How to solve the recursive equation $y^{(n+2)} +(n+1)y^{(n+1)} +tfrac{n(n+1)}{2} y^{(n)}=0$












1














I encounter the problem when I try to get the Taylor series of $arctan x$ at $x=1$. It seems that the method for expanding it at $x=0$ does not work anymore (which is obtained by observing that $arctan' x=frac{1}{1+x^2}=sum_{n=0}^infty (-1)^n x^{2n}$, and then integration over the convergence domain).



So I try to compute the derivatives at $x=1$: Note that if we set $y(x)=arctan x$, then
$$y'=frac{1}{1+x^2},quad y''=frac{-2x}{(1+x^2)^2}implies
(1+x^2)y''=-2x y'.$$

By Leibniz rule, taking derivative of order $n$ at both side, we have
$$
y^{(n+2)}(1)+(n+1)y^{(n+1)}(1)+frac{n(n+1)}{2} y^{(n)}(1)=0.
$$

It is easy to show
$$
y(1)=pi/4,quad y'(1)=1/2,quad y''(1)=-1/2,quad y'''(1)=1/2,quad y''''(1)=0.
$$

I don't know how to get a general formula from the above recursive equation, any ideas?



In fact, I am also searching for a general theory about recursive equations, any reference there?










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    1














    I encounter the problem when I try to get the Taylor series of $arctan x$ at $x=1$. It seems that the method for expanding it at $x=0$ does not work anymore (which is obtained by observing that $arctan' x=frac{1}{1+x^2}=sum_{n=0}^infty (-1)^n x^{2n}$, and then integration over the convergence domain).



    So I try to compute the derivatives at $x=1$: Note that if we set $y(x)=arctan x$, then
    $$y'=frac{1}{1+x^2},quad y''=frac{-2x}{(1+x^2)^2}implies
    (1+x^2)y''=-2x y'.$$

    By Leibniz rule, taking derivative of order $n$ at both side, we have
    $$
    y^{(n+2)}(1)+(n+1)y^{(n+1)}(1)+frac{n(n+1)}{2} y^{(n)}(1)=0.
    $$

    It is easy to show
    $$
    y(1)=pi/4,quad y'(1)=1/2,quad y''(1)=-1/2,quad y'''(1)=1/2,quad y''''(1)=0.
    $$

    I don't know how to get a general formula from the above recursive equation, any ideas?



    In fact, I am also searching for a general theory about recursive equations, any reference there?










    share|cite|improve this question



























      1












      1








      1







      I encounter the problem when I try to get the Taylor series of $arctan x$ at $x=1$. It seems that the method for expanding it at $x=0$ does not work anymore (which is obtained by observing that $arctan' x=frac{1}{1+x^2}=sum_{n=0}^infty (-1)^n x^{2n}$, and then integration over the convergence domain).



      So I try to compute the derivatives at $x=1$: Note that if we set $y(x)=arctan x$, then
      $$y'=frac{1}{1+x^2},quad y''=frac{-2x}{(1+x^2)^2}implies
      (1+x^2)y''=-2x y'.$$

      By Leibniz rule, taking derivative of order $n$ at both side, we have
      $$
      y^{(n+2)}(1)+(n+1)y^{(n+1)}(1)+frac{n(n+1)}{2} y^{(n)}(1)=0.
      $$

      It is easy to show
      $$
      y(1)=pi/4,quad y'(1)=1/2,quad y''(1)=-1/2,quad y'''(1)=1/2,quad y''''(1)=0.
      $$

      I don't know how to get a general formula from the above recursive equation, any ideas?



      In fact, I am also searching for a general theory about recursive equations, any reference there?










      share|cite|improve this question















      I encounter the problem when I try to get the Taylor series of $arctan x$ at $x=1$. It seems that the method for expanding it at $x=0$ does not work anymore (which is obtained by observing that $arctan' x=frac{1}{1+x^2}=sum_{n=0}^infty (-1)^n x^{2n}$, and then integration over the convergence domain).



      So I try to compute the derivatives at $x=1$: Note that if we set $y(x)=arctan x$, then
      $$y'=frac{1}{1+x^2},quad y''=frac{-2x}{(1+x^2)^2}implies
      (1+x^2)y''=-2x y'.$$

      By Leibniz rule, taking derivative of order $n$ at both side, we have
      $$
      y^{(n+2)}(1)+(n+1)y^{(n+1)}(1)+frac{n(n+1)}{2} y^{(n)}(1)=0.
      $$

      It is easy to show
      $$
      y(1)=pi/4,quad y'(1)=1/2,quad y''(1)=-1/2,quad y'''(1)=1/2,quad y''''(1)=0.
      $$

      I don't know how to get a general formula from the above recursive equation, any ideas?



