abstract some of the so called smallness conditions in algebra as well as in category theory,...
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Abstract
Some of the so called smallness conditions in algebra as well as in Category Theory, important and interesting for their own and also tightly related to injectivity, Essential Bounded , Cogenerating set, and Residual Smallness. Here we want to see the relationships between these notions and to study these notions in the class mod(∑ , E). That is all of objects in the Grothendick
Topos E which satisfy ∑, ∑ is a class of equations .
Introduction
In the whole of this talk A is an arbitrary category and M is an arbitrary subclass of its morphisms .
Def. Let A be an arbitrary category and M be an arbitrary subclass of it’s morphisms, also A and B are two objects in A. We say that A is an M- subobject of B provided that there exists an M- morphism m:AB.
Def. One says that M has “good properties” with respect to composition if it is: (1) Isomorphism closed; that is, contains all isomorphism and is closed under composition with isomorphism. (2) Left regular; that is, for f in M with fg=f we have g is an isomorphism. (3) Composition closed; that is, for f:AB and g:BC in M, gf is also in M. (4) Left cancellable; that is, gf is in M, implies f is in M. (5) Right cancellable; that is, gf is in M, implies g is in M.
In this case we say that (A,m) is an M- subobject of B, or (m,B) is an M- extension for A.
The class of all M- subobjects of an object X is denoted by M/X, and the class of all M-extensions of X is denoted by X/M.
We like the class of M-subobjects M/X to behave proper. This holds, if M has good properties.
We can define a binary relation ≤ on M/X as follows:
If (A,m) and (B,n) are two M- subobjets of X, we say that (A,m)≤(B,n) whenever there exists a morphism f:AB such that nf=m. That is
A X
B
m
nf
One can easily see that ≤ is a reflective and transitive relation, but it isn’t antisymmetric. But if M is left regular, ≤ is antisymmetric, too up to
isomorphism. Similarly, one defines a relation ≤ on X/M as follows:
If (m,A) and (n,B) belong to X/ M, we say (m,A)≤(n,B) whenever, there exists a morphism f:A B such that fm = n. That is: X A
B
m
nf
Also it is easily seen that (X/M , ≤) forms a partially ordered class up to the relation ∼. Where (m,A) ∼ (n,B) iff (m,A) ≤ (n,B) and (n,B) ≤ (m,A) .
So from now on, we consider (X/ M ,≤ ) up to ∼.
Def. In Universal Algebra we say that A is subdirectly irreducible if for any morphism f:A ∏i in I Ai with all Pi f epimorphisms, there exists an index i0 in I for which pi0 f is an isomorphism.
The following definition generalizes the above definition and it is seen that these are equivalent for equational categories of algebras.
Def. An object S in a category is called M-subdirectly irreducible if there are an object X with two deferent morphisms f,g:X S s.t. any morphism h with domain S and hf≠hg, belongs to M. See the following:
SX f
gB
hs.t. hf ≠ hg ⇒ h∈M
Def. An M-chain is a family of X/M say {(m i,B i)}i in I which is indexed by a totally ordered set I such that if i ≤j in I then there exists aij:Bi Bj
with aij mi i =m j. Also we have aii = id Bi and for i ≤j ≤k, a ik=ajk aij.
Also when the class of M-subdirectly irreducible objects in a category A forms just only a set we say that A is M-residually small
Def. An M- well ordered chain is an M- chain which is indexed by a totally well ordered set I.
X B0
B1
B2
Bn
Bn+1
m1
m2
mn
mn+1
f01
f12
fn n+1
m0
Essential Boundedness and Residul Smallness
Def. A is said to be M-essentially bounded if for every object A∈A there is a set {m i:AB i : i∈ I } ⊆ M s.t. for any M-essential extension n:AB there exists i0∈I and h:BB i0 with m i0=hn.
The. M*-cowell poweredness implies M-essential boundedness. Conversely, if M=Mono, A is M-well powered and M-essentially bounded, then A is M*-cowell powered.
Def. An M-morphism f:AB is called an M-essential extension of A if any morphism g:BC is in M whenever g f belongs to M. the class of all M- essential extension (of A) is denoted by M* (M*A).
Note. A category A is called M-cowell powered whenever for any object A in A the class of all M-extensions of A forms a set.
The. Let M=Mono and E be another class of morphisms of A s.t. A has (E,M)-factorization diagonalization. Also, let A be E-cowell powered and have a generating set G s.t. for all G∈G, G ப G ∈A. Then, M*-cowell poweredness implies M-residual smallness.
Coro. Under the hypothesis of the former theorems, we can see that residual smallness is a necessary condition to having enough M-
injectives when M=Mono .
The. If A has enough M- injectives then A is M- essentially bounded .
Def. A has M-transferability property if for every pair f, u of morphisms with M-morphism f one has a commutative square
A B
C⇒
A
C
B
D
f
u
f
v
with M-morphism g.
Lem. If A has enough M- injectives then, A fulfills M- transferability property.
Def. We say that A has M-bounds if for any small family {h i:AB i : i∈ I }≤ M there exists an M-morphism h:AB which factors over all h i,s.
g
The. Let A satisfy the M-transferability and M-chain condition, and let M be closed under composition. Then, A has M-bounds.
u
Cogenerating set and Residual Smallness
Def. A set C of objects of a category is a cogenerating set if for every parallel different morphisms m,n:xY there exist C∈C and a morphism f:YC s.t. fm≠fn.
XYm
nC
f
The. Let A have a cogenerating set C and A be M-well powered. Then A is M-residually small.
Prop. For any equational class A , the following conditions are equivalent:
)i (Injectivity is well behaved.
)ii (A has enough injectives .
)iii (Every subdirectly irreducible algebra in A has an injective extension.
)iv (A satisfies M-transferability and M*- cowell poweredness.
The. Let M=Mono and A be well powered with products and a generating set G. Then, having an M-cogenerating set implies M*-cowell poweredness.
( i) A is M-essentially bounded.
Corollary. Let M=Mono and A be well powered and have products and (E,M)-factorization diagonalization for a class E of morphisms for which A is E-well powered. Then T. F. S. A. E.
( ii) A is M*-cowell powered.
(iii) A is M-residually small.
(iv) A has a cogenerating set.
Injectivity of Algebras in a Grothendieck Topos
Now we are going to see and investigate these notions and theorems in mod (∑,E). To do this we compare the two categories mod (∑,E) and
mod).(∑
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