2.1.610 Problem 626

Internal problem ID [9782]
Book : Collection of Kovacic problems
Section : section 1
Problem number : 626
Date solved : Sunday, March 30, 2025 at 02:46:31 PM
CAS classification : [[_2nd_order, _with_linear_symmetries]]

Solved as second order ode using Kovacic algorithm

Time used: 0.458 (sec)

Writing the ode as

$$\begin{array}{}\text{(1)}& (x^4+x^2)y^{\prime \prime }+(2x^3+5x)y^{\prime }-21y& =0\text{(2)}& A{y}^{\prime \prime }+B{y}^{\prime }+Cy& =0\end{array}$$

Comparing (1) and (2) shows that

$$\begin{array}{rl}A& =x^4+x^2\\ \text{(3)}& B& =2x^3+5xC& =-21\end{array}$$

Applying the Liouville transformation on the dependent variable gives

$$\begin{array}{rl}z(x)& =y{e}^{\int \frac{B}{2A}dx}\end{array}$$

Then (2) becomes

$$\begin{array}{}\text{(4)}& z^{″}(x)=rz(x)\end{array}$$

Where $r$ is given by

$$\begin{array}{rl}\text{(5)}& r& =\frac{s}{t}& =\frac{2A{B}^{\prime }-2B{A}^{\prime }+B^2-4AC}{4A^2}\end{array}$$

Substituting the values of $A,B,C$ from (3) in the above and simplifying gives

$$\begin{array}{}\text{(6)}& r& =\frac{78x^2+99}{4{(x^3+x)}^2}\end{array}$$

Comparing the above to (5) shows that

$$\begin{array}{rl}s& =78x^2+99\\ t& =4{(x^3+x)}^2\end{array}$$

Therefore eq. (4) becomes

$$\begin{array}{}\text{(7)}& z^{″}(x)& =\left(\frac{78x^2+99}{4{(x^3+x)}^2}\right)z(x)\end{array}$$

Equation (7) is now solved. After finding $z(x)$ then $y$ is found using the inverse transformation

$$\begin{array}{rl}y& =z\left(x\right)e^{-\int \frac{B}{2A}dx}\end{array}$$

The first step is to determine the case of Kovacic algorithm this ode belongs to. There are 3 cases depending on the order of poles of $r$ and the order of $r$ at $\mathrm{\infty }$. The following table summarizes these cases.

Case	Allowed pole order for $r$	Allowed value for $\mathcal{O}(\mathrm{\infty })$
1	$\{0,1,2,4,6,8,\cdots \}$	$\{\cdots ,-6,-4,-2,0,2,3,4,5,6,\cdots \}$
2	Need to have at least one pole that is either order $2$ or odd order greater than $2$. Any other pole order is allowed as long as the above condition is satisfied. Hence the following set of pole orders are all allowed. $\{1,2\}$,$\{1,3\}$,$\{2\}$,$\{3\}$,$\{3,4\}$,$\{1,2,5\}$.	no condition
3	$\{1,2\}$	$\{2,3,4,5,6,7,\cdots \}$

Table 2.610: Necessary conditions for each Kovacic case

The order of $r$ at $\mathrm{\infty }$ is the degree of $t$ minus the degree of $s$. Therefore

$$\begin{array}{rl}O\left(\mathrm{\infty }\right)& =\text{deg}(t)-\text{deg}(s)\\ & =6-2\\ & =4\end{array}$$

The poles of $r$ in eq. (7) and the order of each pole are determined by solving for the roots of $t=4{(x^3+x)}^2$. There is a pole at $x=0$ of order $2$. There is a pole at $x=i$ of order $2$. There is a pole at $x=-i$ of order $2$. Since there is no odd order pole larger than $2$ and the order at $\mathrm{\infty }$ is $4$ then the necessary conditions for case one are met. Since there is a pole of order $2$ then necessary conditions for case two are met. Since pole order is not larger than $2$ and the order at $\mathrm{\infty }$ is $4$ then the necessary conditions for case three are met. Therefore

$$\begin{array}{rl}L& =[1,2,4,6,12]\end{array}$$

Attempting to find a solution using case $n=1$.

