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Revista de la Unión Matemática Argentina
Print version ISSN 0041-6932On-line version ISSN 1669-9637
Rev. Unión Mat. Argent. vol.48 no.2 Bahía Blanca July/Dec. 2007
The Ladder Construction of Prüfer Modules
Claus Michael Ringel
Dedicated to María Inés Platzeck on the occasion of her 60th birthday
Abstract Let be a ring (associative, with 1). A non-zero module
is said to be a Prüfer module provided there exists a surjective, locally nilpotent endomorphism with kernel of finite length. The aim of this note is to construct Prüfer modules starting from a pair of module homomorphisms
, where
is injective and its cokernel is of finite length. For
the ring of integers, one can construct in this way the ordinary Prüfer groups considered in abelian group theory. Our interest lies in the case that
is an artin algebra.
1. The construction.
Let be a ring (associative, with 1). The modules to be considered will usually be left
-modules. Our main interest will be the case where
is an artin algebra, however the basic construction should be of interest for any ring
. In fact, the standard examples of what we call Prüfer modules are the Prüfer groups in abelian group theory, thus
-modules. Here is the definition of a Prüfer module: it is a non-zero module
which has a surjective, locally nilpotent endomorphism
with kernel of finite length. If
is the kernel of
, we often will write
, and we will denote the kernel of
by
. Observe the slight ambiguity: given a Prüfer module
, not only
but also all non-trivial powers of
and maybe many other endomorphisms will have the required properties (surjectivity, local nilpotency, finite length kernel).
The content of the paper is as follows. In the first section we show that any pair of module homomorphisms , where
is injective with non-zero cokernel of finite length, gives rise to a Prüfer module. Section 2 provides some examples and section 3 outlines the relationship between Prüfer modules and various sorts of self-extensions of finite length modules. The final sections 4 and 5 deal with degenerations in the sense of Riedtmann-Zwara: we will show that this degeneration theory is intimately connected to the existence of Prüfer modules with some splitting property, and we will exhibit an extension of a recent result by Bautista and Pérez. Our interest in the questions considered here was stimulated by a series of lectures by Sverre Smalø [S] at the Mar del Plata conference, March 2006, and we are indebted to him as well as to M.C.R.Butler and G.Zwara for helpful comments.
For the relevance of Prüfer modules when dealing with artin algebras of infinite representation type, we refer to a forthcoming paper [R5]. The appendix to section 3.3 provides some indications in this direction.
1.1. The basic frame. A pair of exact sequences
![U2](/img/revistas/ruma/v48n2/2a0424x.png)
![0 → U w-0→ U → H → 0 0 1](/img/revistas/ruma/v48n2/2a0426x.png)
![v0](/img/revistas/ruma/v48n2/2a0427x.png)
Recall that a commutative square
Next, we will use that the composition of exact squares is exact:
(E1) The composition of two exact squares
1.2. The ladder. Using induction, we obtain in this way modules and pairs of exact sequences
![i ≥ 0.](/img/revistas/ruma/v48n2/2a0436x.png)
We may combine the pushout diagrams constructed inductively and obtain the following ladder of commutative squares:
![⋃ U∞ = iUi](/img/revistas/ruma/v48n2/2a0438x.png)