      In fact, I am also searching for a general theory about recursive equations, any reference there?







      sequences-and-series reference-request taylor-expansion recursion analytic-functions






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      edited Nov 18 at 15:33









      Batominovski

      33.7k33292




      33.7k33292










      asked Nov 18 at 14:32









      van abel

      669523




      669523






















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          If you simply want to find the Taylor series about $x=1$ of $text{arctan}(x)$, then you can use $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{1+x^2}=frac{1}{1+(z+1)^2}=frac{1}{2text{i}}left(frac{1}{1-text{i}+z}-frac{1}{1+text{i}+z}right),,$$
          where $z:=x-1$. This gives
          $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{left(1+frac{z}{1-text{i}}right)^{-1}}{1-text{i}}-frac{left(1+frac{z}{1+text{i}}right)^{-1}}{1+text{i}}right),.$$
          Therefore, if $|z|<sqrt{2}$, then
          $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1-text{i}}right)^{k}}{1-text{i}}-frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1+text{i}}right)^{k}}{1+text{i}}right),.$$
          Simplifying this, we have
          $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,(-1)^k,left(frac{frac1{(1-text{i})^{k+1}}-frac{1}{(1+text{i})^{k+1}}}{2text{i}}right),(x-1)^k$$
          for $xinmathbb{C}$ such that $|x-1|<sqrt{2}$.



          Using $1pmtext{i}=sqrt{2},expleft(pmdfrac{text{i}pi}{4}right)$, we conclude that
          $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,frac{(-1)^k}{2^{frac{k+1}{2}}},sinleft(frac{(k+1)pi}{4}right),(x-1)^k$$
          for all complex numbers $x$ with $|x-1|<sqrt{2}$.
          Integrating the series above, we have
          $$text{arctan}(x)=frac{pi}{4}+sum_{k=1}^{infty},frac{(-1)^{k-1}}{2^{frac{k}{2}},k},sinleft(frac{kpi}{4}right),(x-1)^k,.$$
          for every $xinmathbb{C}$ such that $|x-1|<sqrt{2}$. In this way, it follows that
          $$y^{(k)}(1)=begin{cases}frac{pi}{4}&text{if }k=0,,\
          frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)&text{if }k=1,2,3,ldots,,
          end{cases}$$

          where $y(x):=text{arctan}(x)$.





          However, if you really want to solve the recursion without the expansion above, then let $a_k:=dfrac{y^{(k)}(1)}{(k-1)!}$ for $k=1,2,3,ldots$. From
          $$y^{(k+2)}(1)+(k+1),y^{(k+1)}(1)+frac{k(k+1)}{2},y^{(k)}(1)=0text{ for }k=1,2,3,ldots,,$$
          we divide both sides by $(k+1)!$ to get
          $$a_{k+2}+a_{k+1}+frac{1}{2},a_k=0text{ for }k=1,2,3,ldots,.$$
          The characteristic polynomial of the recursion above is $lambda^2+lambda+dfrac{1}{2}$, whose roots are $dfrac{-1pmtext{i}}{2}$. Therefore, for $k=1,2,3,ldots$,
          $$a_k=p,left(frac{-1+text{i}}{2}right)^k+q,left(frac{-1-text{i}}{2}right)^k$$
          for some fixed $p,qinmathbb{C}$. Since $a_1=dfrac{1}{2}$ and $a_2=-dfrac{1}{2}$, we get $$p=+frac{text{i}}{2}text{ and }q=-frac{text{i}}{2},,$$
          whence
          $$y^{(k)}(1)=frac{left(frac{-1+text{i}}{2}right)^k-,left(frac{-1-text{i}}{2}right)^k}{2text{i}},(k-1)!=frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)$$
          for $k=1,2,3,ldots$.






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            If you simply want to find the Taylor series about $x=1$ of $text{arctan}(x)$, then you can use $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{1+x^2}=frac{1}{1+(z+1)^2}=frac{1}{2text{i}}left(frac{1}{1-text{i}+z}-frac{1}{1+text{i}+z}right),,$$
            where $z:=x-1$. This gives
            $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{left(1+frac{z}{1-text{i}}right)^{-1}}{1-text{i}}-frac{left(1+frac{z}{1+text{i}}right)^{-1}}{1+text{i}}right),.$$
            Therefore, if $|z|<sqrt{2}$, then
            $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1-text{i}}right)^{k}}{1-text{i}}-frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1+text{i}}right)^{k}}{1+text{i}}right),.$$
            Simplifying this, we have
            $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,(-1)^k,left(frac{frac1{(1-text{i})^{k+1}}-frac{1}{(1+text{i})^{k+1}}}{2text{i}}right),(x-1)^k$$
            for $xinmathbb{C}$ such that $|x-1|<sqrt{2}$.



