fcn mobius(n){
   pf:=primeFactors(n);
   sq:=pf.filter1('wrap(f){ (n % (f*f))==0 });  // False if square free
   if(sq==False){ if(pf.len().isEven) 1 else -1 }
   else 0
}
fcn primeFactors(n){  // Return a list of prime factors of n
   acc:=fcn(n,k,acc,maxD){  // k is 2,3,5,7,9,... not optimum
      if(n==1 or k>maxD) acc.close();
      else{
	 q,r:=n.divr(k);   // divr-->(quotient,remainder)
	 if(r==0) return(self.fcn(q,k,acc.write(k),q.toFloat().sqrt()));
	 return(self.fcn(n,k+1+k.isOdd,acc,maxD))  # both are tail recursion
      }
   }(n,2,Sink(List),n.toFloat().sqrt());
   m:=acc.reduce('*,1);      // mulitply factors
   if(n!=m) acc.append(n/m); // opps, missed last factor
   else acc;
}