![wi](/img/revistas/ruma/v48n2/2a0439x.png)
Since all the squares commute, the maps induce a map
which we denote by
:
![U∞ ∕U0](/img/revistas/ruma/v48n2/2a0444x.png)
![U ∞∕U1](/img/revistas/ruma/v48n2/2a0445x.png)
![v∞ : U∞ → U ∞](/img/revistas/ruma/v48n2/2a0446x.png)
![U0](/img/revistas/ruma/v48n2/2a0447x.png)
![U1](/img/revistas/ruma/v48n2/2a0448x.png)
Claim. The map is an isomorphism. Namely, the commutative diagrams
![v- : U ∕U → U ∕U . i i i- 1 i+1 i](/img/revistas/ruma/v48n2/2a0453x.png)
![v-](/img/revistas/ruma/v48n2/2a0454x.png)
![Ui∕Ui- 1](/img/revistas/ruma/v48n2/2a0455x.png)
![i ≥ 1](/img/revistas/ruma/v48n2/2a0456x.png)
![Ui+1∕Ui](/img/revistas/ruma/v48n2/2a0457x.png)
![i ≥ 1](/img/revistas/ruma/v48n2/2a0458x.png)
![-- vi](/img/revistas/ruma/v48n2/2a0459x.png)
It follows: The composition of maps
![p](/img/revistas/ruma/v48n2/2a0461x.png)
![φ](/img/revistas/ruma/v48n2/2a0462x.png)
![U1∕U0.](/img/revistas/ruma/v48n2/2a0463x.png)
![φ](/img/revistas/ruma/v48n2/2a0464x.png)
![φt(Ut∕U0 ) = 0](/img/revistas/ruma/v48n2/2a0465x.png)
![t](/img/revistas/ruma/v48n2/2a0466x.png)
Summary. (a) The maps yield a map
![K](/img/revistas/ruma/v48n2/2a0469x.png)
![Q](/img/revistas/ruma/v48n2/2a0470x.png)
(b) This map induces an isomorphism
. Composing the inverse of this isomorphism with the canonical projection
, we obtain an endomorphism
![H](/img/revistas/ruma/v48n2/2a0476x.png)
![w 0](/img/revistas/ruma/v48n2/2a0477x.png)
![U ∕U ∞ 0](/img/revistas/ruma/v48n2/2a0478x.png)
![φ,](/img/revistas/ruma/v48n2/2a0479x.png)
![H](/img/revistas/ruma/v48n2/2a0480x.png)
![U ∞ ∕U0](/img/revistas/ruma/v48n2/2a0481x.png)
![(U ∞∕U0, φ)](/img/revistas/ruma/v48n2/2a0482x.png)
![(w0, v0)](/img/revistas/ruma/v48n2/2a0483x.png)
![Ui](/img/revistas/ruma/v48n2/2a0484x.png)
If necessary, we will use the following notation: , for all
and
for the Prüfer module. Since
is a Prüfer module with basis the cokernel
of
, we will sometimes write
or even
Remark: Using a terminology introduced for string algebras [R3], we also could say: is expanding,
is contracting.
Lemma. Assume that with
. Then
is generated by
.
Proof: For , the module
is a factor module of
thus by induction,
is generated by
.
1.3. The chessboard. Assume now that both maps are monomorphisms. Then we get the following arrangement of commutative squares:
![U∞ (w0,v0)](/img/revistas/ruma/v48n2/2a04106x.png)
![v∞](/img/revistas/ruma/v48n2/2a04107x.png)
![U ∞ (v0,w0)](/img/revistas/ruma/v48n2/2a04108x.png)
![w∞](/img/revistas/ruma/v48n2/2a04109x.png)
2. Examples.
(1) The classical example: Let be the ring of integers, and
its regular representation. Module homomorphisms
are given by the multiplication with some integer
, thus we denote such a map just by
. Let
and
. If
is odd, then
is the ordinary Prüfer group for the prime
, and
(the subring of
generated by
). If
is even, then
is an elementary abelian 2-group.
(2) Let be the Kronecker algebra over some field
. Let
be simple projective,
indecomposable projective of length 3 and
a non-zero map with cokernel
(one of the indecomposable modules of length 2). The module
is the Prüfer module for
if and only if
otherwise it is a direct sum of copies of
.
(3) Trivial cases: First, let be a split monomorphism. Then the Prüfer module with respect to any map
is just the countable sum of copies of
. Second, let
be an arbitrary monomorphism, let
be an endomorphism. Then
is the countable sum of copies of
.
