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Many generator matrices for constructing extremal binary self-dual codes of different lengths have the form $ G = (I_n \ | \ A), $ where $ I_n $ is the $ n \times n $ identity matrix and $ A $ is the $ n \times n $ matrix fully determined by the first row. In this work, we define a generator matrix in which $ A $ is a block matrix, where the blocks come from group rings and also, $ A $ is not fully determined by the elements appearing in the first row. By applying our construction over $ \mathbb{F}_2+u\mathbb{F}_2 $ and by employing the extension method for codes, we were able to construct new extremal binary self-dual codes of length 68. Additionally, by employing a generalised neighbour method to the codes obtained, we were able to construct many new binary self-dual $ [68, 34, 12] $-codes with the rare parameters $ \gamma = 7, 8 $ and $ 9 $ in $ W_{68, 2}. $ In particular, we find 92 new binary self-dual $ [68, 34, 12] $-codes.
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Table 1.
Codes of length 32 via Theorem 3.1 with the cyclic group
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Table 2.
Codes of length 64 from
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Table 3.
Codes of length 32 via Theorem 3.1 with the dihedral group
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Table 4.
Codes of length 64 from
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Table 5. New codes of length 68 from Theorem 2.4
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