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Let $Rng$ be the category of commutative rings. Let $Loc$ be the category of locally ringed spaces. Let $(X, \mathcal{O}_X)$ be an locally ringed space. Then $\Gamma(X) = \Gamma(X, \mathcal{O}_X)$ is an commutative ring. Hence $\Gamma(X)$ induces an functor $\Gamma\colon Loc \rightarrow Rng^o$, where $Rng^o$ is the oposite category of $Rng$. Does $\Gamma$ have an adjoint functor?

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    I meant to say, in the comment above, "unique morphism of LRS inducing the identity on global sections."2012-11-24

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The $Spec$ functor is the desired adjoint functor to the category of locally ringed spaces (it is right adjoint to $\Gamma$). I think Hartshorne has an exercise where he asks us to prove this when $Loc$ is replaced by the category of schemes.

I like Anton Geraschenko's answer here: https://mathoverflow.net/questions/731/points-in-algebraic-geometry-why-shift-from-m-spec-to-spec/756#756

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    @ZhenLin Thanks! "Lemma 21.6.4 on page 1252 of the book version"2012-11-24