(4) Assume that there exists a split monomorphism , say
and
. Then
![H](/img/revistas/ruma/v48n2/2a04146x.png)
![X](/img/revistas/ruma/v48n2/2a04147x.png)
Remark: Not all Prüfer modules are of ladder type. Consider the generalized Kronecker algebra with countably many arrows
starting at the vertex
and ending in the vertex
. Define a representation
as follows: Let
be a vector space with a countable basis
and let
be defined by
provided
and
otherwise. Let
be the endomorphism of
of
, respectively, which sends
to
and
to
for
Then
is a Prüfer module (with respect to
, but also with respect to any power of
). Obviously,
is a faithful
-module. Assume that
for some maps
with
of finite length. Then
is generated by
, according to Lemma 1.2. However
is of finite length and no finite length
-module is faithful.
3. Ladder extensions.
3.1. The definition. Let be a non-zero module of finite length. A self-extension
is said to be a ladder extension provided there is a commutative diagram with exact rows
![f](/img/revistas/ruma/v48n2/2a04182x.png)
![q](/img/revistas/ruma/v48n2/2a04183x.png)
![f = qv](/img/revistas/ruma/v48n2/2a04184x.png)
![v : U0 → U1.](/img/revistas/ruma/v48n2/2a04185x.png)
This means that we have a commutative diagram with exact rows of the following kind (here ):
![H](/img/revistas/ruma/v48n2/2a04188x.png)
![q : U1 → H,](/img/revistas/ruma/v48n2/2a04189x.png)
![w0](/img/revistas/ruma/v48n2/2a04190x.png)
![v : U → U 0 0 1](/img/revistas/ruma/v48n2/2a04191x.png)
According to section 1 we know: Ladder extensions build up to form Prüfer modules.
Lemma. Let be a commutative ring and
a
-algebra. Then
for any
Proof: We deal with the exact sequence induced by or
, respectively. But
since
Also, any central automorphism of
yields isomorphic extensions
and
. This shows that the extension
only depends on the
-subspace
Remark. Not all self-extensions are ladder extensions. For example: A non-zero self-extension of a simple module over an artinian ring is never a ladder extension!
Proof: Construct the corresponding ladder, thus the corresponding Prüfer module . The module
would be a (serial) module of Loewy length
, with
arbitrary. But the Loewy length of any module over the artinian ring
is bounded by the Loewy length of
thus
cannot exist.
Example. Here is a further example of a self-extension which is not a ladder extension. Consider the following quiver
![β](/img/revistas/ruma/v48n2/2a04218x.png)
![a](/img/revistas/ruma/v48n2/2a04219x.png)
![b](/img/revistas/ruma/v48n2/2a04220x.png)
![Q](/img/revistas/ruma/v48n2/2a04221x.png)
![β3 = 0.](/img/revistas/ruma/v48n2/2a04222x.png)
![^ Q](/img/revistas/ruma/v48n2/2a04223x.png)
![Q](/img/revistas/ruma/v48n2/2a04224x.png)
![D5](/img/revistas/ruma/v48n2/2a04225x.png)
![′ Q](/img/revistas/ruma/v48n2/2a04226x.png)
![Q′](/img/revistas/ruma/v48n2/2a04228x.png)
![H](/img/revistas/ruma/v48n2/2a04231x.png)
![′′ H](/img/revistas/ruma/v48n2/2a04232x.png)
Proposition. Let be an indecomposable module with Auslander-Reiten translate isomorphic to
. Assume that there is a simple submodule
of
with
Then the Auslander-Reiten sequence ending (and starting) in
is a ladder extension.
Proof. Let be the Auslander-Reiten sequence. Denote by
the inclusion map. Since the map
factors through
, there is a commutative diagram with exact rows of the following form:
![Ext1 (S,S ) = 0,](/img/revistas/ruma/v48n2/2a04245x.png)
![v : S → U](/img/revistas/ruma/v48n2/2a04246x.png)
![qv = u.](/img/revistas/ruma/v48n2/2a04247x.png)
![′ H = H [2;w,v ].](/img/revistas/ruma/v48n2/2a04248x.png)
We do not know whether one can delete the assumption about the existence of .
