Monomial isomorphisms of cyclic codes

For codes, there are multiple notions of isomorphism. For example, we can consider isomorphisms that only permute the coordinates of codewords, or isomorphisms that not only permute the coordinates of codewords but also multiply each coordinate by a scalar (not necessarily the same scalar for each coordinate) as it permutes the coordinates. Isomorphisms of cyclic codes of the first kind have been studied in some circumstances-we will call them permutation isomorphisms-and our purpose is to begin the study of the second kind of isomorphism-which we call monomial isomorphisms-for cyclic codes. We give examples of cyclic codes that are monomially isomorphic but not permutationally isomorphic. We also show that the monomial isomorphism problem for cyclic codes of length over reduces to the permutation isomorphism problem for cyclic codes of length over if and only if . Applying known results, this solves the monomial isomorphism problem for cyclic codes satisfying . Additionally, we solve the monomial isomorphism problem for cyclic codes of prime length over all finite fields. Finally, our results also hold for some codes that are not cyclic.