We investigate negacyclic codes over the Galois ring GR(2 a , m ) of length N = 2 k n , where n is odd and k ⩾ 0. We first determine the structure of u -constacyclic codes of length n over the finite chain ring $GR(2^a ,m)[u]/\langle u^{2^k } + 1\rangle $ . Then using a ring isomorphism we obtain the structure of negacyclic codes over GR(2 a , m ) of length N = 2 k n ( n odd) and explore the existence of self-dual negacyclic codes over GR(2 a , m ). A bound for the homogeneous distance of such negacyclic codes is also given.
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