# Problem using finite difference to solve a initial value problem

Hallo, I tried to use 'finite difference' method to solve a Initial Value Problem(IVP). For the two boundaries I used periodical condtion and for the differential operators I used 4th degree center approximations. But as result, I got this thing. Where comes this strange oscillation What do you think could be the problem. Should I use a smaller x stepsize? Will use a forward approximation help? Thanks. Actually I am using matlab's odes15s.

The strange thing is that, if I use a biger x stepsize, say 0.1, i will get a smooth result. With s stepsize 0.06, I will get the result showed in the picture. I tried ode45, which is based on an explicit Runge-Kutta (4,5) formula, the Dormand-Prince pair and ode23tb, which is an implementation of TR-BDF2. I got the same result.

Thanks.

\begin{aligned} \dot{q} & = -\frac{\partial (6*q^2/5*h)}{\partial x}-\frac{3*q}{h^2}+h*h'''-(1+10*cos(pi*t))*h*h'\\ \end{aligned}

\begin{aligned} \dot{h} & = -\frac{\partial q}{\partial x} \end{aligned}

\begin{aligned} h(t,0) = h(t,10),q(t,0) = q(t,10) \end{aligned}

$h(0,x)$, $q(0,x)$ are known.

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This question could do with some improving. At present I don't know it fits in with MO's guidelines (but I'm happy to be corrected). – David Roberts Jul 26 '12 at 1:58
To clarify: are you asking about the theoretical reason why you get this result, or are you asking for help to use Matlab? The former question is on-topic, the latter is not, and a specialised Matlab forum might be better placed to help you. – David Roberts Jul 26 '12 at 5:55
I am asking about the theoretical reason. The ODE have up to third degree differential operators in it and it is nonlinear. – gstar2002 Jul 26 '12 at 10:57
MO isn't a place to diagnose software problems but you could make this into a suitable question but you'll have to supply more details. Could you tell us (1) exactly which differential equation are you studying and what are your boundary conditions? (2) Do you know precisely which method your software is using? `finite difference method' is very vague, to me at least. – Ryan Budney Jul 27 '12 at 16:20
@Ryan, I edit my question. I hope that will turn it to a suitable question here. thanks – gstar2002 Aug 1 '12 at 0:18