OK that works much better, more efficient & no problems of any kind with leaktrace or no_leaks_ok, etc. The reason I thought a hash would be a good idea was that I'm also interested in getting back the corresponding $x, $y, $z at the end. That's why the coeffecients look the way they do in the poly_derivative definition. Is there a way to do that without using a hash? If not I'd still gladly trade that for more efficiency because I can always integrate what I get, or I could just work with the coefficients rather than the zeros.