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  double arctan(double x) {
    
    double zaehler,atg;
    int    i,nenner;
    
    if (x<0)      return -arctan(-x);
    else if (x>1) return Math.PI/2-arctan(1/x);
    else {
      for (i=0; i<3; i++) x/=1+Math.sqrt(1+x*x);
      zaehler=8*x; nenner=1; atg=zaehler;
      for (i=1; i<7; i++) {
        zaehler*=-x*x;
        nenner+=2;
        atg+=zaehler/nenner;
      }
      return atg;
    }
  }