“…it is known that every τ matrix is diagonalized as τ(T n ) = S n Λ n S n , where Λ n is a diagonal matrix constituted by all eigenvalues of τ(T n ), and S n = ([S n ] j,k ) is the real, symmetric, orthogonal matrix defined before, so that S n = S T n = S −1 n . Furthermore, the matrix S n is associated with the fast sine transform of type I (see [13,14] for several other sine/cosine transforms). Indeed the multiplication of a matrix S n times a real vector can be conducted in O(n log n) real operations and the cost is around half of the celebrated discrete fast Fourier transform [15].…”