#include <math.h>
#define JMAX 50

double rtbis(double (*func)(double), double x1, double x2, double xacc)
{
	void nrerror(char error_text[]);
	int j;
	double dx,f,fmid,xmid,rtb;

	f=(*func)(x1);
	fmid=(*func)(x2);
	if (f*fmid >= 0.0) nrerror("Root must be bracketed for bisection in rtbis");
	rtb = f < 0.0 ? (dx=x2-x1,x1) : (dx=x1-x2,x2);
	for (j=1;j<=JMAX;j++) {
		fmid=(*func)(xmid=rtb+(dx *= 0.5));
		if (fmid <= 0.0) rtb=xmid;
		if (fabs(dx) < xacc || fmid == 0.0) return rtb;
		printf("%s%g\n", "intermediate step =   ", rtb); 
	}
	nrerror("Too many bisections in rtbis");
	return 0.0;
}
#undef JMAX