Internal problem ID [1308]
Internal file name [OUTPUT/1309_Sunday_June_05_2022_02_09_35_AM_11476737/index.tex]
Book: Elementary differential equations with boundary value problems. William F. Trench.
Brooks/Cole 2001
Section: Chapter 7 Series Solutions of Linear Second Equations. 7.5 THE METHOD OF
FROBENIUS I. Exercises 7.5. Page 358
Problem number: 17.
ODE order: 2.
ODE degree: 1.
The type(s) of ODE detected by this program : "second order series method. Regular singular point. Difference not integer"
Maple gives the following as the ode type
[[_2nd_order, _with_linear_symmetries]]
\[ \boxed {3 x^{2} y^{\prime \prime }+x \left (x +1\right ) y^{\prime }-y=0} \] With the expansion point for the power series method at \(x = 0\).
The type of the expansion point is first determined. This is done on the homogeneous part of the ODE. \[ 3 x^{2} y^{\prime \prime }+\left (x^{2}+x \right ) y^{\prime }-y = 0 \] The following is summary of singularities for the above ode. Writing the ode as \begin {align*} y^{\prime \prime }+p(x) y^{\prime } + q(x) y &=0 \end {align*}
Where \begin {align*} p(x) &= \frac {x +1}{3 x}\\ q(x) &= -\frac {1}{3 x^{2}}\\ \end {align*}
Combining everything together gives the following summary of singularities for the ode as
Regular singular points : \([0]\)
Irregular singular points : \([\infty ]\)
Since \(x = 0\) is regular singular point, then Frobenius power series is used. The ode is normalized to be \[ 3 x^{2} y^{\prime \prime }+\left (x^{2}+x \right ) y^{\prime }-y = 0 \] Let the solution be represented as Frobenius power series of the form \[ y = \moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n +r} \] Then \begin{align*} y^{\prime } &= \moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) a_{n} x^{n +r -1} \\ y^{\prime \prime } &= \moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) \left (n +r -1\right ) a_{n} x^{n +r -2} \\ \end{align*} Substituting the above back into the ode gives \begin{equation} \tag{1} 3 x^{2} \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) \left (n +r -1\right ) a_{n} x^{n +r -2}\right )+\left (x^{2}+x \right ) \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) a_{n} x^{n +r -1}\right )-\left (\moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n +r}\right ) = 0 \end{equation} Which simplifies to \begin{equation} \tag{2A} \left (\moverset {\infty }{\munderset {n =0}{\sum }}3 x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{1+n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{n +r} a_{n} \left (n +r \right )\right )+\moverset {\infty }{\munderset {n =0}{\sum }}\left (-a_{n} x^{n +r}\right ) = 0 \end{equation} The next step is to make all powers of \(x\) be \(n +r\) in each summation term. Going over each summation term above with power of \(x\) in it which is not already \(x^{n +r}\) and adjusting the power and the corresponding index gives \begin{align*} \moverset {\infty }{\munderset {n =0}{\sum }}x^{1+n +r} a_{n} \left (n +r \right ) &= \moverset {\infty }{\munderset {n =1}{\sum }}a_{n -1} \left (n +r -1\right ) x^{n +r} \\ \end{align*} Substituting all the above in Eq (2A) gives the following equation where now all powers of \(x\) are the same and equal to \(n +r\). \begin{equation} \tag{2B} \left (\moverset {\infty }{\munderset {n =0}{\sum }}3 x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\left (\moverset {\infty }{\munderset {n =1}{\sum }}a_{n -1} \left (n +r -1\right ) x^{n +r}\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{n +r} a_{n} \left (n +r \right )\right )+\moverset {\infty }{\munderset {n =0}{\sum }}\left (-a_{n} x^{n +r}\right ) = 0 \end{equation} The indicial equation is obtained from \(n = 0\). From Eq (2B) this gives \[ 3 x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )+x^{n +r} a_{n} \left (n +r \right )-a_{n} x^{n +r} = 0 \] When \(n = 0\) the above becomes \[ 3 x^{r} a_{0} r \left (-1+r \right )+x^{r} a_{0} r -a_{0} x^{r} = 0 \] Or \[ \left (3 x^{r} r \left (-1+r \right )+x^{r} r -x^{r}\right ) a_{0} = 0 \] Since \(a_{0}\neq 0\) then the above simplifies to \[ \left (3 r^{2}-2 r -1\right ) x^{r} = 0 \] Since the above is true for all \(x\) then the indicial equation becomes \[ 3 r^{2}-2 r -1 = 0 \] Solving for \(r\) gives the roots of the indicial equation as \begin {align*} r_1 &= 1\\ r_2 &= -{\frac {1}{3}} \end {align*}
Since \(a_{0}\neq 0\) then the indicial equation becomes \[ \left (3 r^{2}-2 r -1\right ) x^{r} = 0 \] Solving for \(r\) gives the roots of the indicial equation as \(\left [1, -{\frac {1}{3}}\right ]\).
