23 lines
471 B
C++
23 lines
471 B
C++
#include <cmath>
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#include "nr.h"
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using namespace std;
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DP NR::bessi0(const DP x)
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{
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DP ax,ans,y;
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if ((ax=fabs(x)) < 3.75) {
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y=x/3.75;
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y*=y;
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ans=1.0+y*(3.5156229+y*(3.0899424+y*(1.2067492
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+y*(0.2659732+y*(0.360768e-1+y*0.45813e-2)))));
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} else {
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y=3.75/ax;
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ans=(exp(ax)/sqrt(ax))*(0.39894228+y*(0.1328592e-1
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+y*(0.225319e-2+y*(-0.157565e-2+y*(0.916281e-2
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+y*(-0.2057706e-1+y*(0.2635537e-1+y*(-0.1647633e-1
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+y*0.392377e-2))))))));
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}
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return ans;
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}
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