#include <math.h>

#define EPS 3.0e-11

void gauleg(x1,x2,x,w,n)
double x1,x2,x[],w[];
int n;
{
	int m,j,i;
	double z1,z,xm,xl,pp,p3,p2,p1;

	m=(n+1)/2;
	xm=0.5*(x2+x1);
	xl=0.5*(x2-x1);
	for (i=1;i<=m;i++)  {
		z=cos(3.141592654*(i-0.25)/(n+0.5));
		do {
			p1=1.0;
			p2=0.0;
			for (j=1;j<=n;j++) {
				p3=p2;
				p2=p1;
				p1=((2.0*j-1.0)*z*p2-(j-1.0)*p3)/j;
			}
			pp=n*(z*p1-p2)/(z*z-1.0);
			z1=z;
			z=z1-p1/pp;
		} while (fabs(z-z1) > EPS);
		x[i]=xm-xl*z;
		x[n+1-i]=xm+xl*z;
		w[i]=2.0*xl/((1.0-z*z)*pp*pp);
		w[n+1-i]=w[i];
	}
}

#undef EPS