A difference set (,D) is defined in (2) as a subset D of k elements in a group of order υ with the following properties : (1) if x ∊ , x ≠ 1, there are exactly λ distinct ordered pairs (d1d2) of elements of D such that x = 1d2; (2) if x ∊ , x ≠ 1, there are exactly λ distinct ordered pairs (3d4) of elements of D such that x = 3d4−1.