proposition: Theorem FR_fib k: FR (fib k) = (k./2).-1.+1.
proof: Proof. rewrite - {1} (odd_double_half k);case: (odd k). by rewrite FR_fib2 FR_fib4. by rewrite FR_fib4. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path fintype div bigop binomial) mathcomp(prime finset ssralg ssrnum ssrint)