2.4.52 problem 49
Internal
problem
ID
[8367]
Book
:
Own
collection
of
miscellaneous
problems
Section
:
section
4.0
Problem
number
:
49
Date
solved
:
Sunday, November 10, 2024 at 03:39:32 AM
CAS
classification
:
[[_2nd_order, _with_linear_symmetries], [_2nd_order, _linear, `_with_symmetry_[0,F(x)]`]]
Solve
\begin{align*} x^{2} y^{\prime \prime }+\left (x^{2}+6 x \right ) y^{\prime }+x y&=0 \end{align*}
Using series expansion around \(x=0\)
The type of the expansion point is first determined. This is done on the homogeneous part of
the ODE.
\[ x^{2} y^{\prime \prime }+\left (x^{2}+6 x \right ) y^{\prime }+x y = 0 \]
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 +6}{x}\\ q(x) &= \frac {1}{x}\\ \end{align*}
Table 2.98: Table \(p(x),q(x)\) singularites.
| |
| \(p(x)=\frac {x +6}{x}\) |
| |
|
singularity | type |
| |
| \(x = 0\) | \(\text {``regular''}\) |
| |
| |
| \(q(x)=\frac {1}{x}\) |
| |
|
singularity | type |
| |
| \(x = 0\) | \(\text {``regular''}\) |
| |
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
\[ x^{2} y^{\prime \prime }+\left (x^{2}+6 x \right ) y^{\prime }+x y = 0 \]
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} 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}+6 x \right ) \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) a_{n} x^{n +r -1}\right )+x \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 }}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 }}6 x^{n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{1+n +r} a_{n}\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} \\
\moverset {\infty }{\munderset {n =0}{\sum }}x^{1+n +r} a_{n} &= \moverset {\infty }{\munderset {n =1}{\sum }}a_{n -1} 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 }}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 }}6 x^{n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =1}{\sum }}a_{n -1} x^{n +r}\right ) = 0
\end{equation}
The indicial
equation is obtained from \(n = 0\). From Eq (2B) this gives
\[ x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )+6 x^{n +r} a_{n} \left (n +r \right ) = 0 \]
When \(n = 0\) the above becomes
\[ x^{r} a_{0} r \left (-1+r \right )+6 x^{r} a_{0} r = 0 \]
Or
\[ \left (x^{r} r \left (-1+r \right )+6 x^{r} r \right ) a_{0} = 0 \]
Since \(a_{0}\neq 0\) then the above simplifies to
\[ x^{r} r \left (5+r \right ) = 0 \]
Since the above is true for all \(x\) then the
indicial equation becomes
\[ r \left (5+r \right ) = 0 \]
Solving for \(r\) gives the roots of the indicial equation as
\begin{align*} r_1 &= 0\\ r_2 &= -5 \end{align*}
Since \(a_{0}\neq 0\) then the indicial equation becomes
\[ x^{r} r \left (5+r \right ) = 0 \]
Solving for \(r\) gives the roots of the indicial equation
as \([0, -5]\).
Since \(r_1 - r_2 = 5\) is 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 ) &= C y_{1}\left (x \right ) \ln \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^{n}\\ y_{2}\left (x \right ) &= C y_{1}\left (x \right ) \ln \left (x \right )+\frac {\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n}}{x^{5}} \end{align*}
Or
\begin{align*} y_{1}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n}\\ y_{2}\left (x \right ) &= C y_{1}\left (x \right ) \ln \left (x \right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n -5}\right ) \end{align*}
Where \(C\) above can be zero. We start by finding \(y_{1}\). 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} a_{n} \left (n +r \right ) \left (n +r -1\right )+a_{n -1} \left (n +r -1\right )+6 a_{n} \left (n +r \right )+a_{n -1} = 0
\end{equation}
Solving for \(a_{n}\) from
recursive equation (4) gives
\[ a_{n} = -\frac {a_{n -1}}{n +5+r}\tag {4} \]
Which for the root \(r = 0\) becomes
\[ a_{n} = -\frac {a_{n -1}}{n +5}\tag {5} \]
At this point, it is a good idea to
keep track of \(a_{n}\) in a table both before substituting \(r = 0\) 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
\[ a_{1}=-\frac {1}{6+r} \]
Which for the root \(r = 0\) becomes
\[ a_{1}=-{\frac {1}{6}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) | \(1\) |
| | |
| \(a_{1}\) | \(-\frac {1}{6+r}\) | \(-{\frac {1}{6}}\) |
| | |
