36 lines
683 B
C++
36 lines
683 B
C++
#include <cmath>
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#include "nr.h"
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using namespace std;
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DP NR::dawson(const DP x)
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{
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const int NMAX=6;
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const DP H=0.4, A1=2.0/3.0, A2=0.4, A3=2.0/7.0;
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int i,n0;
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static bool init = true;
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DP d1,d2,e1,e2,sum,x2,xp,xx,ans;
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static Vec_DP c(NMAX);
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if (init) {
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init=false;
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for (i=0;i<NMAX;i++) c[i]=exp(-SQR((2.0*i+1.0)*H));
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}
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if (fabs(x) < 0.2) {
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x2=x*x;
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ans=x*(1.0-A1*x2*(1.0-A2*x2*(1.0-A3*x2)));
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} else {
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xx=fabs(x);
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n0=2*int(0.5*xx/H+0.5);
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xp=xx-n0*H;
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e1=exp(2.0*xp*H);
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e2=e1*e1;
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d1=n0+1;
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d2=d1-2.0;
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sum=0.0;
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for (i=0;i<NMAX;i++,d1+=2.0,d2-=2.0,e1*=e2)
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sum += c[i]*(e1/d1+1.0/(d2*e1));
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ans=0.5641895835*SIGN(exp(-xp*xp),x)*sum;
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}
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return ans;
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}
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