4.57 problem 54
Internal
problem
ID
[7926]
Book
:
Own
collection
of
miscellaneous
problems
Section
:
section
4.0
Problem
number
:
54
Date
solved
:
Monday, October 21, 2024 at 04:35:04 PM
CAS
classification
:
[[_2nd_order, _exact, _linear, _homogeneous]]
Solve
\begin{align*} x y^{\prime \prime }+\left (2-x \right ) y^{\prime }-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.
\[ x y^{\prime \prime }+\left (2-x \right ) y^{\prime }-y = 0 \]
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 -2}{x}\\ q(x) &= -\frac {1}{x}\\ \end{align*}
Table 103: Table \(p(x),q(x)\) singularites.
| |
| \(p(x)=-\frac {x -2}{x}\) |
| |
|
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
\[ x y^{\prime \prime }+\left (2-x \right ) y^{\prime }-y = 0 \]
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} x \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) \left (n +r -1\right ) a_{n} x^{n +r -2}\right )+\left (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 }}x^{n +r -1} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\moverset {\infty }{\munderset {n =0}{\sum }}\left (-x^{n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}2 \left (n +r \right ) a_{n} x^{n +r -1}\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 \(n +r -1\) in each summation term. Going over each summation
term above with power of \(x\) in it which is not already \(x^{n +r -1}\) and adjusting the power and
the corresponding index gives
\begin{align*}
\moverset {\infty }{\munderset {n =0}{\sum }}\left (-x^{n +r} a_{n} \left (n +r \right )\right ) &= \moverset {\infty }{\munderset {n =1}{\sum }}\left (-a_{n -1} \left (n +r -1\right ) x^{n +r -1}\right ) \\
\moverset {\infty }{\munderset {n =0}{\sum }}\left (-a_{n} x^{n +r}\right ) &= \moverset {\infty }{\munderset {n =1}{\sum }}\left (-a_{n -1} x^{n +r -1}\right ) \\
\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 \(n +r -1\).
\begin{equation}
\tag{2B} \left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{n +r -1} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\moverset {\infty }{\munderset {n =1}{\sum }}\left (-a_{n -1} \left (n +r -1\right ) x^{n +r -1}\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}2 \left (n +r \right ) a_{n} x^{n +r -1}\right )+\moverset {\infty }{\munderset {n =1}{\sum }}\left (-a_{n -1} x^{n +r -1}\right ) = 0
\end{equation}
The indicial
equation is obtained from \(n = 0\). From Eq (2B) this gives
\[ x^{n +r -1} a_{n} \left (n +r \right ) \left (n +r -1\right )+2 \left (n +r \right ) a_{n} x^{n +r -1} = 0 \]
When \(n = 0\) the above becomes
\[ x^{-1+r} a_{0} r \left (-1+r \right )+2 r a_{0} x^{-1+r} = 0 \]
Or
\[ \left (x^{-1+r} r \left (-1+r \right )+2 r \,x^{-1+r}\right ) a_{0} = 0 \]
Since \(a_{0}\neq 0\) then the above simplifies to
\[ r \,x^{-1+r} \left (1+r \right ) = 0 \]
Since the above is true for all \(x\) then the
indicial equation becomes
\[ r \left (1+r \right ) = 0 \]
Solving for \(r\) gives the roots of the indicial equation as
\begin{align*} r_1 &= 0\\ r_2 &= -1 \end{align*}
Since \(a_{0}\neq 0\) then the indicial equation becomes
\[ r \,x^{-1+r} \left (1+r \right ) = 0 \]
Solving for \(r\) gives the roots of the indicial equation
as \([0, -1]\).
