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[Help-gsl] a basic question concerning RK4

From: cigdem ozkan
Subject: [Help-gsl] a basic question concerning RK4
Date: Fri, 4 May 2007 02:48:22 -0700 (PDT)

Hey there,
I have a technical question but one has to go through the story to understand 
what I am tyring to achieve. I hope you can find the time to do so. It is very 
basic physics so it only requires patience to read through the rest of my email 
:) Thank you already.
I am trying to simulate the Coulomb repulsion between charged particles. I am 
using RK4 to do this which requires a function. The physics formula of the 
function is:
dx/dt = k*qi*(xi-xj)/sqrt((xi-xj)^2+(yi-yj)^2+(zi-zj)^2)
where qi is the charge of the i-th particle, xi, yi, zi are the coordinates of 
the i-th particle and xj, yj,zj are the coordinates of the j-th particle. 
As an input for the initial values of the positions and charge for each 
particle, I am using arrays (currently only for 4 particles):
q[4] =  { 1.0, 1.0, -1.0, 1.0 } charge 
x0[4] = { 1.0, 2.3, 3.2, 4.0 } init posn x
y0[4] = { 1.0, 2.4, 3.0, 4.0 } init posn y
z0[4] = { 1.0, 2.5, 3.3, 4.2 } init posn z
and by using two "for" loops I can loop through each particle and compute the 
value of the total effect of all the particles on eachother. 
My question is: since there are two values for each coordinate (i and j), how 
can I write the function in an acceptable form for RK4? 
I did try the following:
int func (double t, const double y[], double f[], void *params){
    double mu = *(double *)params;
    double dx = (y[0]-y[1])/r;
    double dy = (y[2]-y[3])/r;
    double dz = (y[4]-y[5])/r;
    f[0] = mu*y[6]*dx/(r*r);
    f[1] = mu*y[6]*dy/(r*r);
    f[2] = mu*y[6]*dz/(r*r);
    return GSL_SUCCESS;
however, I am not convinced this is correct (at all) and as it is I have to 
specify too many initial values for y (ie, y[0], y[1],...y[7]) and it fails 
I would appreciate any feedback especially if someone has HAD to do something 

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