Since \(r_1 - r_2 = {\frac {4}{3}}\) is not an integer, then we can construct two linearly independent solutions \begin {align*} y_{1}\left (x \right ) &= x^{r_{1}} \left (\moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n}\right )\\ y_{2}\left (x \right ) &= x^{r_{2}} \left (\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n}\right ) \end {align*}
Or \begin {align*} y_{1}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{1+n}\\ y_{2}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n -\frac {1}{3}} \end {align*}
We start by finding \(y_{1}\left (x \right )\). Eq (2B) derived above is now used to find all \(a_{n}\) coefficients. The case \(n = 0\) is skipped since it was used to find the roots of the indicial equation. \(a_{0}\) is arbitrary and taken as \(a_{0} = 1\). For \(1\le n\) the recursive equation is \begin{equation} \tag{3} 3 a_{n} \left (n +r \right ) \left (n +r -1\right )+a_{n -1} \left (n +r -1\right )+a_{n} \left (n +r \right )-a_{n} = 0 \end{equation} Solving for \(a_{n}\) from recursive equation (4) gives \[ a_{n} = -\frac {a_{n -1}}{3 n +3 r +1}\tag {4} \] Which for the root \(r = 1\) becomes \[ a_{n} = -\frac {a_{n -1}}{3 n +4}\tag {5} \] At this point, it is a good idea to keep track of \(a_{n}\) in a table both before substituting \(r = 1\) and after as more terms are found using the above recursive equation.
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
For \(n = 1\), using the above recursive equation gives \[ a_{1}=-\frac {1}{4+3 r} \] Which for the root \(r = 1\) becomes \[ a_{1}=-{\frac {1}{7}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{7}}\) |
For \(n = 2\), using the above recursive equation gives \[ a_{2}=\frac {1}{9 r^{2}+33 r +28} \] Which for the root \(r = 1\) becomes \[ a_{2}={\frac {1}{70}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{7}}\) |
| \(a_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{70}\) |
For \(n = 3\), using the above recursive equation gives \[ a_{3}=-\frac {1}{27 r^{3}+189 r^{2}+414 r +280} \] Which for the root \(r = 1\) becomes \[ a_{3}=-{\frac {1}{910}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{7}}\) |
| \(a_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{70}\) |
| \(a_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{910}}\) |
For \(n = 4\), using the above recursive equation gives \[ a_{4}=\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640} \] Which for the root \(r = 1\) becomes \[ a_{4}={\frac {1}{14560}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{7}}\) |
| \(a_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{70}\) |
| \(a_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{910}}\) |
| \(a_{4}\) | \(\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640}\) | \(\frac {1}{14560}\) |
