add numerical recipes library

lib/nr/k_and_r/recipes/dawson.c

@@ -0,0 +1,44 @@
+
+#include <math.h>
+#include "nrutil.h"
+#define NMAX 6
+#define H 0.4
+#define A1 (2.0/3.0)
+#define A2 0.4
+#define A3 (2.0/7.0)
+
+float dawson(x)
+float x;
+{
+	int i,n0;
+	float d1,d2,e1,e2,sum,x2,xp,xx,ans;
+	static float c[NMAX+1];
+	static int init = 0;
+
+	if (init == 0) {
+		init=1;
+		for (i=1;i<=NMAX;i++) c[i]=exp(-SQR((2.0*i-1.0)*H));
+	}
+	if (fabs(x) < 0.2) {
+		x2=x*x;
+		ans=x*(1.0-A1*x2*(1.0-A2*x2*(1.0-A3*x2)));
+	} else {
+		xx=fabs(x);
+		n0=2*(int)(0.5*xx/H+0.5);
+		xp=xx-n0*H;
+		e1=exp(2.0*xp*H);
+		e2=e1*e1;
+		d1=n0+1;
+		d2=d1-2.0;
+		sum=0.0;
+		for (i=1;i<=NMAX;i++,d1+=2.0,d2-=2.0,e1*=e2)
+			sum += c[i]*(e1/d1+1.0/(d2*e1));
+		ans=0.5641895835*SIGN(exp(-xp*xp),x)*sum;
+	}
+	return ans;
+}
+#undef NMAX
+#undef H
+#undef A1
+#undef A2
+#undef A3