Let C be a coherent analytic geometry. Proposition 5.6.1. Any prime of an object is contained in a maximal prime. Proof. Suppose P is a prime of an object X. Let S be the set of prime subobject of X containing P. It is nonempty. Let C be an ascending chain in S. For each prime Q in C let X_{Q} be the localization of X at Q. Then X_{Q} is a local object with the a minimal prime above Q. We obtain a decending sequence of noninitial fractions of X. The intersection U of these fractions is noninitial as the initial object is finitely presented. Let O be any prime of U. Then the strong image of O in each X_{Q} is a prime containg its minimal prime. Thus the strong image of O in X contains each Q. By Zorn's lemma, S has a maximal element, which is a maximal prime object containing P. Propisition 5.6.2. Suppose f: Y > X is an epi. Then any maximal prime of X is in the image Spec(f)(Spec(Y)) of Spec(f). Proof. Suppose P is a maximal prime of X. Let i: X_{P} > X be the localization of X at P. Then X_{P} is an object with a unique prime object (i.e. quasilocal simple object) above P. Let i': T > Y and f': T > X_{P} be the pullback of f and i. Since f is epic and i is a fraction (thus coflat), the pullback f' is epic, thus noninitial. It follows that i' is noninitial. Let Q be any prime of T. Then f^{+1}(i^{+1}(Q)) = (fi')^{+1}(Q) = (if')^{+1}(Q) = P. Thus P is in the image of Spec(f). A subset U of Spec(X) is called stable under specialization (resp. generalization)if any prime contained in (resp. containing) a prime in U is in U. Proposition 5.6.3. Let f: Y > X be a map. Then Spec(f)(Spec(Y)) is closed iff it is stable under specialization. Proof. One direction is clear. Suppose Spec(f)(Spec(Y)) is stable under specialization. Replacing X by the strong image f^{+}(Y) we may assume that f is an epi. We have to show that any prime of X is in Spec(f)(Spec(Y)). By (5.6.1) P is contained in a maximal prime Q, which is in the image by (5.6.2), thus P is in the image as f is stable under specialization. Proposition 5.6.4. Suppose X is a von Neumann regular object. Then a subset U of Spec(X) is closed iff U = Spec(f)(Spec(Y)) for a map f: Y > X. Proof. If U is a closed then by the definition of the topology on Spec(X) it is the image of Spec(f) for a regular mono f to X. Conversely, for any map f, the image Spec(f)(Spec(Y)) is stable under specialization as any prime in a von Neumann regular object is simple, thus maximal. So Spec(f)(Spec(Y)) is closed by (5.6.3). Example 5.6.4.1. We say an analytic category is simply atomic if any simple object is unisimple and any noninitial object is the codomain of a map with a simple domain. Then any simply atomic analytic category is atomic in the sense of [Luo Atomic Categories]. Suppose A is a simply atomic Stone coherent geometry. The spectrum functor Spec coincides with the unifunctor of A. Thus a map is unipotent iff it induces a surjective function on the spectrums. Hence a map is epic iff it is unipotent by (5.6.2) and the fact that A is reduced. It follows that any epi is universal and any map is coflat. Definition 5.6.5. (a) Suppose f: Y > X
is a map. A map g: Z > X is called a precomplement
of f if g is disjoint with f, and any noninitial
map to X that is disjoint with f is not disjoint with g.
Note that a map f is a precomplement of a map g iff g is a precomplement of f. Proposition 5.6.6. If f: Y > X is a coflat quasicomplemenetary map then Spec(f)(Spec(Y)) is an open subset. Proof. Suppose g: Z > X is a map which is disjoint with f and {f, g} is a unipotent cover. Then the assumption implies that Spec(f)(Spec(Y)) = Spec(X)  Spec(g)(Y). Consider a prime P of Spec(f)(Spec(Y)). Let X_{P} be the locaization of X at P. Applying (5.2.13) (Going Up Theorem) the image of Spec(X_{P}) in Spec(X) is in Spec(f)(Y). Thus X_{P} > X is disjoint with g. The fraction X_{P} is an intersection of a collection {U_{i}} of analytic subobjects of X containing the residue of P. Let V_{i} be the pullback of U_{i} along g. Then the intersection of V_{i} is the pullback of X_{p} along g, which is initial. Since the initial object is finite presentable, V_{i} is initial for some i. Thus U_{i} is dijoint with g, which implies that Spec(U_{i}) is contained in Spec(f)(Spec(Y)). Since Spec(U_{i}) contains P, this shows that Spec(f)(Spec(Y)) is an open subset of Spec(X). Proposition 5.6.7. Suppose any finitely presentable map in A is quasicomplementary. If f: Y > X is a coflat finitely presentable map then Spec(f) is an open map. Proof. Consider an open subset U of Spec(Y). Any point p of U is contained in an analyitc subset V of U. The analyitc mono v: V > Y is coflat and finitely presentable, so the composite fv is coflat finitely presentable, thus is also quasicomplementary by assumption. By (5.6.6) fv(V) is an open subset of X, thus is an open neighborhood of f(p) contained in f(U). This shows that f(U) is an open subset, so f is an open map. Remark 5.6.8. The condition of 5.6.7. is satisfied by the opposite
of the category of commutative rings.