For \(n = 5\), using the above recursive equation gives \[ a_{5}=-\frac {1}{243 r^{5}+4050 r^{4}+25785 r^{3}+77850 r^{2}+110472 r +58240} \] Which for the root \(r = 1\) becomes \[ a_{5}=-{\frac {1}{276640}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{7}}\) |
| \(a_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{70}\) |
| \(a_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{910}}\) |
| \(a_{4}\) | \(\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640}\) | \(\frac {1}{14560}\) |
| \(a_{5}\) | \(-\frac {1}{243 r^{5}+4050 r^{4}+25785 r^{3}+77850 r^{2}+110472 r +58240}\) | \(-{\frac {1}{276640}}\) |
Using the above table, then the solution \(y_{1}\left (x \right )\) is \begin {align*} y_{1}\left (x \right )&= x \left (a_{0}+a_{1} x +a_{2} x^{2}+a_{3} x^{3}+a_{4} x^{4}+a_{5} x^{5}+a_{6} x^{6}\dots \right ) \\ &= x \left (1-\frac {x}{7}+\frac {x^{2}}{70}-\frac {x^{3}}{910}+\frac {x^{4}}{14560}-\frac {x^{5}}{276640}+O\left (x^{6}\right )\right ) \end {align*}
Now the second solution \(y_{2}\left (x \right )\) is found. Eq (2B) derived above is now used to find all \(b_{n}\) coefficients. The case \(n = 0\) is skipped since it was used to find the roots of the indicial equation. \(b_{0}\) is arbitrary and taken as \(b_{0} = 1\). For \(1\le n\) the recursive equation is \begin{equation} \tag{3} 3 b_{n} \left (n +r \right ) \left (n +r -1\right )+b_{n -1} \left (n +r -1\right )+b_{n} \left (n +r \right )-b_{n} = 0 \end{equation} Solving for \(b_{n}\) from recursive equation (4) gives \[ b_{n} = -\frac {b_{n -1}}{3 n +3 r +1}\tag {4} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{n} = -\frac {b_{n -1}}{3 n}\tag {5} \] At this point, it is a good idea to keep track of \(b_{n}\) in a table both before substituting \(r = -{\frac {1}{3}}\) and after as more terms are found using the above recursive equation.
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
For \(n = 1\), using the above recursive equation gives \[ b_{1}=-\frac {1}{4+3 r} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{1}=-{\frac {1}{3}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{3}}\) |
For \(n = 2\), using the above recursive equation gives \[ b_{2}=\frac {1}{9 r^{2}+33 r +28} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{2}={\frac {1}{18}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{3}}\) |
| \(b_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{18}\) |
For \(n = 3\), using the above recursive equation gives \[ b_{3}=-\frac {1}{27 r^{3}+189 r^{2}+414 r +280} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{3}=-{\frac {1}{162}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{3}}\) |
| \(b_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{18}\) |
| \(b_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{162}}\) |
For \(n = 4\), using the above recursive equation gives \[ b_{4}=\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{4}={\frac {1}{1944}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{3}}\) |