Looking at poles of order 2. The partial fractions decomposition of $r$ is

$$r=\frac{99}{4x^2}+\frac{21}{16{(x-i)}^2}+\frac{21}{16{(x+i)}^2}+\frac{219i}{16(x-i)}-\frac{219i}{16(x+i)}$$

For the pole at $x=0$ let $b$ be the coefficient of $\frac{1}{x^2}$ in the partial fractions decomposition of $r$ given above. Therefore $b=\frac{99}{4}$. Hence

$$\begin{array}{rl}[\sqrt{r}{]}_c& =0\\ {\alpha }_c^{+}& =\frac{1}{2}+\sqrt{1+4b}& & =\frac{11}{2}\\ {\alpha }_c^{-}& =\frac{1}{2}-\sqrt{1+4b}& & =-\frac{9}{2}\end{array}$$

For the pole at $x=i$ let $b$ be the coefficient of $\frac{1}{{(x-i)}^2}$ in the partial fractions decomposition of $r$ given above. Therefore $b=\frac{21}{16}$. Hence

$$\begin{array}{rl}[\sqrt{r}{]}_c& =0\\ {\alpha }_c^{+}& =\frac{1}{2}+\sqrt{1+4b}& & =\frac{7}{4}\\ {\alpha }_c^{-}& =\frac{1}{2}-\sqrt{1+4b}& & =-\frac{3}{4}\end{array}$$

For the pole at $x=-i$ let $b$ be the coefficient of $\frac{1}{{(x+i)}^2}$ in the partial fractions decomposition of $r$ given above. Therefore $b=\frac{21}{16}$. Hence

$$\begin{array}{rl}[\sqrt{r}{]}_c& =0\\ {\alpha }_c^{+}& =\frac{1}{2}+\sqrt{1+4b}& & =\frac{7}{4}\\ {\alpha }_c^{-}& =\frac{1}{2}-\sqrt{1+4b}& & =-\frac{3}{4}\end{array}$$

Since the order of $r$ at $\mathrm{\infty }$ is $4>2$ then

$$\begin{array}{rl}[\sqrt{r}{]}_{\mathrm{\infty }}& =0\\ {\alpha }_{\mathrm{\infty }}^{+}& =0\\ {\alpha }_{\mathrm{\infty }}^{-}& =1\end{array}$$

The following table summarizes the findings so far for poles and for the order of $r$ at $\mathrm{\infty }$ where $r$ is

$$r=\frac{78x^2+99}{4{(x^3+x)}^2}$$

pole $c$ location	pole order	$[\sqrt{r}{]}_c$	${\alpha }_c^{+}$	${\alpha }_c^{-}$
$0$	$2$	$0$	$\frac{11}{2}$	$-\frac{9}{2}$
$i$	$2$	$0$	$\frac{7}{4}$	$-\frac{3}{4}$
$-i$	$2$	$0$	$\frac{7}{4}$	$-\frac{3}{4}$

Order of $r$ at $\mathrm{\infty }$	$[\sqrt{r}{]}_{\mathrm{\infty }}$	${\alpha }_{\mathrm{\infty }}^{+}$	${\alpha }_{\mathrm{\infty }}^{-}$
$4$	$0$	$0$	$1$

Now that the all $[\sqrt{r}{]}_c$ and its associated ${\alpha }_c^{\pm }$ have been determined for all the poles in the set $\mathrm{\Gamma }$ and $[\sqrt{r}{]}_{\mathrm{\infty }}$ and its associated ${\alpha }_{\mathrm{\infty }}^{\pm }$ have also been found, the next step is to determine possible non negative integer $d$ from these using

$$\begin{array}{rl}d& ={\alpha }_{\mathrm{\infty }}^{s(\mathrm{\infty })}-\sum _{c\in \mathrm{\Gamma }}{\alpha }_c^{s(c)}\end{array}$$

Where $s(c)$ is either $+$ or $-$ and $s(\mathrm{\infty })$ is the sign of ${\alpha }_{\mathrm{\infty }}^{\pm }$. This is done by trial over all set of families $s=(s(c){)}_{c\in \mathrm{\Gamma }\cup \mathrm{\infty }}$ until such $d$ is found to work in finding candidate $\omega$. Trying ${\alpha }_{\mathrm{\infty }}^{-}=1$ then

$$\begin{array}{rl}d& ={\alpha }_{\mathrm{\infty }}^{-}-({\alpha }_{c_1}^{-}+{\alpha }_{c_2}^{+}+{\alpha }_{c_3}^{+})\\ & =1-(-1)\\ & =2\end{array}$$

Since $d$ an integer and $d\ge 0$ then it can be used to find $\omega$ using

$$\begin{array}{rl}\omega & =\sum _{c\in \mathrm{\Gamma }}(s(c)[\sqrt{r}{]}_c+\frac{{\alpha }_c^{s(c)}}{x-c})+s(\mathrm{\infty })[\sqrt{r}{]}_{\mathrm{\infty }}\end{array}$$

