+/*
+ C program for floating point error function
+
+ erf(x) returns the error function of its argument
+ erfc(x) returns 1.0-erf(x)
+
+ erf(x) is defined by
+ ${2 over sqrt(pi)} int from 0 to x e sup {-t sup 2} dt$
+
+ the entry for erfc is provided because of the
+ extreme loss of relative accuracy if erf(x) is
+ called for large x and the result subtracted
+ from 1. (e.g. for x= 10, 12 places are lost).
+
+ There are no error returns.
+
+ Calls exp.
+
+ Coefficients for large x are #5667 from Hart & Cheney (18.72D).
+*/
+
+#define M 7
+#define N 9
+int errno;
+static double torp = 1.1283791670955125738961589031;
+static double p1[] = {
+ 0.804373630960840172832162e5,
+ 0.740407142710151470082064e4,
+ 0.301782788536507577809226e4,
+ 0.380140318123903008244444e2,
+ 0.143383842191748205576712e2,
+ -.288805137207594084924010e0,
+ 0.007547728033418631287834e0,
+};
+static double q1[] = {
+ 0.804373630960840172826266e5,
+ 0.342165257924628539769006e5,
+ 0.637960017324428279487120e4,
+ 0.658070155459240506326937e3,
+ 0.380190713951939403753468e2,
+ 0.100000000000000000000000e1,
+ 0.0,
+};
+static double p2[] = {
+ 0.18263348842295112592168999e4,
+ 0.28980293292167655611275846e4,
+ 0.2320439590251635247384768711e4,
+ 0.1143262070703886173606073338e4,
+ 0.3685196154710010637133875746e3,
+ 0.7708161730368428609781633646e2,
+ 0.9675807882987265400604202961e1,
+ 0.5641877825507397413087057563e0,
+ 0.0,
+};
+static double q2[] = {
+ 0.18263348842295112595576438e4,
+ 0.495882756472114071495438422e4,
+ 0.60895424232724435504633068e4,
+ 0.4429612803883682726711528526e4,
+ 0.2094384367789539593790281779e4,
+ 0.6617361207107653469211984771e3,
+ 0.1371255960500622202878443578e3,
+ 0.1714980943627607849376131193e2,
+ 1.0,
+};
+
+double
+erf(arg) double arg;{
+ double erfc();
+ int sign;
+ double argsq;
+ double d, n;
+ int i;
+
+ errno = 0;
+ sign = 1;
+ if(arg < 0.){
+ arg = -arg;
+ sign = -1;
+ }
+ if(arg < 0.5){
+ argsq = arg*arg;
+ for(n=0,d=0,i=M-1; i>=0; i--){
+ n = n*argsq + p1[i];
+ d = d*argsq + q1[i];
+ }
+ return(sign*torp*arg*n/d);
+ }
+ if(arg >= 10.)
+ return(sign*1.);
+ return(sign*(1. - erfc(arg)));
+}
+
+double
+erfc(arg) double arg;{
+ double erf();
+ double exp();
+ double n, d;
+ int i;
+
+ errno = 0;
+ if(arg < 0.)
+ return(2. - erfc(-arg));
+/*
+ if(arg < 0.5)
+ return(1. - erf(arg));
+*/
+ if(arg >= 10.)
+ return(0.);
+
+ for(n=0,d=0,i=N-1; i>=0; i--){
+ n = n*arg + p2[i];
+ d = d*arg + q2[i];
+ }
+ return(exp(-arg*arg)*n/d);
+}