4.2.9 Example 9 case two
Solve
\begin{align} 2x^{2}y^{\prime \prime }-xy^{\prime }+\left ( 1-2x\right ) y & =0\tag {1}\\ y^{\prime \prime }-\frac {1}{2x}y^{\prime }+\frac {\left ( 1-2x\right ) }{2x^{2}}y & =0\qquad x\neq 0\nonumber \\ y^{\prime \prime }+ay^{\prime }\left ( x\right ) +by & =0\nonumber \end{align}
Hence
\begin{align*} a & =-\frac {1}{2x}\\ b & =\frac {\left ( 1-2x\right ) }{2x^{2}}\end{align*}
It is first transformed to the following ode by eliminating the first derivative
\begin{equation} z^{\prime \prime }=rz \tag {2}\end{equation}
Using what
is known as the Liouville transformation given by
\begin{equation} y=ze^{\frac {-1}{2}\int adx} \tag {3}\end{equation}
Where it can be found that
\(r\) in (2) is
given by
\begin{align} r & =\frac {1}{4}a^{2}+\frac {1}{2}a^{\prime }-b\nonumber \\ & =\frac {1}{4}\left ( -\frac {1}{2x}\right ) ^{2}+\frac {1}{2}\frac {d}{dx}\left ( -\frac {1}{2x}\right ) -\frac {\left ( 1-2x\right ) }{2x^{2}}\nonumber \\ & =\frac {16x-3}{16x^{2}} \tag {4}\end{align}
Hence the DE we will solve using Kovacic algorithm is Eq (2) which is
\begin{equation} z^{\prime \prime }=\frac {16x-3}{16x^{2}}z \tag {5}\end{equation}
Step 0 We need to
find which case it is.
\(r=\frac {s}{t}\) where
\begin{align*} s & =16x-3\\ t & =16x^{2}\end{align*}
The free square factorization of \(t\) is \(t=\left [ 1,x\right ] \). Hence
\begin{equation} m=2 \tag {6}\end{equation}
Since
\(m\) is number of elements in the free square
factorization. in this special case we set
\begin{align*} t_{1} & =1\\ t_{2} & =x \end{align*}
Now
\begin{align*} O\left ( \infty \right ) & =\deg \left ( t\right ) -\deg \left ( s\right ) \\ & =2-1\\ & =1 \end{align*}
There is pole \(x=0\) of order 2. Looking at the cases table, reproduced here
| | | |
| case |
allowed pole order for \(r=\frac {s}{t}\) |
allowed \(O\left ( \infty \right ) \) order |
\(L\) |
| | | |
| 1 |
\(\left \{ 0,1,2,4,6,8,\cdots \right \} \) |
\(\left \{ \cdots ,-8,-6,-4,-2,0,2,3,4,5,6,7,\cdots \right \} \) |
\(\left [ 1\right ] \) |
| | | |
| 2 |
\(\left \{ 2,3,5,7,9,\cdots \right \} \) | no condition | \(\left [ 2\right ] \) |
| | | |
| 3 | \(\left \{ 1,2\right \} \) | \(\left \{ 2,3,4,5,6,7,\cdots \right \} \) | \(\left [ 4,6,12\right ] \) |
| | | |
Shows that only case 2 is possible (\(O\left ( \infty \right ) =1\) is only possible for case 2). Hence \(L=\left [ 2\right ] \).
Step 1
This step has 4 parts (a,b,c,d).