            Using $1pmtext{i}=sqrt{2},expleft(pmdfrac{text{i}pi}{4}right)$, we conclude that
            $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,frac{(-1)^k}{2^{frac{k+1}{2}}},sinleft(frac{(k+1)pi}{4}right),(x-1)^k$$
            for all complex numbers $x$ with $|x-1|<sqrt{2}$.
            Integrating the series above, we have
            $$text{arctan}(x)=frac{pi}{4}+sum_{k=1}^{infty},frac{(-1)^{k-1}}{2^{frac{k}{2}},k},sinleft(frac{kpi}{4}right),(x-1)^k,.$$
            for every $xinmathbb{C}$ such that $|x-1|<sqrt{2}$. In this way, it follows that
            $$y^{(k)}(1)=begin{cases}frac{pi}{4}&text{if }k=0,,\
            frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)&text{if }k=1,2,3,ldots,,
            end{cases}$$

            where $y(x):=text{arctan}(x)$.





            However, if you really want to solve the recursion without the expansion above, then let $a_k:=dfrac{y^{(k)}(1)}{(k-1)!}$ for $k=1,2,3,ldots$. From
            $$y^{(k+2)}(1)+(k+1),y^{(k+1)}(1)+frac{k(k+1)}{2},y^{(k)}(1)=0text{ for }k=1,2,3,ldots,,$$
            we divide both sides by $(k+1)!$ to get
            $$a_{k+2}+a_{k+1}+frac{1}{2},a_k=0text{ for }k=1,2,3,ldots,.$$
            The characteristic polynomial of the recursion above is $lambda^2+lambda+dfrac{1}{2}$, whose roots are $dfrac{-1pmtext{i}}{2}$. Therefore, for $k=1,2,3,ldots$,
            $$a_k=p,left(frac{-1+text{i}}{2}right)^k+q,left(frac{-1-text{i}}{2}right)^k$$
            for some fixed $p,qinmathbb{C}$. Since $a_1=dfrac{1}{2}$ and $a_2=-dfrac{1}{2}$, we get $$p=+frac{text{i}}{2}text{ and }q=-frac{text{i}}{2},,$$
            whence
            $$y^{(k)}(1)=frac{left(frac{-1+text{i}}{2}right)^k-,left(frac{-1-text{i}}{2}right)^k}{2text{i}},(k-1)!=frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)$$
            for $k=1,2,3,ldots$.






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              If you simply want to find the Taylor series about $x=1$ of $text{arctan}(x)$, then you can use $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{1+x^2}=frac{1}{1+(z+1)^2}=frac{1}{2text{i}}left(frac{1}{1-text{i}+z}-frac{1}{1+text{i}+z}right),,$$
              where $z:=x-1$. This gives
              $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{left(1+frac{z}{1-text{i}}right)^{-1}}{1-text{i}}-frac{left(1+frac{z}{1+text{i}}right)^{-1}}{1+text{i}}right),.$$
              Therefore, if $|z|<sqrt{2}$, then
              $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1-text{i}}right)^{k}}{1-text{i}}-frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1+text{i}}right)^{k}}{1+text{i}}right),.$$
              Simplifying this, we have
              $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,(-1)^k,left(frac{frac1{(1-text{i})^{k+1}}-frac{1}{(1+text{i})^{k+1}}}{2text{i}}right),(x-1)^k$$
              for $xinmathbb{C}$ such that $|x-1|<sqrt{2}$.



              Using $1pmtext{i}=sqrt{2},expleft(pmdfrac{text{i}pi}{4}right)$, we conclude that
              $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,frac{(-1)^k}{2^{frac{k+1}{2}}},sinleft(frac{(k+1)pi}{4}right),(x-1)^k$$
              for all complex numbers $x$ with $|x-1|<sqrt{2}$.
              Integrating the series above, we have
              $$text{arctan}(x)=frac{pi}{4}+sum_{k=1}^{infty},frac{(-1)^{k-1}}{2^{frac{k}{2}},k},sinleft(frac{kpi}{4}right),(x-1)^k,.$$
              for every $xinmathbb{C}$ such that $|x-1|<sqrt{2}$. In this way, it follows that
              $$y^{(k)}(1)=begin{cases}frac{pi}{4}&text{if }k=0,,\
              frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)&text{if }k=1,2,3,ldots,,
              end{cases}$$

              where $y(x):=text{arctan}(x)$.





