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Minkowski space refers to a mathematical formulation in four dimensions. However, the mathematics can easily be extended or simplified to create an analogous generalized Minkowski space in any number of dimensions. If , -dimensional Minkowski space is a vector space of real dimension on which there is a constant Minkowski metric of signature or . These generalizations are used in theories where spacetime is assumed to have more or less than dimensions. String theory and M-theory are two examples where . In string theory, there appears conformal field theories with spacetime dimensions.