl' sentenced to 22 years in prison for abusing daughter for three decades""RSJ v The Queen"

            John Burke, 9th Earl of Clanricarde References Navigation menuA General and heraldic dictionary of the peerage and baronetage of the British EmpireLeigh Rayment's Peerage Pages

            Football at the 1986 Brunei Merdeka Games Contents Teams Group stage Knockout stage References Navigation menu"Brunei Merdeka Games 1986".