“…(8.11)]; the recurrence properties of the Weyl curvature tensor of pp-waves are reported in [116]: under certain conditions they are complex recurrent space-times (see [59,61,89]) with closed recurrence parameter. The condition A i C jklm + A j C kilm + A k C ijlm = 0 on a pseudo Riemannian manifold has been extensively studied among others in [33,45,46,48,78,85] [38,36,76,78,91]):…”