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A matrix H of order n = 4t with all entries from the set ﹛1, —1﹜ is Hadamard if HHt = 4tI. The set of Hadamard matrices is 
. A matrix 
is of type I or is skew-Hadamard if H = S — I where St = —S (some authors also use H = S + I). The set of type I members 
is 
. A matrix P is a signed permutation matrix if each row and each column has exactly one non-zero entry, and that entry is from the set ﹛1, —1﹜.
