Question

Find the general solution of the given differential equation on$\left(0,\infty \right)$

${\mathbf{x}}^{\mathbf{2}}\mathbf{y}\mathbf{\text{'}}\mathbf{\text{'}}\mathbf{+}{\mathbf{xy}}^{\mathbf{\text{'}}}\mathbf{+}\left(9x^2-4\right)\mathbf{y}\mathbf{=}\mathbf{0}$

Short answer

The general solution of the given differential equation using the Forbinous Method is

$\mathbf{y}\mathbf{=}{\mathbf{C}}_{\mathbf{1}}{\mathbf{J}}_{\mathbf{2}}\left(3x\right)\mathbf{+}{\mathbf{C}}_{\mathbf{2}}{\mathbf{Y}}_{\mathbf{2}}\left(3x\right)$

Step-by-step solution

Definition of Forbinous method for differential equations.

The Forbinous method is a method for finding an infinite series solution to a second-order ordinary differential equation $x^{2}y\text{'}\text{'}+x{y}^{\text{'}}+\left(9x^2-4\right)y=0$ with and near the regular singular point.

We have the differential equationWhich has a regular singular point at x = 0 and has the form$x^{2}y\text{'}\text{'}+x{y}^{\text{'}}+\left({\alpha }^2{x}^2-{\nu }^2\right)y=0\to \left(1\right)$

Changing the variables of the differential equation

Our aim is to find the general solution for the given differential equation. We can use Forbinous Method to solve this differential equation. First, we change the variables of the differential equation to t instead of x, where

t = ax

And you will end up, regarding , with

$t^2\frac{d^{2}y}{d{t}^2}+t\frac{dy}{dt}+\left(t^2-{\nu }^2\right)y=0$

Assume the solution, according to Forbinous, to be in the form,

$y=\sum _{n=0}^{\infty }{c}_n{t}^{n+r}$

Using Bessel Function of First Kind and Second Kind

Find the first and second derivative, substitute it into the given differential equation and you would end up with two roots, which is r₁ = v and r₂ = -v. Then you can find the two series solution for each case, based on the recurrence relation. Then you will identify the solution to have the form of Bessel Function of First Kind and the two

$$\begin{gathered} y_1\left(x\right)=J_{\nu }\left(\alpha x\right) \\ {y}_2\left(x\right)=J_{-\nu }\left(\alpha x\right) \end{gathered}$$

And you will find that the two solutions are linearly independent for $\nu \ne$integer. For v= integer , we can obtain another solution, which is linearly independent with y₁ (x), using Bessel Function of Second Kind, which is,

$$\begin{gathered} y_3\left(x\right)=Y_{\nu }\left(\alpha x\right) \\ =\frac{\cos \nu \pi J_{\nu }\left(\alpha x\right)-J_{-\nu }\left(\alpha x\right)}{\sin \nu \pi } \end{gathered}$$

Generalized equation of the differential equation

Regarding the given differential equation, we have,

$$\begin{gathered} {\alpha }^2=9\to \alpha =3 \\ {\nu }^2=4\to {\nu }_1=2;{\nu }_2=-2 \end{gathered}$$

Therefore the two solutions are,

$$\begin{gathered} y_1\left(x\right)=J_2\left(3x\right) \\ {y}_2\left(x\right)=J_{-2}\left(3x\right) \end{gathered}$$

which are not linearly independent. We can obtain another solution, which is linearly independent with, using the Bessel Function with Second Kind

$y_3\left(x\right)=Y_2\left(3x\right)$

By using the Superposition Principle, the general solution is

$y=C_1{J}_2\left(3x\right)+C_2{Y}_2\left(3x\right)$

Where C₁, C₂ are arbitrary constants

Therefore the general solution of the given differential equation is$y=C_1{J}_2\left(3x\right)+C_2{Y}_2\left(3x\right)$