For \(n = 2\), using the above recursive equation gives
\[ a_{2}=\frac {1}{\left (6+r \right ) \left (7+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{2}={\frac {1}{42}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) | \(1\) |
| | |
| \(a_{1}\) | \(-\frac {1}{6+r}\) | \(-{\frac {1}{6}}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{42}\) |
| | |
For \(n = 3\), using the above recursive equation gives
\[ a_{3}=-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{3}=-{\frac {1}{336}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(-\frac {1}{6+r}\) | \(-{\frac {1}{6}}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{42}\) |
| | |
| \(a_{3}\) |
\(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) |
\(-{\frac {1}{336}}\) |
| | |
For \(n = 4\), using the above recursive equation gives
\[ a_{4}=\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{4}={\frac {1}{3024}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(-\frac {1}{6+r}\) | \(-{\frac {1}{6}}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{42}\) |
| | |
| \(a_{3}\) | \(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) | \(-{\frac {1}{336}}\) |
| | |
| \(a_{4}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )}\) |
\(\frac {1}{3024}\) |
| | |
For \(n = 5\), using the above recursive equation gives
\[ a_{5}=-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{5}=-{\frac {1}{30240}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(-\frac {1}{6+r}\) |
\(-{\frac {1}{6}}\) |
| | |
| \(a_{2}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{42}\) |
| | |
| \(a_{3}\) | \(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) | \(-{\frac {1}{336}}\) |
| | |
| \(a_{4}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )}\) |
\(\frac {1}{3024}\) |
| | |
| \(a_{5}\) |
\(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )}\) |
\(-{\frac {1}{30240}}\) |
| | |
Using the above table, then the solution \(y_{1}\left (x \right )\) is
\begin{align*} y_{1}\left (x \right )&= 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 \\ &= 1-\frac {x}{6}+\frac {x^{2}}{42}-\frac {x^{3}}{336}+\frac {x^{4}}{3024}-\frac {x^{5}}{30240}+O\left (x^{6}\right ) \end{align*}
Now the second solution \(y_{2}\left (x \right )\) is found. Let
\[ r_{1}-r_{2} = N \]
Where \(N\) is positive integer which is the difference
between the two roots. \(r_{1}\) is taken as the larger root. Hence for this problem we have \(N=5\).
Now we need to determine if \(C\) is zero or not. This is done by finding \(\lim _{r\rightarrow r_{2}}a_{5}\left (r \right )\). If this limit
exists, then \(C = 0\), else we need to keep the log term and \(C \neq 0\). The above table shows that
\begin{align*} a_N &= a_{5} \\ &= -\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )} \end{align*}
Therefore
\begin{align*} \lim _{r\rightarrow r_{2}}-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )}&= \lim _{r\rightarrow -5}-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )}\\ &= -{\frac {1}{120}} \end{align*}
The limit is \(-{\frac {1}{120}}\). Since the limit exists then the log term is not needed and we can set \(C = 0\).
Therefore the second solution has the form
\begin{align*} y_{2}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n +r}\\ &= \moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n -5} \end{align*}
Eq (3) derived above is 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{4} b_{n} \left (n +r \right ) \left (n +r -1\right )+b_{n -1} \left (n +r -1\right )+6 b_{n} \left (n +r \right )+b_{n -1} = 0
\end{equation}
Which for for the root \(r = -5\) becomes
\begin{equation}
\tag{4A} b_{n} \left (n -5\right ) \left (n -6\right )+b_{n -1} \left (n -6\right )+6 b_{n} \left (n -5\right )+b_{n -1} = 0
\end{equation}
Solving for \(b_{n}\) from the recursive equation (4) gives
\[ b_{n} = -\frac {b_{n -1}}{n +5+r}\tag {5} \]
Which
for the root \(r = -5\) becomes
\[ b_{n} = -\frac {b_{n -1}}{n}\tag {6} \]
At this point, it is a good idea to keep track of \(b_{n}\) in a table both before
substituting \(r = -5\) 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
\[ b_{1}=-\frac {1}{6+r} \]
Which for the root \(r = -5\) becomes
