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Causal symmetries

We define a new type of transformation for Lorentzian manifolds characterized by mapping every causal future-directed vector onto a causal future-directed vector. The set of all such transformations, which we call causal symmetries, has the structure of a submonoid. Some of their properties are investigated and we give necessary conditions for a vector field $\xiv$ to be the infinitesimal generator of a one-parameter group of causal symmetries. Some examples are discussed.