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*ROOT Version:* 6.26.06

*Platform:* Not Provided

*Compiler:* Ubuntu

Hello experts from root, I am having quite a hard time trying to run a code in which I need to use the irregular modified cilyndrical Bessel function for the zeroth and first order. I have tried by first defining it as ROOT’s website proposes for special math functions:

[code] #include “Math/SpecFunc.h”

#include

#ifndef PI

#define PI 3.14159265358979323846264338328 /* pi */

#endif

#include “gsl/gsl_sf_bessel.h”

#include

#include <TMath.h>

#include <TSystem.h>

// #include <RooUnfold.h>

Double_t pi=3.14159265358979323846;

Double_t tau=35.92;

Double_t alfa=1/137;

Double_t q0=1;

Double_t mf = 0;

Double_t ef = 10/9;

double cyl_bessel_k(double nu, double x) {

return gsl_sf_bessel_Knu(nu, x);

}

double func2(double* q, double* x, double z, double r, double* p) {

double qq = q[0];

double xx = x[0];

// double a = p[0];

// double b = p[1];

// double c = p[2];

double x1 = (xx/p[1]);

double x2 = (p[1]/xx);

double z1 = (1.0-z);

double epsilon2 = z*z1*qq + pow(mf,2);

double epsilon = sqrt(epsilon2);

// double cyl_bessel_k(double epsilon, double r) = gsl_sf_bessel_Knu(0, epsilon*r);
// double cyl_bessel_k(double epsilon, double r) = gsl_sf_bessel_Knu(1, epsilon*r);

double psiT2 = (6

*alfa)/(4*pow(pi,2))

*ef*((pow(z,2) + pow(z1,2))

*epsilon2*(cyl_bessel_Knu(1,epsilon

*r)) + pow(mf,2)*(cyl_bessel_Knu(0,epsilon

*r)));*

double psiL2 = (6alfa)/(4

double psiL2 = (6

*pow(pi,2))*qq

*ef*(4*pow(z,2)*r)));

*pow(z1,2)*(cyl_bessel_Knu(0,epsilon// double psiTL2 = psiT2 + psiL2;

```
return psiT2;
```

}[/code]

So I need it to give the results for the zeroth and first order while depending on the previous epsilon*r. When I compile it the following error shows: /home/rona/fit2Histo.C:11:10: fatal error: ‘gsl/gsl_sf_bessel.h’ file not found

#include “gsl/gsl_sf_bessel.h”

Feel free to correct me please,

I would really appreciate it if you take the time to solve my problem.

Thanks!