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q-derivative of binomial



The 2019 Stack Overflow Developer Survey Results Are InA contradiction involving derivativeInequality for the Binomial TheoremLimit of Binomial distributionexplaining the derivative of $x^x$Proof derivative equals zero?Derivative of conditional expectationFlaw in proof involving binomial theoremWhat is the derivative of $frac1(1-x)^2$?Proof by derivative that 2=1Error in the derivative proof










0












$begingroup$


Problem



Find the q-derivative of $(a-x)_q^n$ for $n ge 1$.



Answer



This is actually equation (3.11) of Kac and Cheung's Quantum Calculus
$$D_q(a-x)_q^n=-[n](a-qx)_q^n-1.tag3.11$$



My question



I've got a different answer $-[n](a-x)_q^n-1$ by applying the fact that




$$D_q(x-a)_q^n=-[n](x-a)_q^n-1.tag3.5$$




with $a$ replaced by $-a$ (since $a$ can be any number) and the chain rule for $u(x)=alpha x^beta$




$$D_q f(u(x)) = (D_q^beta f) (u(x)) ; D_q u(x) tag1.15$$




with $u(x)=-x$ and $f(x)=(x+a)_q^n$.



Where's my mistake?



I understand the book's derivation (3.11) line by line, so please don't repeat the book's proof. Instead, please point out my mistakes.










share|cite|improve this question









$endgroup$
















    0












    $begingroup$


    Problem



    Find the q-derivative of $(a-x)_q^n$ for $n ge 1$.



    Answer



    This is actually equation (3.11) of Kac and Cheung's Quantum Calculus
    $$D_q(a-x)_q^n=-[n](a-qx)_q^n-1.tag3.11$$



    My question



    I've got a different answer $-[n](a-x)_q^n-1$ by applying the fact that




    $$D_q(x-a)_q^n=-[n](x-a)_q^n-1.tag3.5$$




    with $a$ replaced by $-a$ (since $a$ can be any number) and the chain rule for $u(x)=alpha x^beta$




    $$D_q f(u(x)) = (D_q^beta f) (u(x)) ; D_q u(x) tag1.15$$




    with $u(x)=-x$ and $f(x)=(x+a)_q^n$.



    Where's my mistake?



    I understand the book's derivation (3.11) line by line, so please don't repeat the book's proof. Instead, please point out my mistakes.










    share|cite|improve this question









    $endgroup$














      0












      0








      0





      $begingroup$


      Problem



      Find the q-derivative of $(a-x)_q^n$ for $n ge 1$.



      Answer



      This is actually equation (3.11) of Kac and Cheung's Quantum Calculus
      $$D_q(a-x)_q^n=-[n](a-qx)_q^n-1.tag3.11$$



      My question



      I've got a different answer $-[n](a-x)_q^n-1$ by applying the fact that




      $$D_q(x-a)_q^n=-[n](x-a)_q^n-1.tag3.5$$




      with $a$ replaced by $-a$ (since $a$ can be any number) and the chain rule for $u(x)=alpha x^beta$




      $$D_q f(u(x)) = (D_q^beta f) (u(x)) ; D_q u(x) tag1.15$$




      with $u(x)=-x$ and $f(x)=(x+a)_q^n$.



      Where's my mistake?



      I understand the book's derivation (3.11) line by line, so please don't repeat the book's proof. Instead, please point out my mistakes.










      share|cite|improve this question









      $endgroup$




      Problem



      Find the q-derivative of $(a-x)_q^n$ for $n ge 1$.



      Answer



      This is actually equation (3.11) of Kac and Cheung's Quantum Calculus
      $$D_q(a-x)_q^n=-[n](a-qx)_q^n-1.tag3.11$$



      My question



      I've got a different answer $-[n](a-x)_q^n-1$ by applying the fact that




      $$D_q(x-a)_q^n=-[n](x-a)_q^n-1.tag3.5$$




      with $a$ replaced by $-a$ (since $a$ can be any number) and the chain rule for $u(x)=alpha x^beta$




      $$D_q f(u(x)) = (D_q^beta f) (u(x)) ; D_q u(x) tag1.15$$




      with $u(x)=-x$ and $f(x)=(x+a)_q^n$.



      Where's my mistake?



