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[B] R. L. Bishop: Clairaut submersions. Differential Geometry, in Honor of K. Yano, S.  Kobayashi et al. (eds.), Tokyo, Kinokuniya, 1972, pp. 21–31. MR 0334078 | Zbl 0246.53048

[BP] J. K. Beem and P. E. Parker: Values of pseudoriemannian sectional curvature. Comment. Math. Helv. 59 (1984), 319–331. DOI 10.1007/BF02566352 | MR 0749111

[CL] E. A. Coddington and N. Levinson: Theory of Ordinary Differential Equations. New York, McGraw-Hill, 1955. MR 0069338

[CP] L. A. Cordero and P. E. Parker: Symmetries of sectional curvature on 3-manifolds. (to appear). MR 1362194

[CPIII] L. A. Cordero and P. E. Parker: Left-invariant Lorentzian metrics on 3-dimensional Lie groups. to appear. MR 1459412

[EP] R. H. Escobales and P. E. Parker: Geometric consequences of the normal curvature cohomology class in umbilic foliations. Indiana U. Math. J. 37 (1988), 389–408. DOI 10.1512/iumj.1988.37.37020 | MR 0963509

[HE] S. W. Hawking and G. F. R. Ellis: The Large Scale Structure of Space-time. Cambridge, U.P., 1973. MR 0424186

[ON] B. O’Neill: Semi-riemannian Geometry. New York, Academic Press, 1983. MR 0719023

[W] J. A. Wolf: Spaces of Constant Curvature. $5^{\mathrm th}$ ed. Boston, Publish or Perish, 1984. MR 0928600 | Zbl 0556.53033