5.1. Coherent Analytic Geometries
Any coherent analytic
category is 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 |
iI}
is an analytic cover for X. Let V be the intersection of
these {Vi}. Then by (3.1.10)
we have X = {(Vi)c}
= V, so V = 0. Since
0
is finitely copresentable by (4.3.2.c),
there is a finite subset J
I such that {Vi} | iI}
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. –
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 (Vi)c.
Thus P 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).
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 tpT. By (a) the generic map t°pT:
P(X) --> X factors through pX. Thus
tpT.
factors through the strong mono s. Suppose
me = t
is the epi-strong-mono factorization of t. Since
pT
is epic, mepT = tpT is the epi-strong-mono
factorization of tpT. Thus m is the smallest strong
mono such that tpT can be factored through. Since tpT
factors through s, m factors throughout s, thus t
= me factors through
s as desired.
(c) It follows from (b) that S(X) is the unique generic
primary strong subobject of X.
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).
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 | iI}
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 mtrt
= m and ntrt = 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 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.
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).
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