| \(b_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{18}\) |
| \(b_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{162}}\) |
| \(b_{4}\) | \(\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640}\) | \(\frac {1}{1944}\) |
For \(n = 5\), using the above recursive equation gives \[ b_{5}=-\frac {1}{243 r^{5}+4050 r^{4}+25785 r^{3}+77850 r^{2}+110472 r +58240} \] Which for the root \(r = -{\frac {1}{3}}\) becomes \[ b_{5}=-{\frac {1}{29160}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(-\frac {1}{4+3 r}\) | \(-{\frac {1}{3}}\) |
| \(b_{2}\) | \(\frac {1}{9 r^{2}+33 r +28}\) | \(\frac {1}{18}\) |
| \(b_{3}\) | \(-\frac {1}{27 r^{3}+189 r^{2}+414 r +280}\) | \(-{\frac {1}{162}}\) |
| \(b_{4}\) | \(\frac {1}{81 r^{4}+918 r^{3}+3699 r^{2}+6222 r +3640}\) | \(\frac {1}{1944}\) |
| \(b_{5}\) | \(-\frac {1}{243 r^{5}+4050 r^{4}+25785 r^{3}+77850 r^{2}+110472 r +58240}\) | \(-{\frac {1}{29160}}\) |
Using the above table, then the solution \(y_{2}\left (x \right )\) is \begin {align*} y_{2}\left (x \right )&= x \left (b_{0}+b_{1} x +b_{2} x^{2}+b_{3} x^{3}+b_{4} x^{4}+b_{5} x^{5}+b_{6} x^{6}\dots \right ) \\ &= \frac {1-\frac {x}{3}+\frac {x^{2}}{18}-\frac {x^{3}}{162}+\frac {x^{4}}{1944}-\frac {x^{5}}{29160}+O\left (x^{6}\right )}{x^{\frac {1}{3}}} \end {align*}
Therefore the homogeneous solution is \begin{align*} y_h(x) &= c_{1} y_{1}\left (x \right )+c_{2} y_{2}\left (x \right ) \\ &= c_{1} x \left (1-\frac {x}{7}+\frac {x^{2}}{70}-\frac {x^{3}}{910}+\frac {x^{4}}{14560}-\frac {x^{5}}{276640}+O\left (x^{6}\right )\right ) + \frac {c_{2} \left (1-\frac {x}{3}+\frac {x^{2}}{18}-\frac {x^{3}}{162}+\frac {x^{4}}{1944}-\frac {x^{5}}{29160}+O\left (x^{6}\right )\right )}{x^{\frac {1}{3}}} \\ \end{align*} Hence the final solution is \begin{align*} y &= y_h \\ &= c_{1} x \left (1-\frac {x}{7}+\frac {x^{2}}{70}-\frac {x^{3}}{910}+\frac {x^{4}}{14560}-\frac {x^{5}}{276640}+O\left (x^{6}\right )\right )+\frac {c_{2} \left (1-\frac {x}{3}+\frac {x^{2}}{18}-\frac {x^{3}}{162}+\frac {x^{4}}{1944}-\frac {x^{5}}{29160}+O\left (x^{6}\right )\right )}{x^{\frac {1}{3}}} \\ \end{align*}
The solution(s) found are the following \begin{align*} \tag{1} y &= c_{1} x \left (1-\frac {x}{7}+\frac {x^{2}}{70}-\frac {x^{3}}{910}+\frac {x^{4}}{14560}-\frac {x^{5}}{276640}+O\left (x^{6}\right )\right )+\frac {c_{2} \left (1-\frac {x}{3}+\frac {x^{2}}{18}-\frac {x^{3}}{162}+\frac {x^{4}}{1944}-\frac {x^{5}}{29160}+O\left (x^{6}\right )\right )}{x^{\frac {1}{3}}} \\ \end{align*}
Verification of solutions
\[ y = c_{1} x \left (1-\frac {x}{7}+\frac {x^{2}}{70}-\frac {x^{3}}{910}+\frac {x^{4}}{14560}-\frac {x^{5}}{276640}+O\left (x^{6}\right )\right )+\frac {c_{2} \left (1-\frac {x}{3}+\frac {x^{2}}{18}-\frac {x^{3}}{162}+\frac {x^{4}}{1944}-\frac {x^{5}}{29160}+O\left (x^{6}\right )\right )}{x^{\frac {1}{3}}} \] Verified OK.
