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In page Spectrum of a ring:

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of local rings. Thus Spec {\displaystyle \operatorname {Spec} } even defines a contravariant functor from the category of commutative rings to the category of locally ringed spaces. In fact it is the universal such functor, and hence can be used to define the functor Spec {\displaystyle \operatorname {Spec} } up to natural isomorphism.[citation needed]