Determine the convolution of the following pairs of signals by means of the Fourier transform: (a) X1(n) = δ(n - 1), X2(n) = [1 + δ(n)]u(n) (b) X1(n) = u(n), X2(n) = δ(n) + jδ(n) (c) X1(n) = δ(n), X2(n) = cos(ωn)u(n) (d) X1(n) = nu(n), X2(n) = δ(n - 1)