5.1. Coherent Analytic Geometries
Any coherent analytic
category is a spatial
(4.2.2) and perfect
(4.1.2.b). Thus a locally
disjunctable coherent analytic category is a spatial analytic
geometry, which is simply called a coherent
analytic geometry.
In the following we assume A is a coherent
analytic geometry. Recall that (3.6.9)
if X is an object. an open subset U of Spec(X)
is affine open if it is determined by an analytic subobject of X.
The set of affine open subsets is a base for the topology on Spec(X).
Proposition 5.1.1. (a) Spec(X)
is a coherent space for any object
X.
(b} For any map f: Y ®
X the continuous mapping Spec(f): Spec(Y)
® Spec(X) is a mapping
of coherent spaces.
Proof. (a) Suppose {Vi} is a set of disjunctable
strong subobjects such that {(Vi)c}iÎI
is an analytic cover for X. Let V be the intersection of
these {Vi}. Then by (3.1.10)
we have X = ØØ{(Vi)c}
= ØV, so V = 0. Since
0 is finitely copresentable by (4.3.2.c),
there is a finite subset J Í
I such that {Vi}iÎJ
has intersection 0. Applying (3.1.10)
again we see that {(Vi)c} is a finite
analytic cover for X. This shows that Spec(X) is quasi-compact.
Thus any affine open subset is also quasi-compact. Since affine open subsets
form a base for Spec(X), any finite intersection of quasi-compact
open subsets is quasi-compact. This means that Spec(X) is
coherent.
(b) The pullback of analytic subobject of X along f is
an analytic subobject of Y. Since affine open subsets form a base
for Spec(X), this implies that inverse image of quasi-compact
open subset is quasi-compact open. Thus Spec(f) is a mapping
of coherent spaces. –n
Proposition 5.1.2. (a) If X is
quasi-primary then the fraction hull
P(X) of X is the intersection of all non-initial analytic
monos.
(b) An object is primary iff
its rational hull R(X)
is quasi-simple; if X
is primary then R(X) = Q(X) = P(X);
(c) An object is integral
iff its rational hull is simple;
any integral object X has a generic
residue P(X) ® X
which is an epic coflat simple fraction.
Proof. (a) Consider the intersection P of non-initial
analytic subobjects of X, which is a non-initial fraction of X
containing P(X). We have to prove that P is quasi-simple,
or equivalently, that P is pseudo-simple
(3.3.10). By (3.3.8.a)
it suffices to prove that any non-initial strong subobject of P
is unipotent. Since P ®
X is coflat, any strong subobject
of P is induced from X by (1.5.4).
Since any unipotent strong subobject of X induces a unipotent strong
subobject of P, we only need to prove that P is disjoint
with any non-unipotent strong subobject V of X. Since A
is locally
disjunctable, V is an intersection of proper disjunctable
strong subobjects {Vi}. Since V is non-unipotent,
at least one complement (Vi)c is non-initial,
and P Í (Vi)c.
Thus P Ç V = 0. This shows
that P is pseudo-simple, thus P = P(X).
(b) If X is primary then any non-initial analytic mono is epic.
Thus R(X) = P(X) by (a), and P(X)
is primary by (3.3.8). Any object X
is a quotient of its rational hull R(X). If R(X)
is quasi-simple, it is primary by (3.3.8),
thus X as a quotient of R(X) is primary by (3.2.2.a)
(c) If X is integral then any proper strong subobject of the
reduced object X is non-unipotent
by (3.1.2). According to the proof of (a),
P(X) is disjoint with any non-unipotent strong
subobject V of X. Since P(X) ®
X is coflat, any strong subobject of P(X) is induced
from X. This means that the only proper strong subobject of P(X)
is 0 , thus P(X) is simple, which is a generic residue
of X; and P(X) ® X
is an epic coflat fraction as P(X) = R(X)
by (b) and R(X) ®
X is epic coflat. Conversely if P(X) is simple the X
as a quotient of integral object R(X) = P(X)
is integral by (3.2.3.a). n
Proposition 5.1.3. Suppose X
is a quasi-primary object with the fractional hull pX:
P(X) ® X. Let s:
S(X) ® X be the
strong image of pX.
(a) P(X) is generic quasi-simple and any generic map
from a quasi-simple object to X factors through pX
uniquely.
(b) S(X) is generic primary and any generic map from
a primary object to X factors through s uniquely.
(c) Any quasi-primary object has a unique generic primary strong subobject.