The above gives

$$\begin{array}{rl}\omega & =((-)[\sqrt{r}{]}_{c_1}+\frac{{\alpha }_{c_1}^{-}}{x-c_1})+((+)[\sqrt{r}{]}_{c_2}+\frac{{\alpha }_{c_2}^{+}}{x-c_2})+((+)[\sqrt{r}{]}_{c_3}+\frac{{\alpha }_{c_3}^{+}}{x-c_3})+(-)[\sqrt{r}{]}_{\mathrm{\infty }}\\ & =-\frac{9}{2x}+\frac{7}{4(x-i)}+\frac{7}{4(x+i)}+(-)\left(0\right)\\ & =-\frac{9}{2x}+\frac{7}{4(x-i)}+\frac{7}{4(x+i)}\\ & =-\frac{9}{2x}+\frac{7x}{2x^2+2}\end{array}$$

Now that $\omega$ is determined, the next step is find a corresponding minimal polynomial $p(x)$ of degree $d=2$ to solve the ode. The polynomial $p(x)$ needs to satisfy the equation

$$\begin{array}{}\text{(1A)}& p^{″}+2\omega p^{\prime }+({\omega }^{\prime }+{\omega }^2-r)p=0\end{array}$$

Let

$$\begin{array}{}\text{(2A)}& p(x)& =x^2+a_{1}x+a_0\end{array}$$

Substituting the above in eq. (1A) gives

$$\begin{array}{rl}\left(2\right)+2(-\frac{9}{2x}+\frac{7}{4(x-i)}+\frac{7}{4(x+i)})(2x+a_1)+((\frac{9}{2x^2}-\frac{7}{4{(x-i)}^2}-\frac{7}{4{(x+i)}^2})+{(-\frac{9}{2x}+\frac{7}{4(x-i)}+\frac{7}{4(x+i)})}^2-\left(\frac{78x^2+99}{4{(x^3+x)}^2}\right))& =0\\ \frac{(x^2+1)(2x{a}_0-16x-9a_1)}{{(x+i)}^2{(-x+i)}^{2}x}=0\end{array}$$

Solving for the coefficients $a_i$ in the above using method of undetermined coefficients gives

$$\{a_0=8,a_1=0\}$$

Substituting these coefficients in $p(x)$ in eq. (2A) results in

$$\begin{array}{rl}p(x)& =x^2+8\end{array}$$

Therefore the first solution to the ode $z^{″}=rz$ is

$$\begin{array}{rl}{z}_1(x)& =p{e}^{\int \omega dx}\\ & =(x^2+8){\mathrm{e}}^{\int (-\frac{9}{2x}+\frac{7}{4(x-i)}+\frac{7}{4(x+i)})dx}\\ & =(x^2+8){\mathrm{e}}^{-\frac{9\ln \left(x\right)}{2}+\frac{7\ln (x^2+1)}{4}}\\ & =\frac{(x^2+8){(x^2+1)}^{7/4}}{x^{9/2}}\end{array}$$

The first solution to the original ode in $y$ is found from

$$\begin{array}{rl}{y}_1& =z_1{e}^{\int -\frac{1}{2}\frac{B}{A}dx}\\ & =z_1{e}^{-\int \frac{1}{2}\frac{2x^3+5x}{x^4+x^2}dx}\\ & =z_1{e}^{-\frac{5\ln \left(x\right)}{2}+\frac{3\ln (x^2+1)}{4}}\\ & =z_1\left(\frac{{(x^2+1)}^{3/4}}{x^{5/2}}\right)\end{array}$$

Which simplifies to

$$y_1=\frac{{(x^2+1)}^{5/2}(x^2+8)}{x^7}$$

The second solution $y_2$ to the original ode is found using reduction of order

$$y_2=y_1\int \frac{e^{\int -\frac{B}{A}dx}}{y_1^2}dx$$

Substituting gives

$$\begin{array}{rl}{y}_2& =y_1\int \frac{e^{\int -\frac{2x^3+5x}{x^4+x^2}dx}}{{\left(y_1\right)}^2}dx\\ & =y_1\int \frac{e^{-5\ln \left(x\right)+\frac{3\ln (x^2+1)}{2}}}{{\left(y_1\right)}^2}dx\\ & =y_1(-\frac{(35x^6+140x^4+168x^2+64)x^5{\mathrm{e}}^{-5\ln \left(x\right)+\frac{3\ln (x^2+1)}{2}}}{35{(x^2+1)}^4(x^2+8)})\end{array}$$

Therefore the solution is

$$\begin{array}{rl}y& =c_1{y}_1+c_2{y}_2\\ & =c_1\left(\frac{{(x^2+1)}^{5/2}(x^2+8)}{x^7}\right)+c_2\left(\frac{{(x^2+1)}^{5/2}(x^2+8)}{x^7}(-\frac{(35x^6+140x^4+168x^2+64)x^5{\mathrm{e}}^{-5\ln \left(x\right)+\frac{3\ln (x^2+1)}{2}}}{35{(x^2+1)}^4(x^2+8)})\right)\end{array}$$

Will add steps showing solving for IC soon.

