On the decomposition of self-dual codes over with an automorphism of odd prime order

We present a general theory of decomposing self-dual codes over that have an automorphism of odd prime order. Using this decomposition we classify all Lee-extremal/optimal self-dual codes of lengths 9 to 16 with an odd order automorphism and partially classify those of lengths 17 to 20. In particular we find 945 inequivalent Lee-extremal self-dual codes of length 20.