Proof. (a) P(X) is generic quasi-simple by
(4.2.5.c). It is easy to see that any generic
map from a quasi-simple object to X factors through any non-initial
analytic subobject of X, therefore it also factors through the intersection
pX of these analytic monos, which is P(X)
by (5.1.2.a).
(b) The quasi-simple object P(X) is primary (3.3.8),
thus its quotient S(X) is also primary. S(X)
is a generic strong subobject of X because any non-initial analytic
subobject of X contains P(X), therefore not disjoint
with S(X). We prove that S(X) has the required
universal property. Consider a generic map t: T ®
X with T primary. Let pT: P(T)
® T be the fractional hull of
T. Since pT is generic, so is the composite t°pT.
By (a) the generic map t°pT:
P(X) ® X factors
through pX. Thus t°pT.
factors through the strong mono s. Suppose m°e
= t is the epi-strong-mono factorization of t. Since pT
is epic, m° e°pT
= t°pT
is the epi-strong-mono factorization of t°pT.
Thus m is the smallest strong mono such that t°pT
can be factored through. Since t°pT
factors through s, m factors throughout s, thus t
= m°e factors
through s as desired.
(c) It follows from (b) that S(X) is the unique generic
primary strong subobject of X. n
Proposition 5.1.4. (a) Any map f:
S ® X with a simple domain factors
through a unique residue.
(b) Any simple subobject is contained in a unique residue.
(c) A simple subobject is a residue iff it is maximal.
Proof. (a) The epi S ®
f+1(S) is generic by (3.3.4.f),
so it factors through the generic residue of f+1(S).
The uniqueness follows from (3.4.4.f).
(b) follows from (a).
(c) It follows from (b) that any maximal simple subobject is a residue.
The other direction has been noticed in (3.4.4.d).
n
Proposition 5.1.5. (a) Any colimits
of reduced objects is reduced.
(b) Any cofiltered limits of reduced object is reduced.
Proof. (a) Since the full subcategory of reduced objects is a
coreflective subobject of A , it is closed
under colimits.
(b) Let {ri: X ®
Xi}iÎI
be a cofiltered limits of reduced objects in A.
We have to prove that any proper strong subobject U of X
is non-unipotent. Since any proper strong subobject is contained in a proper
regular subobject, and any proper regular subobject is an intersection
of proper finitely cogenerated regular subobject by (4.1.3.a),
we may assume that U is a finitely cogenerated regular subobject.
So let us assume that U is the equalizer of a pair of distinct maps
(m, n): X ® T where
T is finitely copresentable. Since X is a cofiltered limits
and T is finitely copresentable, we can find some t in I
and a pair (mt, nt): Xt ®
T of maps such that mt°rt
= m and nt°rt
= n. We may assume that t is an initial object in I.
Let Ut be the equalizer of (mt, nt).
Then the pullback of Ut along rt: X ®
Xt is U. Since the proper regular subobject Ut
is an intersection of proper disjunctable strong subobject, and rt
does not factors through Ut, we can find a proper disjunctable
subobject V of Xt containing Ut
such that rt does not factor through rt.
Let V be the pullback of Vt along rt,
and Vi be the pullback of Vt along
Xi ® Xt.
Then Vi and V are proper disjunctable strong subobjects
and U Í V, and Vc
is the cofiltered limit of (Vi)c. Since
each Xi is reduced and Vi is proper,
each (Vi)c is non-initial. Since the
initial object is finitely copresentable, this implies that Vc
is non-initial. Thus V is not unipotent, and hence U is not
unipotent as desired. n
Proposition 5.1.6. (a) An object is
integral iff it is a quotient of a simple object.
(b) A non-initial object is primary iff it is a quotient of a quasi-simple
object.
(c) An object is reduced iff it is a quotient of coproducts of simple
objects.
Proof. (a) The condition is sufficient because any quotient of a simple
object is integral. Conversely any integral object is a quotient of its
rational hull, which is simple (5.1.2.c).
(b) The condition is sufficient because any quotient of a quasi-simple
object is primary (3.3.8). Conversely any
primary object is a quotient of its rational hull, which is quasi-simple
(5.1.2.b).
(c) Any coproducts of simple objects is reduced (5.1.5.a).
Conversely, assume X is a reduced object. Let T be the coproduct
of all the residues pi: Pi ®
X of X. Denote by t: T ®
X the map induced by pi. Then T is reduced
(5.1.5.a). We prove that t is epic. It suffices
to show that t is unipotent as by assumption X is reduced.
Any map s: S ® X with
a simple domain factors through a unique residue of X by (5.1.4.a).
So s factors through t. Since the class of simple objects
is unipotent dense, it follows that t is unipotent by (2.2.10).
n
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