“…We denote by ∇, R, S, κ and C the Levi-Civita connection, the Riemann-Christoffel curvature tensor, the Ricci tensor, the scalar curvature and the Weyl conformal curvature tensor of (M, g), respectively. We refer to [5,11,14,17] for precise definitions of the symbols used. Let (M , g) and ( N , g), dim M = p, dim N = n − p, 1 p < n, be semi-Riemannian manifolds and F a positive smooth function on M .…”