#include <stdio.h>
#include <data.h>
#include <math.h>
#include <utilities.h>
  
// Add your special include files here.
// For example, if you want to fit with functions from the NAG library, 
// add the header file for the NAG functions here.
#include <NAG\OCN_s.h>
  
// Add code here for other Origin C functions that you want to define in this file,
// and access in your fitting function.

double factorial(int x)
{
  int i;
  double factx = 1;

  for(i=1; i<=x ; i++ )
    factx *= i;
  return factx;
}


double bessel_i2(double y)
{
int v=2.0;
double Isum;
for(int s = 1; s < 101; s++) 
	Isum += (1.0/(factorial(s)*factorial(s+v)))*pow((y/2.0),2.0*s + v); 
return Isum;
}


// You can access C functions defined in other files, if those files are loaded and compiled 
// in your workspace, and the functions have been prototyped in a header file that you have
// included above. 

// You can access NLSF object methods and properties directly in your function code.

// You should follow C-language syntax in defining your function. 
// For instance, if your parameter name is P1, you cannot use p1 in your function code. 
// When using fractions, remember that integer division such as 1/2 is equal to 0, and not 0.5
// Use 0.5 or 1/2.0 to get the correct value.

// For more information and examples, please refer to the "User-Defined Fitting Function" 
// section of the Origin Help file.


//----------------------------------------------------------
// FRAP Model for Diffusion Type-Uniform Circular Beam
void _nlsfAxelrod_eqn14(
// Fit Parameter(s):
double tau, double K, double countk,
// Independent Variable(s):
double t,
// Dependent Variable(s):
double& fk)
{
	// Beginning of editable part
	double dsum=0;
	double y=2.0*tau/t;
	double bessel_i0=nag_bessel_i0(y); 
	
	for(int k=0;k<=countk;k++)
	{	
	int w=2*k+2;	
	dsum += (pow(-1, k) * factorial(w) * factorial(k+1) * pow(tau/t,k+2.0))/(pow(factorial(k),2.0) * pow(factorial(k+2),2));	
	}
		
	fk = 1- (tau/t) * exp(-y) * (bessel_i0+ bessel_i2(y)) + 2*dsum;
	// End of editable part
}