A method for constructing inequivalent self-dual codes with applications to length 56
Vassil Yorgov · IEEE Transactions on Information Theory · 1987
Necessary and sufficient conditions are presented for two doubly even codes with an automorphism of odd prime order to be equivalent under certain assumptions for the codes. As an application we obtain all extremal doubly even codes of length56with an automorphism of order13. Sixteen such codes are inequivalent.