              However, if you really want to solve the recursion without the expansion above, then let $a_k:=dfrac{y^{(k)}(1)}{(k-1)!}$ for $k=1,2,3,ldots$. From
              $$y^{(k+2)}(1)+(k+1),y^{(k+1)}(1)+frac{k(k+1)}{2},y^{(k)}(1)=0text{ for }k=1,2,3,ldots,,$$
              we divide both sides by $(k+1)!$ to get
              $$a_{k+2}+a_{k+1}+frac{1}{2},a_k=0text{ for }k=1,2,3,ldots,.$$
              The characteristic polynomial of the recursion above is $lambda^2+lambda+dfrac{1}{2}$, whose roots are $dfrac{-1pmtext{i}}{2}$. Therefore, for $k=1,2,3,ldots$,
              $$a_k=p,left(frac{-1+text{i}}{2}right)^k+q,left(frac{-1-text{i}}{2}right)^k$$
              for some fixed $p,qinmathbb{C}$. Since $a_1=dfrac{1}{2}$ and $a_2=-dfrac{1}{2}$, we get $$p=+frac{text{i}}{2}text{ and }q=-frac{text{i}}{2},,$$
              whence
              $$y^{(k)}(1)=frac{left(frac{-1+text{i}}{2}right)^k-,left(frac{-1-text{i}}{2}right)^k}{2text{i}},(k-1)!=frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)$$
              for $k=1,2,3,ldots$.






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                2












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                2






                If you simply want to find the Taylor series about $x=1$ of $text{arctan}(x)$, then you can use $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{1+x^2}=frac{1}{1+(z+1)^2}=frac{1}{2text{i}}left(frac{1}{1-text{i}+z}-frac{1}{1+text{i}+z}right),,$$
                where $z:=x-1$. This gives
                $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{left(1+frac{z}{1-text{i}}right)^{-1}}{1-text{i}}-frac{left(1+frac{z}{1+text{i}}right)^{-1}}{1+text{i}}right),.$$
                Therefore, if $|z|<sqrt{2}$, then
                $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1-text{i}}right)^{k}}{1-text{i}}-frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1+text{i}}right)^{k}}{1+text{i}}right),.$$
                Simplifying this, we have
                $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,(-1)^k,left(frac{frac1{(1-text{i})^{k+1}}-frac{1}{(1+text{i})^{k+1}}}{2text{i}}right),(x-1)^k$$
                for $xinmathbb{C}$ such that $|x-1|<sqrt{2}$.



                Using $1pmtext{i}=sqrt{2},expleft(pmdfrac{text{i}pi}{4}right)$, we conclude that
                $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,frac{(-1)^k}{2^{frac{k+1}{2}}},sinleft(frac{(k+1)pi}{4}right),(x-1)^k$$
                for all complex numbers $x$ with $|x-1|<sqrt{2}$.
                Integrating the series above, we have
                $$text{arctan}(x)=frac{pi}{4}+sum_{k=1}^{infty},frac{(-1)^{k-1}}{2^{frac{k}{2}},k},sinleft(frac{kpi}{4}right),(x-1)^k,.$$
                for every $xinmathbb{C}$ such that $|x-1|<sqrt{2}$. In this way, it follows that
                $$y^{(k)}(1)=begin{cases}frac{pi}{4}&text{if }k=0,,\
                frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)&text{if }k=1,2,3,ldots,,
                end{cases}$$

                where $y(x):=text{arctan}(x)$.





                However, if you really want to solve the recursion without the expansion above, then let $a_k:=dfrac{y^{(k)}(1)}{(k-1)!}$ for $k=1,2,3,ldots$. From
                $$y^{(k+2)}(1)+(k+1),y^{(k+1)}(1)+frac{k(k+1)}{2},y^{(k)}(1)=0text{ for }k=1,2,3,ldots,,$$
                we divide both sides by $(k+1)!$ to get
                $$a_{k+2}+a_{k+1}+frac{1}{2},a_k=0text{ for }k=1,2,3,ldots,.$$
                The characteristic polynomial of the recursion above is $lambda^2+lambda+dfrac{1}{2}$, whose roots are $dfrac{-1pmtext{i}}{2}$. Therefore, for $k=1,2,3,ldots$,
                $$a_k=p,left(frac{-1+text{i}}{2}right)^k+q,left(frac{-1-text{i}}{2}right)^k$$
                for some fixed $p,qinmathbb{C}$. Since $a_1=dfrac{1}{2}$ and $a_2=-dfrac{1}{2}$, we get $$p=+frac{text{i}}{2}text{ and }q=-frac{text{i}}{2},,$$
                whence
                $$y^{(k)}(1)=frac{left(frac{-1+text{i}}{2}right)^k-,left(frac{-1-text{i}}{2}right)^k}{2text{i}},(k-1)!=frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)$$
                for $k=1,2,3,ldots$.