      I understand the book's derivation (3.11) line by line, so please don't repeat the book's proof. Instead, please point out my mistakes.







      fake-proofs quantum-calculus






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




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      asked Mar 21 at 20:28









      GNUSupporter 8964民主女神 地下教會GNUSupporter 8964民主女神 地下教會

      14k82651




      14k82651




















          1 Answer
          1






          active

          oldest

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          0












          $begingroup$

          To clear this question from the unanswered queue, I am going to answer my own question.



          It turns out that I had overlooked the line $(a-x)_q^n ne (-1)^n(x-a)_q^n$, where



          $$(x-a)_q^n = begincases
          1 & text if n = 0, tag3.4 \
          (x-a)(x-qa) cdots (x-q^n-1 a) & text if n ge 1.
          endcases$$



          My attempt is actually addressing $D_q (-x + a)_q^n$ instead of $D_q (a-x)^n$.



          N.B. My attempted answer "$D_q (-x + a)_q^n = -[n](a-x)_q^n-1$" is not even correct due to the same reason. It should be "$D_q (-x + a)_q^n = -[n](-x+a)_q^n-1$" instead.






          share|cite|improve this answer









          $endgroup$













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            1 Answer
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            active

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            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

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            active

            oldest

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            0












            $begingroup$

            To clear this question from the unanswered queue, I am going to answer my own question.



            It turns out that I had overlooked the line $(a-x)_q^n ne (-1)^n(x-a)_q^n$, where



            $$(x-a)_q^n = begincases
            1 & text if n = 0, tag3.4 \
            (x-a)(x-qa) cdots (x-q^n-1 a) & text if n ge 1.
            endcases$$



            My attempt is actually addressing $D_q (-x + a)_q^n$ instead of $D_q (a-x)^n$.



            N.B. My attempted answer "$D_q (-x + a)_q^n = -[n](a-x)_q^n-1$" is not even correct due to the same reason. It should be "$D_q (-x + a)_q^n = -[n](-x+a)_q^n-1$" instead.






            share|cite|improve this answer









            $endgroup$

















              0












              $begingroup$

              To clear this question from the unanswered queue, I am going to answer my own question.



              It turns out that I had overlooked the line $(a-x)_q^n ne (-1)^n(x-a)_q^n$, where



              $$(x-a)_q^n = begincases
              1 & text if n = 0, tag3.4 \
              (x-a)(x-qa) cdots (x-q^n-1 a) & text if n ge 1.
              endcases$$



              My attempt is actually addressing $D_q (-x + a)_q^n$ instead of $D_q (a-x)^n$.



              N.B. My attempted answer "$D_q (-x + a)_q^n = -[n](a-x)_q^n-1$" is not even correct due to the same reason. It should be "$D_q (-x + a)_q^n = -[n](-x+a)_q^n-1$" instead.






              share|cite|improve this answer









              $endgroup$















                0












                0








                0





                $begingroup$

                To clear this question from the unanswered queue, I am going to answer my own question.



                It turns out that I had overlooked the line $(a-x)_q^n ne (-1)^n(x-a)_q^n$, where



                $$(x-a)_q^n = begincases
                1 & text if n = 0, tag3.4 \
                (x-a)(x-qa) cdots (x-q^n-1 a) & text if n ge 1.
                endcases$$



                My attempt is actually addressing $D_q (-x + a)_q^n$ instead of $D_q (a-x)^n$.



                N.B. My attempted answer "$D_q (-x + a)_q^n = -[n](a-x)_q^n-1$" is not even correct due to the same reason. It should be "$D_q (-x + a)_q^n = -[n](-x+a)_q^n-1$" instead.






                share|cite|improve this answer









                $endgroup$



                To clear this question from the unanswered queue, I am going to answer my own question.



                It turns out that I had overlooked the line $(a-x)_q^n ne (-1)^n(x-a)_q^n$, where



                $$(x-a)_q^n = begincases
                1 & text if n = 0, tag3.4 \
                (x-a)(x-qa) cdots (x-q^n-1 a) & text if n ge 1.
                endcases$$



                My attempt is actually addressing $D_q (-x + a)_q^n$ instead of $D_q (a-x)^n$.



                N.B. My attempted answer "$D_q (-x + a)_q^n = -[n](a-x)_q^n-1$" is not even correct due to the same reason. It should be "$D_q (-x + a)_q^n = -[n](-x+a)_q^n-1$" instead.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Mar 30 at 10:15









                GNUSupporter 8964民主女神 地下教會GNUSupporter 8964民主女神 地下教會

                14k82651




                14k82651



























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