[Help-gsl] QAG Integration with variable parameters

[Anders Misfeldt]

Hi,

I'm using gsl_integration_qags to calculate some integrals, but I've 
encountered a problem I can't seem to find out.

I want to calculate the following integral

Int (from 0 to 100) x^3/(exp(x-eps) + 1) dx

I'm trying to make multiple calculations for different values of a, but 
how can I call gsl_integration_qags with a different a? As until now 
I've just made the variable a in my function, but I'm making a for 
statement, so that I get multiple results out. And I'm kinda stuck.

Can someone help me out here?

My code is:


#include <stdio.h>
#include <math.h>
#include <gsl/gsl_integration.h>
#define real double

real calls = 0;

real f (real x, void * params) {
  real alpha = *(real *) params;
  calls+=1;
  return x*x*x/(exp(x-eps)+1);
}

int main(void) {
  gsl_integration_workspace * w
    = gsl_integration_workspace_alloc(100000);
  real res,a,b,acc,error,expected,alpha;
  int key,i;
  size_t limit;

  acc=0.00001;
  expected=0.339047; //disregard this
  alpha=1.0;
  limit=100000;
  key=1;
  calls=0;

  gsl_function F;
  F.function=&f;
  F.params=&alpha;

  a=0;
  b=100;

  for (i=0;i<=20;i++) {
     eps=i;
     gsl_integration_qags(&F,a,b,acc,acc,limit,w,&res,&error);
     printf("%g \n", res);
  }
  return 0;
}