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/* Copyright (C) 2017 - IIT Bombay - FOSSEE
This file must be used under the terms of the CeCILL.
This source file is licensed as described in the file COPYING, which
you should have received as part of this distribution. The terms
are also available at
http://www.cecill.info/licences/Licence_CeCILL_V2-en.txt
Organization: FOSSEE, IIT Bombay
Author: Brijesh Gupta C R
Email: toolbox@scilab.in
*/
#include <stdio.h>
#include "math.h"
#include "besselk.h"
#define ACC 40.0
#define BIGNO 1.0e10
#define BIGNI 1.0e-10
static double bessi0( double x )
{
double ax,ans;
double y;
if ((ax=fabs(x)) < 3.75) {
y=x/3.75,y=y*y;
ans=1.0+y*(3.5156229+y*(3.0899424+y*(1.2067492
+y*(0.2659732+y*(0.360768e-1+y*0.45813e-2)))));
} else {
y=3.75/ax;
ans=(exp(ax)/sqrt(ax))*(0.39894228+y*(0.1328592e-1
+y*(0.225319e-2+y*(-0.157565e-2+y*(0.916281e-2
+y*(-0.2057706e-1+y*(0.2635537e-1+y*(-0.1647633e-1
+y*0.392377e-2))))))));
}
return ans;
}
static double bessi1( double x)
{
double ax,ans;
double y;
if ((ax=fabs(x)) < 3.75) {
y=x/3.75,y=y*y;
ans=ax*(0.5+y*(0.87890594+y*(0.51498869+y*(0.15084934
+y*(0.2658733e-1+y*(0.301532e-2+y*0.32411e-3))))));
} else {
y=3.75/ax;
ans=0.2282967e-1+y*(-0.2895312e-1+y*(0.1787654e-1
-y*0.420059e-2));
ans=0.39894228+y*(-0.3988024e-1+y*(-0.362018e-2
+y*(0.163801e-2+y*(-0.1031555e-1+y*ans))));
ans *= (exp(ax)/sqrt(ax));
}
return x < 0.0 ? -ans : ans;
}
static double bessk0( double x )
{
double y,ans;
if (x <= 2.0) {
y=x*x/4.0;
ans=(-log(x/2.0)*bessi0(x))+(-0.57721566+y*(0.42278420
+y*(0.23069756+y*(0.3488590e-1+y*(0.262698e-2
+y*(0.10750e-3+y*0.74e-5))))));
} else {
y=2.0/x;
ans=(exp(-x)/sqrt(x))*(1.25331414+y*(-0.7832358e-1
+y*(0.2189568e-1+y*(-0.1062446e-1+y*(0.587872e-2
+y*(-0.251540e-2+y*0.53208e-3))))));
}
return ans;
}
static double bessk1( double x )
{
double y,ans;
if (x <= 2.0) {
y=x*x/4.0;
ans=(log(x/2.0)*bessi1(x))+(1.0/x)*(1.0+y*(0.15443144
+y*(-0.67278579+y*(-0.18156897+y*(-0.1919402e-1
+y*(-0.110404e-2+y*(-0.4686e-4)))))));
} else {
y=2.0/x;
ans=(exp(-x)/sqrt(x))*(1.25331414+y*(0.23498619
+y*(-0.3655620e-1+y*(0.1504268e-1+y*(-0.780353e-2
+y*(0.325614e-2+y*(-0.68245e-3)))))));
}
return ans;
}
double bessk( int n, double x )
{
int j;
double bk,bkm,bkp,tox;
if (n < 0 || x == 0.0)
{
double dblank;
return( dblank );
}
if (n == 0)
return( bessk0(x) );
if (n == 1)
return( bessk1(x) );
tox=2.0/x;
bkm=bessk0(x);
bk=bessk1(x);
for (j=1;j<n;j++) {
bkp=bkm+j*tox*bk;
bkm=bk;
bk=bkp;
}
return bk;
}
void dbesselka(double* inp1,int size1, double* inp2,int size2, double* oup)
{
int i;
if(size1 != size2)
printf("Error! arguments #1 and #2 have incompatible dimensions.");
for(i = 0; i<size1;i++)
{
oup[i] = bessk(inp1[i],inp2[i]);
}
}
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