Since \(r_1 - r_2 = 1\) 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} \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 -1}\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 )+2 a_{n} \left (n +r \right )-a_{n -1} = 0
\end{equation}
Solving for \(a_{n}\) from
recursive equation (4) gives
\[ a_{n} = \frac {a_{n -1}}{n +1+r}\tag {4} \]
Which for the root \(r = 0\) becomes
\[ a_{n} = \frac {a_{n -1}}{n +1}\tag {5} \]
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
\[ a_{1}=\frac {1}{2+r} \]
Which for the root \(r = 0\) becomes
\[ a_{1}={\frac {1}{2}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) | \(1\) |
| | |
| \(a_{1}\) | \(\frac {1}{2+r}\) | \(\frac {1}{2}\) |
| | |
For \(n = 2\), using the above recursive equation gives
\[ a_{2}=\frac {1}{\left (2+r \right ) \left (3+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{2}={\frac {1}{6}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) | \(1\) |
| | |
| \(a_{1}\) | \(\frac {1}{2+r}\) | \(\frac {1}{2}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{6}\) |
| | |
For \(n = 3\), using the above recursive equation gives
\[ a_{3}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{3}={\frac {1}{24}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(\frac {1}{2+r}\) | \(\frac {1}{2}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{6}\) |
| | |
| \(a_{3}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) |
\(\frac {1}{24}\) |
| | |
For \(n = 4\), using the above recursive equation gives
\[ a_{4}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{4}={\frac {1}{120}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(\frac {1}{2+r}\) | \(\frac {1}{2}\) |
| | |
| \(a_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{6}\) |
| | |
| \(a_{3}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) | \(\frac {1}{24}\) |
| | |
| \(a_{4}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )}\) |
\(\frac {1}{120}\) |
| | |
For \(n = 5\), using the above recursive equation gives
\[ a_{5}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right ) \left (5+r \right ) \left (6+r \right )} \]
Which for the root \(r = 0\) becomes
\[ a_{5}={\frac {1}{720}} \]
And the table
now becomes
| | |
| \(n\) |
\(a_{n ,r}\) |
\(a_{n}\) |
| | |
| \(a_{0}\) |
\(1\) |
\(1\) |
| | |
| \(a_{1}\) |
\(\frac {1}{2+r}\) |
\(\frac {1}{2}\) |
| | |
| \(a_{2}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{6}\) |
| | |
| \(a_{3}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) | \(\frac {1}{24}\) |
| | |
| \(a_{4}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )}\) |
\(\frac {1}{120}\) |
| | |
| \(a_{5}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right ) \left (5+r \right ) \left (6+r \right )}\) |
\(\frac {1}{720}\) |
| | |
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}{2}+\frac {x^{2}}{6}+\frac {x^{3}}{24}+\frac {x^{4}}{120}+\frac {x^{5}}{720}+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 \(N=1\).
Now we need to determine if \(C\) is zero or not. This is done by finding \(\lim _{r\rightarrow r_{2}}a_{1}\left (r \right )\). 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_{1} \\ &= \frac {1}{2+r} \end{align*}
Therefore
\begin{align*} \lim _{r\rightarrow r_{2}}\frac {1}{2+r}&= \lim _{r\rightarrow -1}\frac {1}{2+r}\\ &= 1 \end{align*}
The limit is \(1\). 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 -1} \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 )+2 \left (n +r \right ) b_{n}-b_{n -1} = 0
\end{equation}
Which for for the root \(r = -1\) becomes
\begin{equation}
\tag{4A} b_{n} \left (n -1\right ) \left (n -2\right )-b_{n -1} \left (n -2\right )+2 \left (n -1\right ) b_{n}-b_{n -1} = 0
\end{equation}
Solving for \(b_{n}\) from the recursive equation (4) gives
\[ b_{n} = \frac {b_{n -1}}{n +1+r}\tag {5} \]
Which
for the root \(r = -1\) 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 \(r = -1\) 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
\[ b_{1}=\frac {1}{2+r} \]
Which for the root \(r = -1\) becomes
\[ b_{1}=1 \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) | \(1\) |
| | |
| \(b_{1}\) | \(\frac {1}{2+r}\) | \(1\) |
| | |
For \(n = 2\), using the above recursive equation gives
\[ b_{2}=\frac {1}{\left (2+r \right ) \left (3+r \right )} \]
Which for the root \(r = -1\) becomes
\[ b_{2}={\frac {1}{2}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) | \(1\) |
| | |
| \(b_{1}\) | \(\frac {1}{2+r}\) | \(1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{2}\) |
| | |
For \(n = 3\), using the above recursive equation gives
\[ b_{3}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )} \]
Which for the root \(r = -1\) becomes
\[ b_{3}={\frac {1}{6}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(\frac {1}{2+r}\) | \(1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) |
\(\frac {1}{6}\) |
| | |
For \(n = 4\), using the above recursive equation gives
\[ b_{4}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )} \]