3.2. Standard self-extensions.
Let be an
-module, say with an exact sequence
, where
denotes a projective cover of
. We know that
![Hom (u,H ) : Hom (P H, H ) → Hom (ΩH, H )](/img/revistas/ruma/v48n2/2a04257x.png)
![φ : P H → H](/img/revistas/ruma/v48n2/2a04258x.png)
![φu.](/img/revistas/ruma/v48n2/2a04259x.png)
![p : P H → H](/img/revistas/ruma/v48n2/2a04260x.png)
![P H](/img/revistas/ruma/v48n2/2a04261x.png)
![φ′ : P H → P H](/img/revistas/ruma/v48n2/2a04262x.png)
![φ = pφ′.](/img/revistas/ruma/v48n2/2a04263x.png)
![φu = pφ′u](/img/revistas/ruma/v48n2/2a04264x.png)
![Hom (ΩH, p).](/img/revistas/ruma/v48n2/2a04265x.png)
Thus we can consider
![Hom (ΩH, H )∕Im (Hom (u, H )) = Ext1 (H, H ).](/img/revistas/ruma/v48n2/2a04267x.png)
![Ext1(H, H )s](/img/revistas/ruma/v48n2/2a04268x.png)
Proposition. Standard self-extensions are ladder extensions.
Proof. Here is the usual diagram in which way a map yields a self-extension of
![f](/img/revistas/ruma/v48n2/2a04272x.png)
![p](/img/revistas/ruma/v48n2/2a04273x.png)
![′ f = pw](/img/revistas/ruma/v48n2/2a04274x.png)
![′ w : ΩH → P H :](/img/revistas/ruma/v48n2/2a04275x.png)
3.3. Modules of projective dimension 1.
Proposition. If the projective dimension of is at most 1, then any self-extension of
is standard, thus a ladder extension.
Proof: Consider a module with a projective presentation
Any self-extension of
is given by a diagram of the following kind:
![P ′](/img/revistas/ruma/v48n2/2a04283x.png)
![p : P → H](/img/revistas/ruma/v48n2/2a04284x.png)
![f ′ : P ′ → P](/img/revistas/ruma/v48n2/2a04285x.png)
![f = pf ′.](/img/revistas/ruma/v48n2/2a04286x.png)
![′ H [2] = H [2;u, f ].](/img/revistas/ruma/v48n2/2a04287x.png)
Corollary. If is a hereditary ring, any self-extension is standard, thus a ladder extension.
Example of a ladder extension which is not standard. Consider the quiver
![δα = 0 = γβ = γ α - δβ.](/img/revistas/ruma/v48n2/2a04291x.png)
![H = (β : a → b)](/img/revistas/ruma/v48n2/2a04292x.png)
![α](/img/revistas/ruma/v48n2/2a04293x.png)
![ΩH](/img/revistas/ruma/v48n2/2a04294x.png)
![PH → H](/img/revistas/ruma/v48n2/2a04295x.png)
![ΩH = (γ : b → c).](/img/revistas/ruma/v48n2/2a04296x.png)
![H](/img/revistas/ruma/v48n2/2a04298x.png)
![f : ΩH → H](/img/revistas/ruma/v48n2/2a04299x.png)
![P H](/img/revistas/ruma/v48n2/2a04300x.png)
![Hom (ΩH, PH )](/img/revistas/ruma/v48n2/2a04301x.png)
![u](/img/revistas/ruma/v48n2/2a04302x.png)
![-- f : ΩH ∕K → H](/img/revistas/ruma/v48n2/2a04303x.png)
![p : PH ∕u (K ) → H](/img/revistas/ruma/v48n2/2a04304x.png)
![K = S (c)](/img/revistas/ruma/v48n2/2a04305x.png)
![f](/img/revistas/ruma/v48n2/2a04306x.png)
Appendix. Here, we want to indicate that the Corollary can be used in order to obtain a conceptual proof of the second Brauer-Thrall conjecture for hereditary artin algebras.
