Frobenius Method to solve $x(1 - x)y'' - 3xy' - y = 0$












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So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.



$$ x(1 - x)y'' - 3xy' - y = 0 $$










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$endgroup$

















    2












    $begingroup$


    So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.



    $$ x(1 - x)y'' - 3xy' - y = 0 $$










    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$


      So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.



      $$ x(1 - x)y'' - 3xy' - y = 0 $$










      share|cite|improve this question











      $endgroup$




      So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.



      $$ x(1 - x)y'' - 3xy' - y = 0 $$







      ordinary-differential-equations power-series frobenius-method






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      edited Nov 30 '16 at 0:17







      user137731

















      asked Oct 28 '12 at 5:04









      ILoveMathILoveMath

      1




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          $begingroup$

          The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.



          Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
          $$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
          $$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
          $$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
          $$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
          The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
          Plugging in the appropriate series expansions, we get that
          $$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
          Hence, we get that
          $$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
          First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
          Hence, the solution is given by
          $$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
          This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
          begin{align}
          y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
          & = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
          & = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
          & = a_1 dfrac{x}{(1-x)^2}
          end{align}






          share|cite|improve this answer









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            0












            $begingroup$

            The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
            $$
            x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
            implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
            $$

            Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
            into that formula to find
            begin{align}
            frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
            \
            implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
            \
            implies u&=-frac1x+ln|x(1-x)|
            end{align}

            so that the second basis solution is
            $$
            y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
            $$






            share|cite|improve this answer









            $endgroup$













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              2 Answers
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              4












              $begingroup$

              The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.



              Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
              $$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
              $$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
              $$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
              $$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
              The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
              Plugging in the appropriate series expansions, we get that
              $$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
              Hence, we get that
              $$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
              First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
              Hence, the solution is given by
              $$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
              This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
              begin{align}
              y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
              & = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
              & = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
              & = a_1 dfrac{x}{(1-x)^2}
              end{align}






              share|cite|improve this answer









              $endgroup$


















                4












                $begingroup$

                The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.



                Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
                $$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
                $$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
                $$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
                $$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
                The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
                Plugging in the appropriate series expansions, we get that
                $$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
                Hence, we get that
                $$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
                First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
                Hence, the solution is given by
                $$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
                This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
                begin{align}
                y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
                & = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
                & = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
                & = a_1 dfrac{x}{(1-x)^2}
                end{align}






                share|cite|improve this answer









                $endgroup$
















                  4












                  4








                  4





                  $begingroup$

                  The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.



                  Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
                  $$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
                  $$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
                  $$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
                  $$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
                  The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
                  Plugging in the appropriate series expansions, we get that
                  $$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
                  Hence, we get that
                  $$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
                  First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
                  Hence, the solution is given by
                  $$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
                  This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
                  begin{align}
                  y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
                  & = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
                  & = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
                  & = a_1 dfrac{x}{(1-x)^2}
                  end{align}






                  share|cite|improve this answer









                  $endgroup$



                  The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.



                  Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
                  $$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
                  $$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
                  $$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
                  $$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
                  The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
                  Plugging in the appropriate series expansions, we get that
                  $$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
                  Hence, we get that
                  $$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
                  First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
                  Hence, the solution is given by
                  $$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
                  This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
                  begin{align}
                  y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
                  & = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
                  & = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
                  & = a_1 dfrac{x}{(1-x)^2}
                  end{align}







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                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Oct 28 '12 at 5:23







                  user17762






























                      0












                      $begingroup$

                      The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
                      $$
                      x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
                      implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
                      $$

                      Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
                      into that formula to find
                      begin{align}
                      frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
                      \
                      implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
                      \
                      implies u&=-frac1x+ln|x(1-x)|
                      end{align}

                      so that the second basis solution is
                      $$
                      y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
                      $$






                      share|cite|improve this answer









                      $endgroup$


















                        0












                        $begingroup$

                        The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
                        $$
                        x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
                        implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
                        $$

                        Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
                        into that formula to find
                        begin{align}
                        frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
                        \
                        implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
                        \
                        implies u&=-frac1x+ln|x(1-x)|
                        end{align}

                        so that the second basis solution is
                        $$
                        y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
                        $$






                        share|cite|improve this answer









                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
                          $$
                          x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
                          implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
                          $$

                          Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
                          into that formula to find
                          begin{align}
                          frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
                          \
                          implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
                          \
                          implies u&=-frac1x+ln|x(1-x)|
                          end{align}

                          so that the second basis solution is
                          $$
                          y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
                          $$






                          share|cite|improve this answer









                          $endgroup$



                          The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
                          $$
                          x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
                          implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
                          $$

                          Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
                          into that formula to find
                          begin{align}
                          frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
                          \
                          implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
                          \
                          implies u&=-frac1x+ln|x(1-x)|
                          end{align}

                          so that the second basis solution is
                          $$
                          y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
                          $$







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Dec 20 '18 at 18:42









                          LutzLLutzL

                          59.8k42057




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