Which for the root \(r = -1\) becomes
\[ b_{4}={\frac {1}{24}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(\frac {1}{2+r}\) | \(1\) |
| | |
| \(b_{2}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) | \(\frac {1}{6}\) |
| | |
| \(b_{4}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )}\) |
\(\frac {1}{24}\) |
| | |
For \(n = 5\), using the above recursive equation gives
\[ b_{5}=\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right ) \left (5+r \right ) \left (6+r \right )} \]
Which for the root \(r = -1\) becomes
\[ b_{5}={\frac {1}{120}} \]
And the table
now becomes
| | |
| \(n\) |
\(b_{n ,r}\) |
\(b_{n}\) |
| | |
| \(b_{0}\) |
\(1\) |
\(1\) |
| | |
| \(b_{1}\) |
\(\frac {1}{2+r}\) |
\(1\) |
| | |
| \(b_{2}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right )}\) | \(\frac {1}{2}\) |
| | |
| \(b_{3}\) | \(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right )}\) | \(\frac {1}{6}\) |
| | |
| \(b_{4}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (5+r \right ) \left (4+r \right )}\) |
\(\frac {1}{24}\) |
| | |
| \(b_{5}\) |
\(\frac {1}{\left (2+r \right ) \left (3+r \right ) \left (4+r \right ) \left (5+r \right ) \left (6+r \right )}\) |
\(\frac {1}{120}\) |
| | |
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} \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}{2}+\frac {x^{2}}{6}+\frac {x^{3}}{24}+\frac {x^{4}}{120}+\frac {x^{5}}{720}+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} \\
\end{align*}
Hence the final solution is
\begin{align*}
y &= y_h \\
&= c_1 \left (1+\frac {x}{2}+\frac {x^{2}}{6}+\frac {x^{3}}{24}+\frac {x^{4}}{120}+\frac {x^{5}}{720}+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} \\
\end{align*}
4.57.1 Maple step by step solution
\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & x \left (\frac {d}{d x}y^{\prime }\right )+\left (2-x \right ) y^{\prime }-y=0 \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 2 \\ {} & {} & \frac {d}{d x}y^{\prime } \\ \bullet & {} & \textrm {Isolate 2nd derivative}\hspace {3pt} \\ {} & {} & \frac {d}{d x}y^{\prime }=\frac {y}{x}+\frac {\left (x -2\right ) y^{\prime }}{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} \\ {} & {} & \frac {d}{d x}y^{\prime }-\frac {\left (x -2\right ) y^{\prime }}{x}-\frac {y}{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 {x -2}{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}}}=2 \\ {} & \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} \\ {} & {} & x \left (\frac {d}{d x}y^{\prime }\right )+\left (2-x \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 =0..1 \\ {} & {} & 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 \cdot \left (\frac {d}{d x}y^{\prime }\right )\hspace {3pt}\textrm {to series expansion}\hspace {3pt} \\ {} & {} & x \cdot \left (\frac {d}{d x}y^{\prime }\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}{d x}y^{\prime }\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 (1+r \right ) x^{-1+r}+\left (\moverset {\infty }{\munderset {k =0}{\sum }}\left (a_{k +1} \left (k +1+r \right ) \left (k +2+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 (1+r \right )=0 \\ \bullet & {} & \textrm {Values of r that satisfy the indicial equation}\hspace {3pt} \\ {} & {} & r \in \left \{-1, 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 +2+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 +2+r} \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =-1 \\ {} & {} & a_{k +1}=\frac {a_{k}}{k +1} \\ \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}}{k +1}\right ] \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =0 \\ {} & {} & a_{k +1}=\frac {a_{k}}{k +2} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =0 \\ {} & {} & \left [y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k}, a_{k +1}=\frac {a_{k}}{k +2}\right ] \\ \bullet & {} & \textrm {Combine solutions and rename parameters}\hspace {3pt} \\ {} & {} & \left [y=\left (\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k -1}\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 +2}\right ] \end {array} \]
4.57.2 Maple trace
Methods for second order ODEs:
4.57.3 Maple dsolve solution
Solving time : 0.017 (sec)
Leaf size : 44
dsolve(x*diff(diff(y(x),x),x)+(2-x)*diff(y(x),x)-y(x) = 0,y(x),
series,x=0)
\[
y = c_1 \left (1+\frac {1}{2} x +\frac {1}{6} x^{2}+\frac {1}{24} x^{3}+\frac {1}{120} x^{4}+\frac {1}{720} x^{5}+\operatorname {O}\left (x^{6}\right )\right )+\frac {c_2 \left (1+x +\frac {1}{2} x^{2}+\frac {1}{6} x^{3}+\frac {1}{24} x^{4}+\frac {1}{120} x^{5}+\operatorname {O}\left (x^{6}\right )\right )}{x}
\]
4.57.4 Mathematica DSolve solution
Solving time : 0.024 (sec)
Leaf size : 62
AsymptoticDSolveValue[{x*D[y[x],{x,2}]+(2-x)*D[y[x],x]-y[x]==0,{}},
y[x],{x,0,5}]
\[
y(x)\to c_1 \left (\frac {x^3}{24}+\frac {x^2}{6}+\frac {x}{2}+\frac {1}{x}+1\right )+c_2 \left (\frac {x^4}{120}+\frac {x^3}{24}+\frac {x^2}{6}+\frac {x}{2}+1\right )
\]