Assume that there is no generic module. We show: Any indecomposable module is a brick without self-extensions. Assume that there is an indecomposable module which is not a brick or which does have self-extensions. If
is not a brick, then the brick paper [R2] shows that there are bricks
with self-extensions. Thus, we see that there always is a brick
with self-extensions. Take any non-zero self-extension of
. According to 3.2, such a self-extension is standard, thus a ladder extension, thus we obtain a corresponding Prüfer module
. The process of simplification [R1] shows that all the modules
are indecomposable. Thus
is not of finite type and therefore there exists a generic module [R5].
But if any indecomposable module is a brick without self-extensions, the quadratic form is weakly positive. Ovsienko asserts that then there are only finitely many positive roots, thus the algebra is of bounded representation type and therefore of finite representation type.
3.4. Warning. A Prüfer module is not necessarily determined by
, even if it is of ladder type.
As an example take the generalized Kronecker quiver with vertices and three arrows
. and let
be the two-dimensional indecomposable representation annihilated by
and
. Consider a projective cover
, let
be its kernel, say with inclusion map
0 → ΩH ![]() ![]() | (*) |
The ladders to be considered are given by the various maps such that the image of
is not contained in
(otherwise, the induced self-extension of
will split). In order to specify a self-extension
of
, we require that
is annihilated say by
.
We will consider several copies of . If
is a generator, let us denote
thus,
is a basis of
.
We start with generated by
and consider the exact sequence
as displayed above. We see that
is a basis of
Now, let us consider two maps , here we denote the generator of
by
The first map
is given by
and
The second map
is defined by
and
.
Note that , thus
and actually this is precisely the self-extension of
annihilated by
An easy calculation shows that (and even
) is annihilated by
, whereas
is faithful. The following displays may be helpful; always, we exhibit the modules:
![f](/img/revistas/ruma/v48n2/2a04363x.png)
Now the corresponding display for the homomorphism .
4. Degenerations.
Definition: Let be finite length modules. Call
a degeneration of
provided there is an exact sequence of the form
with
of finite length. (such a sequence will be called a Riedtmann-Zwara sequence). The map
is called a corresponding steering map. (Note that in case we deal with modules over a finite dimensional
-algebra and
is an algebraically closed field, then this notion of degeneration coincides with the usual one, as Zwara [Z2] has shown.)
The proof of the following result is essentially due to Zwara, he used this argument in order to show that is a degeneration of
if and only if there is an exact sequence
(a co-Riedtmann-Zwara sequence) with
of finite length.
Proposition. Let be
-modules of finite length. The following conditions are equivalent:
- (1)
is a degeneration of
.
- (2) There is a Prüfer module
and some natural number
such that
for all
- (3) There is a Prüfer module
and some natural number
such that
.
Here is the recipe how to obtain a Prüfer module starting from a degeneration: If
is a degeneration of
, say with steering module
, then there exists a monomorphism
with cokernel
. The Prüfer module
we are looking for is
Proof of the implication (3) (1). Assume that there is a Prüfer module
such that
We get the following two exact sequences
![]() | (1) |
in the first, the map is given by applying
, in the second the map
is given by applying
In both sequences, we can replace
by
Thus we obtain as first sequence a new Riedtmann-Zwara sequence, and as second sequence a dual Riedtmann-Zwara sequence:
![]() | (2) |
note that both use the same steering module, namely Thus:
Remark. We see: The module is a degeneration of
if and only if there exists a module
and an exact sequence
.
Proof of the proposition. We need further properties of exact squares:
- (E2) For any map
, and any module
, the following diagram is exact:
- (E3) Let
is split mono.
- (E4) Assume that we have the following exact square
is a split monomorphism, then the sequence
Proofs. (E2) is obvious. (E3): Since is injective,
is injective. Let
be the cokernel of
. We obtain the map
by forming the induced exact sequence of
, using the zero map
. But such an induced exact sequence splits. (E4) Assume that
Then
There is the following lemma (again, see Zwara [Z1]):
Lemma (Existence of nilpotent steering maps.) If there is an exact sequence then there is an exact sequence
such that the map
is nilpotent.
