 of length
 of length  over GF
 over GF  is the maximum distance separable if and only if either
 is the maximum distance separable if and only if either  is a prime, in which case
 is a prime, in which case  is equivalent, up to a coordinate permutation, to an extended Reed-Solomon code, or
 is equivalent, up to a coordinate permutation, to an extended Reed-Solomon code, or  is a trivial code of dimension
 is a trivial code of dimension  . Hence there exists a nontrivial cyclic extended Reed-Solomon code of length
 . Hence there exists a nontrivial cyclic extended Reed-Solomon code of length  over GF
 over GF  if and only if
 if and only if  is a prime.
 is a prime.