Invariance of domain is a theorem in topology that relates to the concept of continuous functions between topological spaces, particularly finite-dimensional Euclidean spaces.
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Invariance of domain says that a continuous map that is an injective function between topological manifolds without boundary of the same dimension is an open map. It is useful in the Penrose singularity theorem: projecting a compact achronal boundary along timelike curves onto a Cauchy hypersurface gives a nonempty subset that is both open and closed, forcing that connected Cauchy hypersurface to be compact.