Structure of repeated-root constacyclic codes of length 3ps3ps and their duals

Let p≠3 be any prime. A classification of constacyclic codes of length 3ps3ps over the finite field FpmFpm 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 3ps3ps exist only when p=2p=2, and in such case, those self-dual codes are listed.