Structure of repeated-root constacyclic codes of length and their duals

Let p ≠ 3 be any prime. A classification of constacyclic codes of length 3 p s over the finite field F p m is provided. Based on this, the structures in terms of polynomial generators of all such constacyclic codes and their duals are established. Among other results, we show that self-dual cyclic codes of length 3 p s exist only when p = 2 , and in such case, those self-dual codes are listed.