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New Self-Dual Codes of length 68 from a 2 by 2 block matrix Construction and Group Rings

Many generator matrices for constructing extremal binary self-dual codes of different lengths have the form G=(I|A), where I is the n by n identity matrix and A is the n by 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 F_2+uF_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.