Equation Icon  ( ) S1 = S0 ⋅ cos(ωxt + φx) = S0 1 ei(ωxt+φx) + e− i(ωxt+ φx) N∑ N∑ (2 ) = 12 aia∗j ei(ωi− ωj)t ⋅ ei(ωxt+φx) + e−i(ωxt+ φx) . (78 ) i=0 j=0