✓ Maple. Time used: 0.018 (sec). Leaf size: 41

ode:=x^2*(x^2+1)*diff(diff(y(x),x),x)+x*(2*x^2+5)*diff(y(x),x)-21*y(x) = 0; 
dsolve(ode,y(x), singsol=all);
 

$$y=\frac{c_1{(x^2+1)}^{5/2}(x^2+8)+35(x^6+4x^4+\frac{24}{5}{x}^2+\frac{64}{35})c_2}{x^7}$$

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)] 
checking if the LODE is missing y 
-> Trying a Liouvillian solution using Kovacics algorithm 
   A Liouvillian solution exists 
   Reducible group (found an exponential solution) 
   Reducible group (found another exponential solution) 
<- Kovacics algorithm successful
 

Maple step by step

$$\begin{array}{lll}& & \text{Let’s solve}\\ & & x^2(x^2+1)\left(\frac{d}{dx}\frac{d}{dx}y\left(x\right)\right)+x(2x^2+5)\left(\frac{d}{dx}y\left(x\right)\right)-21y\left(x\right)=0\\ \bullet & & \text{Highest derivative means the order of the ODE is}2\\ & & \frac{d}{dx}\frac{d}{dx}y\left(x\right)\\ \bullet & & \text{Isolate 2nd derivative}\\ & & \frac{d}{dx}\frac{d}{dx}y\left(x\right)=\frac{21y\left(x\right)}{x^2(x^2+1)}-\frac{(2x^2+5)\left(\frac{d}{dx}y\left(x\right)\right)}{x(x^2+1)}\\ \bullet & & \text{Group terms with}y\left(x\right)\text{on the lhs of the ODE and the rest on the rhs of the ODE; ODE is linear}\\ & & \frac{d}{dx}\frac{d}{dx}y\left(x\right)+\frac{(2x^2+5)\left(\frac{d}{dx}y\left(x\right)\right)}{x(x^2+1)}-\frac{21y\left(x\right)}{x^2(x^2+1)}=0\\ ◻& & \text{Check to see if}{x}_0\text{is a regular singular point}\\ & \circ & \text{Define functions}\\ & & [P_2\left(x\right)=\frac{2x^2+5}{x(x^2+1)},P_3\left(x\right)=-\frac{21}{x^2(x^2+1)}]\\ & \circ & x\cdot P_2\left(x\right)\text{is analytic at}x=0\\ & & (x\cdot P_2\left(x\right))|{}_{x=0}=5\\ & \circ & x^2\cdot P_3\left(x\right)\text{is analytic at}x=0\\ & & (x^2\cdot P_3\left(x\right))|{}_{x=0}=-21\\ & \circ & x=0\text{is a regular singular point}\\ & & \text{Check to see if}{x}_0\text{is a regular singular point}\\ & & x_0=0\\ \bullet & & \text{Multiply by denominators}\\ & & x^2(x^2+1)\left(\frac{d}{dx}\frac{d}{dx}y\left(x\right)\right)+x(2x^2+5)\left(\frac{d}{dx}y\left(x\right)\right)-21y\left(x\right)=0\\ \bullet & & \text{Assume series solution for}y\left(x\right)\\ & & y\left(x\right)=\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k{x}^{k+r}\\ ◻& & \text{Rewrite ODE with series expansions}\\ & \circ & \text{Convert}{x}^m\cdot \left(\frac{d}{dx}y\left(x\right)\right)\text{to series expansion for}m=1..3\\ & & x^m\cdot \left(\frac{d}{dx}y\left(x\right)\right)=\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k(k+r)x^{k+r-1+m}\\ & \circ & \text{Shift index using}k->k+1-m\\ & & x^m\cdot \left(\frac{d}{dx}y\left(x\right)\right)=\stackrel{\mathrm{\infty }}{\sum _{k=-1+m}}{a}_{k+1-m}(k+1-m+r)x^{k+r}\\ & \circ & \text{Convert}{x}^m\cdot \left(\frac{d}{dx}\frac{d}{dx}y\left(x\right)\right)\text{to