                share|cite|improve this answer












                If you simply want to find the Taylor series about $x=1$ of $text{arctan}(x)$, then you can use $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{1+x^2}=frac{1}{1+(z+1)^2}=frac{1}{2text{i}}left(frac{1}{1-text{i}+z}-frac{1}{1+text{i}+z}right),,$$
                where $z:=x-1$. This gives
                $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{left(1+frac{z}{1-text{i}}right)^{-1}}{1-text{i}}-frac{left(1+frac{z}{1+text{i}}right)^{-1}}{1+text{i}}right),.$$
                Therefore, if $|z|<sqrt{2}$, then
                $$frac{text{d}}{text{d}x},text{arctan}(x)=frac{1}{2text{i}},left(frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1-text{i}}right)^{k}}{1-text{i}}-frac{sumlimits_{k=0}^infty,(-1)^k,left(frac{z}{1+text{i}}right)^{k}}{1+text{i}}right),.$$
                Simplifying this, we have
                $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,(-1)^k,left(frac{frac1{(1-text{i})^{k+1}}-frac{1}{(1+text{i})^{k+1}}}{2text{i}}right),(x-1)^k$$
                for $xinmathbb{C}$ such that $|x-1|<sqrt{2}$.



                Using $1pmtext{i}=sqrt{2},expleft(pmdfrac{text{i}pi}{4}right)$, we conclude that
                $$frac{text{d}}{text{d}x},text{arctan}(x)=sum_{k=0}^infty,frac{(-1)^k}{2^{frac{k+1}{2}}},sinleft(frac{(k+1)pi}{4}right),(x-1)^k$$
                for all complex numbers $x$ with $|x-1|<sqrt{2}$.
                Integrating the series above, we have
                $$text{arctan}(x)=frac{pi}{4}+sum_{k=1}^{infty},frac{(-1)^{k-1}}{2^{frac{k}{2}},k},sinleft(frac{kpi}{4}right),(x-1)^k,.$$
                for every $xinmathbb{C}$ such that $|x-1|<sqrt{2}$. In this way, it follows that
                $$y^{(k)}(1)=begin{cases}frac{pi}{4}&text{if }k=0,,\
                frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)&text{if }k=1,2,3,ldots,,
                end{cases}$$

                where $y(x):=text{arctan}(x)$.





                However, if you really want to solve the recursion without the expansion above, then let $a_k:=dfrac{y^{(k)}(1)}{(k-1)!}$ for $k=1,2,3,ldots$. From
                $$y^{(k+2)}(1)+(k+1),y^{(k+1)}(1)+frac{k(k+1)}{2},y^{(k)}(1)=0text{ for }k=1,2,3,ldots,,$$
                we divide both sides by $(k+1)!$ to get
                $$a_{k+2}+a_{k+1}+frac{1}{2},a_k=0text{ for }k=1,2,3,ldots,.$$
                The characteristic polynomial of the recursion above is $lambda^2+lambda+dfrac{1}{2}$, whose roots are $dfrac{-1pmtext{i}}{2}$. Therefore, for $k=1,2,3,ldots$,
                $$a_k=p,left(frac{-1+text{i}}{2}right)^k+q,left(frac{-1-text{i}}{2}right)^k$$
                for some fixed $p,qinmathbb{C}$. Since $a_1=dfrac{1}{2}$ and $a_2=-dfrac{1}{2}$, we get $$p=+frac{text{i}}{2}text{ and }q=-frac{text{i}}{2},,$$
                whence
                $$y^{(k)}(1)=frac{left(frac{-1+text{i}}{2}right)^k-,left(frac{-1-text{i}}{2}right)^k}{2text{i}},(k-1)!=frac{(-1)^{k-1},(k-1)!}{2^{frac{k}{2}}},sinleft(frac{kpi}{4}right)$$
                for $k=1,2,3,ldots$.







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                answered Nov 18 at 15:25









                Batominovski

                33.7k33292




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