Optimal subcodes and optimum distance profiles of self-dual codes

Binary optimal codes often contain optimal or near-optimal subcodes. In this paper we show that this is true for the family of self-dual codes. One approach is to compute the optimum distance profiles (ODPs) of linear codes, which was introduced by Luo et al. (2010). One of our main results is the development of general algorithms, called the Chain Algorithms, for finding ODPs of linear codes. Then we determine the ODPs for the Type II codes of lengths up to 24 and the extremal Type II codes of length 32, give a partial result of the ODP of the extended quadratic residue code q48q48 of length 48. We also show that there does not exist a [48,k,16][48,k,16] subcode of q48q48 for k⩾17k⩾17, and we find a first example of a doubly-even   self-complementary [48,16,16][48,16,16] code.