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Difference sets are used to give a novel method for constructing structured regular quasi-cyclic low-density parity-check codes. Let F_q be a field with q elements and assume that D={d_1,..., d_k} is a (v,k,1)-difference set for Z_v with d_1<d_2<...<d_k. Depending on v=q-1, v≥ 2d_k or v≥ 2d_{k-1}, three code construction methods are given that produce regular 4-cycle free codes, that is codes...