We give a necessary and sufficient condition for an n-dimensional Riemannian manifold to be isometrically immersed into one of the Lorentzian products S n ×R 1 or H n ×R 1 . This condition is expressed in terms of its first and second fundamental forms, the tangent and normal projections of the vertical vector field. As applications, we give an equivalent condition in a spinorial way and we deduce the existence of a one-parameter family of isometric maximal deformation of a given maximal surface obtained by rotating the shape operator.
Isometric Immersions into Lorentzian Products 1271Moreover, this immersion is unique up to a global isometry of M n (κ) × R which preserves the orientation of R. Int. J. Geom. Methods Mod. Phys. 2011.08:1269-1290. Downloaded from www.worldscientific.com by YALE UNIVERSITY on 07/07/15. For personal use only. 1272 J. Roth
The compatibility equationsLet (M n , g) be an oriented Riemannian hypersurface of M n (κ) × R 1 with normal unit vector field ν. We denote by ∂ t the vertical vector of M n (κ) × R 1 , that is, the unit vector field giving the orientation of R 1 in M n (κ) × R 1 . The projection of ∂ t on T M is denoted by T and its normal part is f . Therefore, we have ∂ t = T + f ν, with f = − ∂ t , ν since ν, ν = −1. We can compute the curvature tensor of M n (κ)× R 1 for vector fields tangent to M . We have the following proposition.
Proposition 2.1. For any vector fields X, Y, Z, W ∈ Γ(T M), we haveProof. Let X, Y, Z, W ∈ Γ(T M). We can write these vector fields as follows:with X, Y , Z and W tangent to M n (κ) and x, y, z and w some real-valued functions.Since ∂ t is parallel, we haveMoreover, since the vector fields X, Y, Z and W are tangent to M and ∂ t , ∂ t = −1, we have x = − X, T , y = − Y, T , z = − Z, T and w = − W, T . So, a straightforward computation yields the first identity. For the second identity, we denote, ν = ν + n∂ t with ν tangent to M n (κ). Clearly, we see that n = − ν, ∂ t = f . So, we havewhich gives easily the wanted identity. Int. J. Geom. Methods Mod. Phys. 2011.08:1269-1290. Downloaded from www.worldscientific.com by YALE UNIVERSITY on 07/07/15. For personal use only.