part (a) Here the fixed parts \(e_{fixed},\theta _{fixed}\) are calculated using
\begin{align*} e_{fixed} & =\frac {1}{4}\left ( \min \left ( O\left ( \infty \right ) ,2\right ) -\deg \left ( t\right ) -3\deg \left ( t_{1}\right ) \right ) \\ \theta _{fixed} & =\frac {1}{4}\left ( \frac {t^{\prime }}{t}+3\frac {t_{1}^{\prime }}{t_{1}}\right ) \end{align*}
Using \(O\left ( \infty \right ) =1,t=16x^{2},t_{1}=1\) the above gives
\begin{align*} e_{fixed} & =\frac {1}{4}\left ( \min \left ( 1,2\right ) -2-3\left ( 0\right ) \right ) \\ & =\frac {1}{4}\left ( 1-2\right ) \\ & =-\frac {1}{4}\\ \theta _{fixed} & =\frac {1}{4}\left ( \frac {\frac {d}{dx}\left ( 16x^{2}\right ) }{16x^{2}}+3\left ( 0\right ) \right ) \\ & =\frac {1}{2x}\end{align*}
part (b)
Here the values \(e_{i},\theta _{i}\) are found for \(i=1\cdots k_{2}\) where \(k_{2}\) is the number of roots of \(t_{2}=x\). In other words, the
number of poles of \(r\) that are of order \(2\). There is one pole of order \(2.\) Hence \(k_{2}=1\). the coefficient of \(\frac {1}{\left ( x-0\right ) ^{2}}\)
in the partial fractions of \(r=\frac {16x-3}{16x^{2}}=\frac {1}{x}-\frac {3}{16}\frac {1}{\left ( x-0\right ) ^{2}}\). Therefore \(b=-\frac {3}{16}\). Hence \(e_{1}=\sqrt {1+4b}=\sqrt {1+4\left ( -\frac {3}{16}\right ) }=\allowbreak \frac {1}{2}\) and \(\theta _{1}=\frac {e_{1}}{x-0}=\frac {1}{2x}\). Hence
\begin{align*} e & =\left \{ \frac {1}{2}\right \} \\ \theta & =\left \{ \frac {1}{2x}\right \} \end{align*}
Part (c)
This part applied only to case 1. It is used to generate \(e_{i},\theta _{i}\) for poles of \(r\) order \(4,6,8,\cdots \) if
any exist. Since only case 2 exist in this example. This is skipped. Hence \(k_{2}\) stays
\(0\).
Part(d)
Now we need to find \(e_{0},\theta _{0}\). Since this is not case 1 and since it is not \(O\left ( \infty \right ) >2\) and not \(O\left ( \infty \right ) =2\), then
\begin{align*} e_{0} & =0\\ \theta _{0} & =0 \end{align*}
Hence now we have
\begin{align*} e & =\left \{ 0,\frac {1}{2}\right \} \\ \theta & =\left \{ 0,\frac {1}{2x}\right \} \end{align*}
The above are arranged such that \(e_{0}\) is the first entry. Same for \(\theta \). This to keep the same
notation as in the paper. The above complete step 1, which is to generates the candidate \(e^{\prime }s\)
and \(\theta ^{\prime }s\). In step 2, these are used to generate trials \(d\) and \(\theta \) and find from them \(P\left ( x\right ) \) polynomial if
possible.
Step 2
In this step, we now have all the \(e_{i},\theta _{i}\) values found above in addition to \(e_{fix},\theta _{fix}\).
Starting with \(n=2\). Since case 2 only applies here. And since we have \(k_{2}=1\) then there are \(\left ( n+1\right ) ^{k_{2}+1}=3^{2}=9\)
sets \(s\) to try. The first set \(s\) is
\[ s=\left \{ \frac {-n}{2},\frac {-n}{2}\right \} =\left \{ -1,-1\right \} \]
Now we generate trial
\(d\) using
\begin{equation} d=\left ( n\right ) \left ( e_{fix}\right ) +s_{0}e_{0}-\sum _{i=1}^{k_{2}}s_{i}e_{i}\nonumber \end{equation}
Since
\(k_{2}=1\) then the above
becomes
\begin{equation} d=\left ( n\right ) \left ( e_{fix}\right ) +s_{0}e_{0}-s_{1}e_{1} \tag {7}\end{equation}
If
\(d\geq 0\) then we go and find a trial
\(\Theta \). We need to have both
\(d,\Theta \) to go to the next
step.