series expansion for}m=2..4\\ & & x^m\cdot \left(\frac{d}{dx}\frac{d}{dx}y\left(x\right)\right)=\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k(k+r)(k+r-1)x^{k+r-2+m}\\ & \circ & \text{Shift index using}k->k+2-m\\ & & x^m\cdot \left(\frac{d}{dx}\frac{d}{dx}y\left(x\right)\right)=\stackrel{\mathrm{\infty }}{\sum _{k=-2+m}}{a}_{k+2-m}(k+2-m+r)(k+1-m+r)x^{k+r}\\ & & \text{Rewrite ODE with series expansions}\\ & & a_0(7+r)(-3+r)x^r+a_1(8+r)(-2+r)x^{1+r}+(\stackrel{\mathrm{\infty }}{\sum _{k=2}}(a_k(k+r+7)(k+r-3)+a_{k-2}(k-2+r)(k+r-1))x^{k+r})=0\\ \bullet & & a_0\text{cannot be 0 by assumption, giving the indicial equation}\\ & & (7+r)(-3+r)=0\\ \bullet & & \text{Values of r that satisfy the indicial equation}\\ & & r\in \{-7,3\}\\ \bullet & & \text{Each term must be 0}\\ & & a_1(8+r)(-2+r)=0\\ \bullet & & \text{Solve for the dependent coefficient(s)}\\ & & a_1=0\\ \bullet & & \text{Each term in the series must be 0, giving the recursion relation}\\ & & a_k(k+r+7)(k+r-3)+a_{k-2}(k-2+r)(k+r-1)=0\\ \bullet & & \text{Shift index using}k->k+2\\ & & a_{k+2}(k+9+r)(k+r-1)+a_k(k+r)(k+r+1)=0\\ \bullet & & \text{Recursion relation that defines series solution to ODE}\\ & & a_{k+2}=-\frac{a_k(k+r)(k+r+1)}{(k+9+r)(k+r-1)}\\ \bullet & & \text{Recursion relation for}r=-7\text{; series terminates at}k=6\\ & & a_{k+2}=-\frac{a_k(k-7)(k-6)}{(k+2)(k-8)}\\ \bullet & & \text{Solution for}r=-7\\ & & [y\left(x\right)=\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k{x}^{k-7},a_{k+2}=-\frac{a_k(k-7)(k-6)}{(k+2)(k-8)},a_1=0]\\ \bullet & & \text{Recursion relation for}r=3\\ & & a_{k+2}=-\frac{a_k(k+3)(k+4)}{(k+12)(k+2)}\\ \bullet & & \text{Solution for}r=3\\ & & [y\left(x\right)=\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k{x}^{k+3},a_{k+2}=-\frac{a_k(k+3)(k+4)}{(k+12)(k+2)},a_1=0]\\ \bullet & & \text{Combine solutions and rename parameters}\\ & & [y\left(x\right)=(\stackrel{\mathrm{\infty }}{\sum _{k=0}}{a}_k{x}^{k-7})+(\stackrel{\mathrm{\infty }}{\sum _{k=0}}{b}_k{x}^{k+3}),a_{k+2}=-\frac{a_k(k-7)(k-6)}{(k+2)(k-8)},a_1=0,b_{k+2}=-\frac{b_k(k+3)(k+4)}{(k+12)(k+2)},b_1=0]\end{array}$$

✓ Mathematica. Time used: 0.431 (sec). Leaf size: 126

ode=x^2*(1+x^2)*D[y[x],{x,2}]+x*(5+2*x^2)*D[y[x],x]-21*y[x]==0; 
ic={}; 
DSolve[{ode,ic},y[x],x,IncludeSingularSolutions->True]
 

$$y(x)\to (x^2+8)\exp ({\int }_1^x-\frac{2K[1{]}^2+9}{2(K[1{]}^3+K[1])}dK[1]-\frac{1}{2}{\int }_1^x\frac{2K[2{]}^2+5}{K[2{]}^3+K[2]}dK[2])(c_2{\int }_1^x\frac{\exp (-2{\int }_1^{K[3]}-\frac{2K[1{]}^2+9}{2(K[1{]}^3+K[1])}dK[1])}{{(K[3{]}^2+8)}^2}dK[3]+c_1)$$

✗ Sympy

from sympy import * 
x = symbols("x") 
y = Function("y") 
ode = Eq(x**2*(x**2 + 1)*Derivative(y(x), (x, 2)) + x*(2*x**2 + 5)*Derivative(y(x), x) - 21*y(x),0) 
ics = {} 
dsolve(ode,func=y(x),ics=ics)
 

False