# Differential equation

• Nov 6th 2008, 01:17 PM
jokke22
Differential equation
I have a diff. equation given as;

$X{n+1} - \frac {{3}} {{2}}Xn + X{n-1} = n^{2} + 1$

$n > 1$

What is the general solution
$X^{s}{n}$
to the assosiated diff. equation? (Thinking)

Any help/pointers would be greatly appriciated!
• Jul 17th 2016, 06:51 AM
Rebesques
Re: Differential equation
This is in fact the discrete analogue to a differential equation, called a difference equation.

Begin by searching for solutions to the associated homogeneous equation: $x_{n+1}-(3/2)x_n+x_{n-1}=0, n\in \mathbb{N}$
Set $x_n=r^n, n\in \mathbb{N}$ and substitute to obtain the quadratic $r^2-(3/2)r+1=0$, with solutions
$r_{1,2}=\frac{3}{4}\pm{\rm i}\frac{\sqrt{7}}{4}=a\pm {\rm i}b$.

The general solution to the homogeneous equation becomes $x_n=c_1a^n\cos(bn)+c_2a^n\sin(bn), n\in \mathbb{N}$, where the $c_i,i=1,2$ are constants.

Now we seek for a particular solution $x_n^p$ to the original equation. Since the right hand side is a quadratic polynomial in $n$, we can try $x_n^p=n^2+cn+d,$ where $c,d$ are to be determined by substitution.

After this is done, the general solution to the original equation is $x_n=c_1a^n\cos(bn)+c_2a^n\sin(bn)+n^2+cn+d, n\in \mathbb{N}$.