A characterization of a (56,28) extremal self-dual code (Corresp.)

A
(54,27)
code is characterized in terms of a
27 \times 27
matrix made up of circulant
3 \times 3
matrices. The minimum distance of this code is found to be
11
, and its weight distribution is computed. The
(54,27)d=11
code is extended to a
(56,28)d=12
external self-dual code. A
(54,27)d=10
self-dual code is also described in terms of the incidence matrix of a
(27,13,6)
group difference set defined on GF
(3^{3})
.