\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & 3 x^{2} y^{\prime \prime }+\left (x^{2}+x \right ) y^{\prime }-y=0 \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 2 \\ {} & {} & y^{\prime \prime } \\ \bullet & {} & \textrm {Isolate 2nd derivative}\hspace {3pt} \\ {} & {} & y^{\prime \prime }=\frac {y}{3 x^{2}}-\frac {\left (x +1\right ) y^{\prime }}{3 x} \\ \bullet & {} & \textrm {Group terms with}\hspace {3pt} y\hspace {3pt}\textrm {on the lhs of the ODE and the rest on the rhs of the ODE; ODE is linear}\hspace {3pt} \\ {} & {} & y^{\prime \prime }+\frac {\left (x +1\right ) y^{\prime }}{3 x}-\frac {y}{3 x^{2}}=0 \\ \square & {} & \textrm {Check to see if}\hspace {3pt} x_{0}=0\hspace {3pt}\textrm {is a regular singular point}\hspace {3pt} \\ {} & \circ & \textrm {Define functions}\hspace {3pt} \\ {} & {} & \left [P_{2}\left (x \right )=\frac {x +1}{3 x}, P_{3}\left (x \right )=-\frac {1}{3 x^{2}}\right ] \\ {} & \circ & x \cdot P_{2}\left (x \right )\textrm {is analytic at}\hspace {3pt} x =0 \\ {} & {} & \left (x \cdot P_{2}\left (x \right )\right )\bigg | {\mstack {}{_{x \hiderel {=}0}}}=\frac {1}{3} \\ {} & \circ & x^{2}\cdot P_{3}\left (x \right )\textrm {is analytic at}\hspace {3pt} x =0 \\ {} & {} & \left (x^{2}\cdot P_{3}\left (x \right )\right )\bigg | {\mstack {}{_{x \hiderel {=}0}}}=-\frac {1}{3} \\ {} & \circ & x =0\textrm {is a regular singular point}\hspace {3pt} \\ & {} & \textrm {Check to see if}\hspace {3pt} x_{0}=0\hspace {3pt}\textrm {is a regular singular point}\hspace {3pt} \\ {} & {} & x_{0}=0 \\ \bullet & {} & \textrm {Multiply by denominators}\hspace {3pt} \\ {} & {} & 3 x^{2} y^{\prime \prime }+x \left (x +1\right ) y^{\prime }-y=0 \\ \bullet & {} & \textrm {Assume series solution for}\hspace {3pt} y \\ {} & {} & y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k +r} \\ \square & {} & \textrm {Rewrite ODE with series expansions}\hspace {3pt} \\ {} & \circ & \textrm {Convert}\hspace {3pt} x^{m}\cdot y^{\prime }\hspace {3pt}\textrm {to series expansion for}\hspace {3pt} m =1..2 \\ {} & {} & x^{m}\cdot y^{\prime }=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} \left (k +r \right ) x^{k +r -1+m} \\ {} & \circ & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +1-m \\ {} & {} & x^{m}\cdot y^{\prime }=\moverset {\infty }{\munderset {k =-1+m}{\sum }}a_{k +1-m} \left (k +1-m +r \right ) x^{k +r} \\ {} & \circ & \textrm {Convert}\hspace {3pt} x^{2}\cdot y^{\prime \prime }\hspace {3pt}\textrm {to series expansion}\hspace {3pt} \\ {} & {} & x^{2}\cdot y^{\prime \prime }=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} \left (k +r \right ) \left (k +r -1\right ) x^{k +r} \\ & {} & \textrm {Rewrite ODE with series expansions}\hspace {3pt} \\ {} & {} & a_{0} \left (1+3 r \right ) \left (-1+r \right ) x^{r}+\left (\moverset {\infty }{\munderset {k =1}{\sum }}\left (a_{k} \left (3 k +3 r +1\right ) \left (k +r -1\right )+a_{k -1} \left (k +r -1\right )\right ) x^{k +r}\right )=0 \\ \bullet & {} & a_{0}\textrm {cannot be 0 by assumption, giving the indicial equation}\hspace {3pt} \\ {} & {} & \left (1+3 r \right ) \left (-1+r \right )=0 \\ \bullet & {} & \textrm {Values of r that satisfy the indicial equation}\hspace {3pt} \\ {} & {} & r \in \left \{1, -\frac {1}{3}\right \} \\ \bullet & {} & \textrm {Each term in the series must be 0, giving the recursion relation}\hspace {3pt} \\ {} & {} & 3 \left (k +r -1\right ) \left (\left (k +r +\frac {1}{3}\right ) a_{k}+\frac {a_{k -1}}{3}\right )=0 \\ \bullet & {} & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +1 \\ {} & {} & 3 \left (k +r \right ) \left (\left (k +\frac {4}{3}+r \right ) a_{k +1}+\frac {a_{k}}{3}\right )=0 \\ \bullet & {} & \textrm {Recursion relation that defines series solution to ODE}\hspace {3pt} \\ {} & {} & a_{k +1}=-\frac {a_{k}}{3 k +4+3 r} \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =1 \\ {} & {} & a_{k +1}=-\frac {a_{k}}{3 k +7} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =1 \\ {} & {} & \left [y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k +1}, a_{k +1}=-\frac {a_{k}}{3 k +7}\right ] \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =-\frac {1}{3} \\ {} & {} & a_{k +1}=-\frac {a_{k}}{3 k +3} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =-\frac {1}{3} \\ {} & {} & \left [y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k -\frac {1}{3}}, a_{k +1}=-\frac {a_{k}}{3 k +3}\right ] \\ \bullet & {} & \textrm {Combine solutions and rename parameters}\hspace {3pt} \\ {} & {} & \left [y=\left (\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{1+k}\right )+\left (\moverset {\infty }{\munderset {k =0}{\sum }}b_{k} x^{k -\frac {1}{3}}\right ), a_{1+k}=-\frac {a_{k}}{3 k +7}, b_{1+k}=-\frac {b_{k}}{3 k +3}\right ] \end {array} \]
Maple trace Kovacic algorithm successful
`Methods for second order ODEs: --- Trying classification methods --- trying a quadrature checking if the LODE has constant coefficients checking if the LODE is of Euler type trying a symmetry of the form [xi=0, eta=F(x)] checking if the LODE is missing y -> Trying a Liouvillian solution using Kovacics algorithm A Liouvillian solution exists Reducible group (found an exponential solution) Group is reducible, not completely reducible Solution has integrals. Trying a special function solution free of integrals... -> Trying a solution in terms of special functions: -> Bessel -> elliptic -> Legendre -> Whittaker -> hyper3: Equivalence to 1F1 under a power @ Moebius <- hyper3 successful: received ODE is equivalent to the 1F1 ODE <- Whittaker successful <- special function solution successful -> Trying to convert hypergeometric functions to elementary form... <- elementary form for at least one hypergeometric solution is achieved - returning with no uncomputed integrals <- Kovacics algorithm successful`
✓ Solution by Maple
Time used: 0.016 (sec). Leaf size: 45
Order:=6; dsolve(3*x^2*diff(y(x),x$2)+x*(1+x)*diff(y(x),x)-y(x)=0,y(x),type='series',x=0);
\[ y \left (x \right ) = \frac {c_{1} \left (1-\frac {1}{3} x +\frac {1}{18} x^{2}-\frac {1}{162} x^{3}+\frac {1}{1944} x^{4}-\frac {1}{29160} x^{5}+\operatorname {O}\left (x^{6}\right )\right )}{x^{\frac {1}{3}}}+c_{2} x \left (1-\frac {1}{7} x +\frac {1}{70} x^{2}-\frac {1}{910} x^{3}+\frac {1}{14560} x^{4}-\frac {1}{276640} x^{5}+\operatorname {O}\left (x^{6}\right )\right ) \]
✓ Solution by Mathematica
Time used: 0.003 (sec). Leaf size: 86
AsymptoticDSolveValue[3*x^2*y''[x]+x*(1+x)*y'[x]-y[x]==0,y[x],{x,0,5}]
\[ y(x)\to c_1 x \left (-\frac {x^5}{276640}+\frac {x^4}{14560}-\frac {x^3}{910}+\frac {x^2}{70}-\frac {x}{7}+1\right )+\frac {c_2 \left (-\frac {x^5}{29160}+\frac {x^4}{1944}-\frac {x^3}{162}+\frac {x^2}{18}-\frac {x}{3}+1\right )}{\sqrt [3]{x}} \]