Re: [Help-gsl] RK4 Help required

[Pau Cervera Badia]

Since gsl solvers solve systems of first order differential equations of 
the type:

x' = f(x,t) (where the prima is the time derivative)

where x is a vector of positions, and f(x,t) is the vector of forces you 
have to implement via the func that is passed via gsl_odeiv_system to 
the gsl solver, you should consider making a vector x of 3 times longer 
than the number of charges you have.

For instance, you can order your components in a 12-vector as,

x_0, y_0, z_0, x_1, y_1, z_1, x_2, y_2, z_2, x_3, y_3, z_3.


To help you to extract the right components, you can define the macros,

#define xindex(i) 3 * i
#define yindex(i) 3 * i + 1
#define zindex(z) 3 * i + 2

and access the position of particle i as

double x_i = sys[xindex(i)];
double y_i = sys[yindex(i)];
double z_i = sys[zindex(i)];

when needed.

Good luck,

cigdem ozkan wrote:
> Hi again,
> I am posting a smaller version of my question rearding rk4 since I didnt get a 
> reply and I am really anxious for some feedback.
>
> I have the following fuction:
> dxi/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. 
> Input for the initial values: 
> 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
> QUESTION:
> Since there are two values for each coordinate (i and 
> j), how can I write the function in an acceptable form for RK4? 
> I would appreciate any feedback concerning RK4.
> Thanks for your time,
> C.
>
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--
Pau Cervera i Badia (e-mail address@hidden)
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