\(\Theta \) is found using
\begin{equation} \Theta =\left ( n\right ) \left ( \theta _{fix}\right ) +\sum _{i=0}^{k_{2}}s_{i}\theta _{i} \tag {8}\end{equation}
Hence the first trial
\(d\) is (using Eq (7)) and recalling that
\(e_{fix}=-\frac {1}{4},\theta _{fixed}=\frac {1}{2x}\)
gives
\begin{align*} d & =\left ( 2\right ) \left ( -\frac {1}{4}\right ) +\left ( -1\right ) \left ( 0\right ) -\left ( -1\right ) \left ( \frac {1}{2}\right ) \\ & =0 \end{align*}
We can use this \(d.\)From Eq (8)
\begin{align*} \Theta & =\left ( 2\right ) \left ( \frac {1}{2x}\right ) +s_{0}\theta _{0}+s_{1}\theta _{1}\\ & =\left ( 2\right ) \left ( \frac {1}{2x}\right ) +\left ( -1\right ) \left ( 0\right ) +\left ( -1\right ) \frac {1}{2x}\\ & =\frac {1}{2x}\end{align*}
Since this is case 2 (\(n=2\)) then we need to first find \(P\left ( x\right ) \). The degree is \(d=0\). Hence constant. Say \(P=1\). But
we need to verify this is valid. Setting up the equation
\[ P^{\prime \prime \prime }+3\Theta P^{\prime \prime }+\left ( 3\Theta ^{2}+3\Theta ^{\prime }-4r\right ) P^{\prime }+\left ( \Theta ^{\prime \prime }+3\Theta \Theta ^{\prime }+\Theta ^{3}-4r\Theta -2r^{\prime }\right ) P=0 \]
Which simplifies to (since
\(P=1\))
\[ \Theta ^{\prime \prime }+3\Theta \Theta ^{\prime }+\Theta ^{3}-4r\Theta -2r^{\prime }=0 \]
Using
\(\Theta =\frac {1}{2x},r=\frac {16x-3}{16x^{2}}\)
the above reduces to
\[ 0=0 \]
Hence
\(P\left ( x\right ) =1\) can be used. Now let
\begin{align*} \phi & =\Theta +\frac {P^{\prime }}{P}\\ & =\frac {1}{2x}\end{align*}
We now need to solve for \(\omega \) from (notice that original Kovacic paper has \(+\) and not \(-\) after first
term in the following equation. The \(+\) is from Smith paper. It seems to have been a typo in
original paper as this version gives the correct solution).
\begin{align*} \omega ^{2}-\phi \omega +\left ( \frac {1}{2}\phi ^{\prime }+\frac {1}{2}\phi ^{2}-r\right ) & =0\\ \omega ^{2}-\frac {1}{2x}\omega -\frac {1}{8x^{2}}-\frac {1}{16x^{2}}\left ( 16x-3\right ) & =0 \end{align*}
Solving (and picking first root) gives
\[ \omega =\frac {1}{4x}\left ( 1+4\sqrt {x}\right ) \]
Before using this, we verify it satisfies
\(\omega ^{\prime }+\omega ^{2}=r\)\begin{align*} \frac {d}{dx}\left ( \frac {1}{4x}\left ( 1+4\sqrt {x}\right ) \right ) +\left ( \frac {1}{4x}\left ( 1+4\sqrt {x}\right ) \right ) ^{2} & =\frac {16x-3}{16x^{2}}\\ \frac {1}{16x^{2}}\left ( 16x-3\right ) & =\frac {16x-3}{16x^{2}}\end{align*}
Verified OK. Hence solution is
\begin{align*} z & =e^{\int \omega dx}\\ & =x^{\frac {1}{4}}e^{2\sqrt {x}}\end{align*}
Hence first solution to given ODE is
\begin{align*} y_{1} & =ze^{\frac {-1}{2}\int adx}\\ & =x^{\frac {1}{4}}e^{2\sqrt {x}}e^{-\frac {1}{2}\int -\frac {1}{2x}dx}\\ & =\sqrt {x}e^{2\sqrt {x}}\end{align*}
Second solution \(y_{2}\) can now be find by reduction of order.