\[ b_{1}=-1 \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) | \(1\) |
| | |
| \(b_{1}\) | \(-\frac {1}{6+r}\) | \(-1\) |
| | |
For \(n = 2\), using the above recursive equation gives
\[ b_{2}=\frac {1}{\left (6+r \right ) \left (7+r \right )} \]
Which for the root \(r = -5\) becomes
\[ b_{2}={\frac {1}{2}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) | \(1\) |
| | |
| \(b_{1}\) | \(-\frac {1}{6+r}\) | \(-1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{2}\) |
| | |
For \(n = 3\), using the above recursive equation gives
\[ b_{3}=-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )} \]
Which for the root \(r = -5\) becomes
\[ b_{3}=-{\frac {1}{6}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(-\frac {1}{6+r}\) | \(-1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) |
\(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) |
\(-{\frac {1}{6}}\) |
| | |
For \(n = 4\), using the above recursive equation gives
\[ b_{4}=\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )} \]
Which for the root \(r = -5\) becomes
\[ b_{4}={\frac {1}{24}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(-\frac {1}{6+r}\) | \(-1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) | \(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) | \(-{\frac {1}{6}}\) |
| | |
| \(b_{4}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )}\) |
\(\frac {1}{24}\) |
| | |
For \(n = 5\), using the above recursive equation gives
\[ b_{5}=-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )} \]
Which for the root \(r = -5\) becomes
\[ b_{5}=-{\frac {1}{120}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(-\frac {1}{6+r}\) |
\(-1\) |
| | |
| \(b_{2}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) | \(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right )}\) | \(-{\frac {1}{6}}\) |
| | |
| \(b_{4}\) |
\(\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right )}\) |
\(\frac {1}{24}\) |
| | |
| \(b_{5}\) |
\(-\frac {1}{\left (6+r \right ) \left (7+r \right ) \left (8+r \right ) \left (9+r \right ) \left (10+r \right )}\) |
\(-{\frac {1}{120}}\) |
| | |
Using the above table, then the solution \(y_{2}\left (x \right )\) is
\begin{align*} y_{2}\left (x \right )&= 1 \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-x +\frac {x^{2}}{2}-\frac {x^{3}}{6}+\frac {x^{4}}{24}-\frac {x^{5}}{120}+O\left (x^{6}\right )}{x^{5}} \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 \left (1-\frac {x}{6}+\frac {x^{2}}{42}-\frac {x^{3}}{336}+\frac {x^{4}}{3024}-\frac {x^{5}}{30240}+O\left (x^{6}\right )\right ) + \frac {c_2 \left (1-x +\frac {x^{2}}{2}-\frac {x^{3}}{6}+\frac {x^{4}}{24}-\frac {x^{5}}{120}+O\left (x^{6}\right )\right )}{x^{5}} \\
\end{align*}
Hence the final solution is
\begin{align*}
y &= y_h \\
&= c_1 \left (1-\frac {x}{6}+\frac {x^{2}}{42}-\frac {x^{3}}{336}+\frac {x^{4}}{3024}-\frac {x^{5}}{30240}+O\left (x^{6}\right )\right )+\frac {c_2 \left (1-x +\frac {x^{2}}{2}-\frac {x^{3}}{6}+\frac {x^{4}}{24}-\frac {x^{5}}{120}+O\left (x^{6}\right )\right )}{x^{5}} \\
\end{align*}
Maple step by step solution
\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & x^{2} \left (\frac {d^{2}}{d x^{2}}y \left (x \right )\right )+\left (x^{2}+6 x \right ) \left (\frac {d}{d x}y \left (x \right )\right )+x y \left (x \right )=0 \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 2 \\ {} & {} & \frac {d^{2}}{d x^{2}}y \left (x \right ) \\ \bullet & {} & \textrm {Isolate 2nd derivative}\hspace {3pt} \\ {} & {} & \frac {d^{2}}{d x^{2}}y \left (x \right )=-\frac {y \left (x \right )}{x}-\frac {\left (6+x \right ) \left (\frac {d}{d x}y \left (x \right )\right )}{x} \\ \bullet & {} & \textrm {Group terms with}\hspace {3pt} y \left (x \right )\hspace {3pt}\textrm {on the lhs of the ODE and the rest on the rhs of the ODE; ODE is linear}\hspace {3pt} \\ {} & {} & \frac {d^{2}}{d x^{2}}y \left (x \right )+\frac {\left (6+x \right ) \left (\frac {d}{d x}y \left (x \right )\right )}{x}+\frac {y \left (x \right )}{x}=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 {6+x}{x}, P_{3}\left (x \right )=\frac {1}{x}\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}}}=6 \\ {} & \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}}}=0 \\ {} & \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} \\ {} & {} & \left (\frac {d^{2}}{d x^{2}}y \left (x \right )\right ) x +\left (6+x \right ) \left (\frac {d}{d x}y \left (x \right )\right )+y \left (x \right )=0 \\ \bullet & {} & \textrm {Assume series solution for}\hspace {3pt} y \left (x \right ) \\ {} & {} & y \left (x \right )=\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 \left (\frac {d}{d x}y \left (x \right )\right )\hspace {3pt}\textrm {to series expansion for}\hspace {3pt} m =0..1 \\ {} & {} & x^{m}\cdot \left (\frac {d}{d x}y \left (x \right )\right )=\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 \left (\frac {d}{d x}y \left (x \right )\right )=\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 \cdot \left (\frac {d^{2}}{d x^{2}}y \left (x \right )\right )\hspace {3pt}\textrm {to series expansion}\hspace {3pt} \\ {} & {} & x \cdot \left (\frac {d^{2}}{d x^{2}}y \left (x \right )\right )=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} \left (k +r \right ) \left (k +r -1\right ) x^{k +r -1} \\ {} & \circ & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +1 \\ {} & {} & x \cdot \left (\frac {d^{2}}{d x^{2}}y \left (x \right )\right )=\moverset {\infty }{\munderset {k =-1}{\sum }}a_{k +1} \left (k +1+r \right ) \left (k +r \right ) x^{k +r} \\ & {} & \textrm {Rewrite ODE with series expansions}\hspace {3pt} \\ {} & {} & a_{0} r \left (5+r \right ) x^{-1+r}+\left (\moverset {\infty }{\munderset {k =0}{\sum }}\left (a_{k +1} \left (k +1+r \right ) \left (k +6+r \right )+a_{k} \left (k +1+r \right )\right ) x^{k +r}\right )=0 \\ \bullet & {} & a_{0}\textrm {cannot be 0 by assumption, giving the indicial equation}\hspace {3pt} \\ {} & {} & r \left (5+r \right )=0 \\ \bullet & {} & \textrm {Values of r that satisfy the indicial equation}\hspace {3pt} \\ {} & {} & r \in \left \{-5, 0\right \} \\ \bullet & {} & \textrm {Each term in the series must be 0, giving the recursion relation}\hspace {3pt} \\ {} & {} & \left (k +1+r \right ) \left (a_{k +1} \left (k +6+r \right )+a_{k}\right )=0 \\ \bullet & {} & \textrm {Recursion relation that defines series solution to ODE}\hspace {3pt} \\ {} & {} & a_{k +1}=-\frac {a_{k}}{k +6+r} \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =-5 \\ {} & {} & a_{k +1}=-\frac {a_{k}}{k +1} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =-5 \\ {} & {} & \left [y \left (x \right )=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k -5}, a_{k +1}=-\frac {a_{k}}{k +1}\right ] \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =0 \\ {} & {} & a_{k +1}=-\frac {a_{k}}{k +6} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =0 \\ {} & {} & \left [y \left (x \right )=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k}, a_{k +1}=-\frac {a_{k}}{k +6}\right ] \\ \bullet & {} & \textrm {Combine solutions and rename parameters}\hspace {3pt} \\ {} & {} & \left [y \left (x \right )=\left (\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k -5}\right )+\left (\moverset {\infty }{\munderset {k =0}{\sum }}b_{k} x^{k}\right ), a_{k +1}=-\frac {a_{k}}{k +1}, b_{k +1}=-\frac {b_{k}}{k +6}\right ] \end {array} \]
Maple trace
`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)]
<- linear_1 successful`
Maple dsolve solution
Solving time : 0.024 (sec)
Leaf size : 44
dsolve(x^2*diff(diff(y(x),x),x)+(x^2+6*x)*diff(y(x),x)+x*y(x) = 0,y(x),
series,x=0)
\[
y = c_{1} \left (1-\frac {1}{6} x +\frac {1}{42} x^{2}-\frac {1}{336} x^{3}+\frac {1}{3024} x^{4}-\frac {1}{30240} x^{5}+\operatorname {O}\left (x^{6}\right )\right )+\frac {c_{2} \left (2880-2880 x +1440 x^{2}-480 x^{3}+120 x^{4}-24 x^{5}+\operatorname {O}\left (x^{6}\right )\right )}{x^{5}}
\]
Mathematica DSolve solution
Solving time : 0.029 (sec)
Leaf size : 68
AsymptoticDSolveValue[{x^2*D[y[x],{x,2}]+(6*x+x^2)*D[y[x],x]+x*y[x]==0,{}},
y[x],{x,0,5}]
\[
y(x)\to c_2 \left (\frac {x^4}{3024}-\frac {x^3}{336}+\frac {x^2}{42}-\frac {x}{6}+1\right )+c_1 \left (\frac {1}{x^5}-\frac {1}{x^4}+\frac {1}{2 x^3}-\frac {1}{6 x^2}+\frac {1}{24 x}\right )
\]