Null Sectional Curvatures of Warped Products
  In this paper, we investigate the null (light-like) sectional curvatures of Lorentzian warped product manifolds. We derive the formulas for the null sectional curvature of many well-known warped product space-time models such as multiply generalized Robertson-Walker space-times, generalized Kasner spacetimes and standard static space-times. 
Bengi R. Yavuz, B\"ulent \"unal and Fernando Dobarro
math.DG