Método de Euler VI (variando dt)

#include <stdio.h>
#include <math.h>

#define x0 2

#define v0 1

#define t0 0
#define tf 2.5

#define dt0 0.01
#define dtf 1e-6
#define ddt -1e-4

#define w02 3
#define Ndt ((dtf-dt0)/ddt)

double dvdt(double x, double v, double t)
{
    return -w02*x;
}

double E(double x, double v)
{
    return v*v/w02 + x*x;
}

int main(void)
{
    double x, xn,
           t, tn,
           v, vn,
           dt;

    FILE *h = fopen("./eulerdt.dat","w+");

    for(int k = 0; k < Ndt; k++)
    {
        dt = dt0 + k * ddt;

        x = x0; t = t0; v = v0;

        double E0 = E(x,v);

        for (int i = 0; i < ((tf-t0)/dt); ++i)
        {
            vn = v + dvdt(x,v,t)*dt;
            xn = x + vn*dt;
            tn = t + dt;

            x = xn;
            v = vn;
            t = tn;
        }
        fprintf(h, "%f %f\n", dt, E(x,v)/E0-1);
    }

    fclose(h);
    return 0;
}