> Is "closed form" here supposed to mean "closed form we can evaluate without needing arbitrary-precision arithmetic"?
You don't need arbitrary-precision arithmetic to evaluate Binet's formula - big integer arithmetic suffices (simply do your calculations in the ring Z[X]/(X^2-X-1) ).
You don't need arbitrary-precision arithmetic to evaluate Binet's formula - big integer arithmetic suffices (simply do your calculations in the ring Z[X]/(X^2-X-1) ).