sub divisors {
  my($n,@f) = @_;
  my @d = (1);
  while (@f) {
    my $p = shift(@f);
    if (!@f || $p != $f[0]) {
      # Factor appears once.  Multiply through.
      push @d, map { $p * $_ } @d;
    } else {
      my $e = 1;
      do { shift(@f); $e++; } while @f && $p == $f[0];
      # Factor appears $e times.  Multiply through p, p^2, p^3, ...
      my @t = @d;
      for (1 .. $e) {
        @t = map { $p * $_ } @t;
        push @d, @t;
      }
    }
  }
  @d = sort { $a <=> $b } @d;
  @d;
}