// EXPONENTIAL FUNCTION FOR FITTING WITH MRQMIN ROUTINE
// FITS TO THE FORMULA: 
//	y[x] = a[1]*G / (1 + (1-G)*a[3]*a[1]/a[2])
//	where G = exp(-a[2]*x)

#include <math.h>

void beta(float x, float a[], float *y, float dyda[], int na)
{
	float G,V;

	G = exp(a[2]*x);
	V = a[1]/(G + a[3]*a[1]*(G-1)/a[2]);	
	*y = V;
	dyda[1] = (V/a[1])*(1 + V*a[3]*(1-G)/a[2]);
	dyda[2] = V*(a[1]*a[3]*((G-1)/a[2] - x)/a[2] - x);
	dyda[3] = -V*V*(G-1)/a[2];
}