On extremal self-dual quaternary codes of lengths 18 to 28. I

A general decomposition theorem is given for self-dual codes over finite fields that have a permutation automorphism of a given form. Such a code can be decomposed as a direct sum of subcodes that may be viewed as shorter-length codes over extension fields where the dual of each direct summand is also a direct summand. Situations in which it is easy to distinguish such codes are also presented. These results are used to enumerate some of the extremal quaternary self-dual codes of lengths 18, 20, 22, 26 and 28