4. Morphisms in a Metric Site  Definition 4.1. A metric presite C is separable if the following condition is satisfied:  Suppose f: (X, x) ® (S, s) and g: (Y, y) ® (S, s) are two morphisms of pointed objects, then there exist a pointed object (W, w) and two morphisms p: (W, w) ® (X, x) and q: (W, w) ® (Y, y) such that fp = gq.  All the metric sites given in (1.5.2), (2.2.1) - (2.2.3), (3.11) are separable. A separable metric presite C has the important properties that any monomorphism is injective, and surjective morphisms are stable under base extension:  Proposition 4.2. Any monomorphism in a separable metric presite C is injective.  Proof. Suppose a morphism f: Y ® X is not injective. There are two distinct points y and y' of |Y| such that f(y) = f(y'). Since C is separable, we can find a pointed object (W, w) and two morphisms p: (W, w) ® (Y, y) and q: (W, w) ® (Y, y') such that fp = fq. Since p ¹ q, the morphism f is not a monomorphism.  Proposition 4.3. Suppose f: X ® S, g: Y ® S are two objects over an object S in a separable metric presite. Suppose the fibre product X ×S Y of X and Y over S exists, with the projections p: X ×S Y ® X and q: X ×S Y ® Y. We have  (a) |q(X ×S Y)| = g-1(|f(X)|).  (b) If |f(X)| is open or closed, then |q(X ×S Y)| is open or closed.  (c) If f is surjective, then q: X ×S Y ® Y is surjective.  (d) If f is a monomorphism, then q: X ×S Y ® Y is injective.  Proof. (a) can be checked directly using (4.1); (b) and (c) follow from (a); (d) follows from (4.2) because q is a monomorphism. n  Proposition 4.4. Suppose (C, tC) is a metric presite and D a full subcategory of C. Suppose tC is an extension of tD = tC|D. Then (C, tC) is separable if and only if (D, tD) is separable.  Proof. First suppose (D, tD) is separable. Suppose f: (X, x) ® (S, s) and g: (Y, y) ® (S, s) are two morphisms of pointed objects of C. Since |X|, |Y| are D-exact, there are two morphisms f': (X', x') ® (X, x) and g': (Y', y') ® (Y, y) of pointed objects with X', Y' Î D by (1.6.a). We obtain two morphisms u = f f': (X', x') ® (S, s) and v = gg': (Y', y') ® (S, s). Since |S| is D-exact, u is connected to v in D/(|S|, s) by (1.6.b). In the notation of (1.9) this means that the triple (X', x', u) is equivalent to (Y', y', v). Since D is separable, it is easy to see that (X', x', u) is equivalent to (Y', y', v) if and only if (X', x', u) ~ (Y', y', v). Thus we can find two morphisms p: (Z, z) ® (X', x') and q: (Z, z) ® (Y', y') with Z Î D such that up = vq. The compositions f'p: (Z, z) ® (X, x) and g'q: (Z, z) ® (Y, y) are what we are looking for. This shows that (C, tC) is separable.  Conversely assume that (C, tC) is separable. Suppose f: (X, x) ® (S, s) and g: (Y, y) ® (S, s) are two morphisms of pointed object of D. Since C is separable, there are two morphisms p: (Z, z) ® (X, x) and q: (Z, z) ® (Y, y) of pointed objects of C. Since |Z| is D-exact, there is a morphism h: (Z', z') ® (Z, z) with Z' Î D by (1.6.a). Then ph: (Z', z') ® (X, x) and qh: (Z', z') ® (Y, y) are what we are looking for. This proves that (D, tD) is separable. n  Remark 4.5. It follows from (4.4) that if C is an ordinary metric (resp. strict metric) site, then C^ (resp. C~ or D(C)) is separable if and only if C is separable.  Finally we introduce the important notion of a separated morphism in a metric site. Suppose C is a metric site with fibre products and a terminate object Z. Let f: X ® Y be a morphism in C. The diagonal morphism of f is the unique morphism Df: X ® X ×Y X whose composition with both projection maps p, q: X ×Y X ® X is the identity map of X.  Definition 4.6. A morphism f: X ® Y in C is called separated (or X is separated over Y) if the diagonal morphism Df: X ® X ×Y X is universally closed (i.e., Df is a closed morphism which remains closed under any base extension). An object X is called separated if it is separated over the final object Z.  Remark 4.7. In categorical geometry a separated object plays the role of a Hausdorff space in topology. Indeed, if the pretopology t of a metric site C preserves fibre products and |Z| is a one-point space (e.g., C is the metric site of topological spaces, or ringed spaces), then one can prove that an object X is separated if and only if its underlying space is Hausdorff.  Remark 4.8. A monomorphism in a category is regular if it occurs as an equalizer of a pair of morphisms with the same domain and codomain. Many natural metric sites (with fibre products) have the good property that any regular monomorphism is universally bicontinuous (e.g., the metric sites of local ringed spaces or schemes). Since the diagonal morphism Df for any morphism f is a regular monomorphism, it follows that a morphism f: X ® Y in these sites is separated if and only if |Df(X)| is a closed subset of |X ×Y X|.  Following the notation and method of [EGAI, Ch. I, §3] we can prove the following general properties for separated morphisms:  Proposition 4.9. (cf. [Hartshorne 1977, p.99]). (a) Any monomorphism is separated.  (b) A composition of two separated morphisms is separated.  (c) Separated morphisms are stable under base extension.  (d) If f: X ® Y and f': X' ® Y' are separated morphisms over a base object S, then the product morphism f × f': X ×S X' ® Y ×S Y' is also separated.  (e) If the composition gf of two morphisms is separated, f is separated.  (f) A morphism f: X ® Y is separated if and only if |Y| can be covered by open effective subsets Vi such that f-1(Vi) ® Vi is separated for each i. n  Corollary 4.10. (a) Let f: X ® S be a separated morphism. Suppose U Í |X| and V Í |S| are effective subsets and f(U) Í V. Then U is separated over V.  (b) An object X is separated if and only for any object S, any morphism f: X ® S is separated.  (c) If f: X ® S is a separated morphism and S is separated, then X is separated.  (d) Any effective subobject of a separated object X is separated. n  A metric site C is called separated if it has fibre products and a terminate object such that any object (or equivalently by (4.10.b), any morphism) in C is separated. An ordinary, strict, separated metric site C is called separately complete if any separated Dedekind cut in D(C) is representable.  Definition 4.11. Suppose C is an ordinary strict, separated metric site. A separated completion of C is an ordinary, separately complete metric site C ' containing C as a base.  The following separated version of Theorem 3.8 can be proved in a similar fashion (the last assertion follows from (4.9.d)).  Theorem 4.12. Any ordinary, strict, separated metric site C has a separated completion, which is uniquely determined by C up to equivalence. The collection of separated Dedekind cuts forms a separated completion of C. Fibre products exist in any separated completion of C. n  Example 4.12.1. Fibre products exist in the metric sites of complex analytic spaces, separated prevarieties and separated schemes because each of these metric sites is a separated completion of a strict, separated metric site.  Example 4.12.2. Fibre products exist in the metric site of separated local ringed spaces (this follows from (2.10) and (4.9.d)).    [Content][References][Notations][Home]