Proof: We can decompose such that the given map
maps
into
,
into
and such that the induced map
belongs to the radical of the category, whereas the induced map
is an isomorphism. We obtain the following pair of exact squares
![U ′ 1](/img/revistas/ruma/v48n2/2a04443x.png)
![U1](/img/revistas/ruma/v48n2/2a04444x.png)
Assume that a monomorphism with cokernel
and
is given. Consider also the canonical embedding
and form the ladder
for this pair of monomorphisms
. The modules
are just the modules we are looking for: As we know, there is a Prüfer module
with
being the kernel of
We construct the maps explicitly as follows:
and
using the recipe (E2). Thus we obtain the following sequence of exact squares:
In particular, we have
Note that the composition is of the form
for some
.
We also have the following sequence of exact squares:
The composition of these exact squares yields an exact square
![Y [n ]](/img/revistas/ruma/v48n2/2a04466x.png)
![Xn](/img/revistas/ruma/v48n2/2a04467x.png)
Since the composition is of the form
and
it follows that
is a split monomorphism, see (E3).
Also, we can consider the following two exact squares, with (the upper square is exact, according to (E2)):
![ht](/img/revistas/ruma/v48n2/2a04474x.png)
![U](/img/revistas/ruma/v48n2/2a04476x.png)
![X (∞ )](/img/revistas/ruma/v48n2/2a04480x.png)
![Y [∞ ]](/img/revistas/ruma/v48n2/2a04481x.png)
![′ n Un (w,w ) = U ⊕ X .](/img/revistas/ruma/v48n2/2a04482x.png)
![φ](/img/revistas/ruma/v48n2/2a04483x.png)
![λ ∈ k](/img/revistas/ruma/v48n2/2a04485x.png)
![[η 0]](/img/revistas/ruma/v48n2/2a04486x.png)
![η = (1 + λφ )-1](/img/revistas/ruma/v48n2/2a04487x.png)
Corollary. Assume that is a degeneration of
. Then there exists a Prüfer module
such that
is isomorphic to
for some natural number
.
5. Application: The theorem of Bautista-Pérez.
Here we assume that we deal with an artin algebra , and all the modules are
-modules of finite length.
Proposition. Let be a module with
and assume there is given an exact sequence
. Then the cokernel of any monomorphism
is a degeneration of
.
Corollary (Bautista-Pérez). Let be modules, and let
and
be cokernels of monomorphisms
Assume that both
and
Then the modules
and
are isomorphic.
Both assertions are well-known in case is an algebraically closed field: in this case, the conclusion of the proposition just asserts that
is a degeneration of
in the sense of algebraic geometry. The main point here is to deal with the general case when
is an arbitrary artin algebra. The corollary stated above (under the additional assumptions that
is projective and that
are contained in the radical of
) is due to Bautista and Pérez [BP] and this result was presented by Smalø with a new proof [S] at Mar del Plata.
We need the following well-known lemma.
Lemma. Let be a module with
Let
be a sequence of inclusions of modules with
for all
Then there is a natural number
such that
is a split monomorphism for all
Let us use it in order to finish the proof of the proposition. Let and
the given monomorphism with cokernel
. Let
be an additional monomorphism, say with cokernel
. Thus we are in the setting of section 1. We apply the Lemma to the chain of inclusions
![n](/img/revistas/ruma/v48n2/2a04530x.png)
![wn : Un → Un+1](/img/revistas/ruma/v48n2/2a04531x.png)
![U n+1](/img/revistas/ruma/v48n2/2a04532x.png)
![U ⊕ W. n](/img/revistas/ruma/v48n2/2a04533x.png)
![U n+1](/img/revistas/ruma/v48n2/2a04535x.png)
![U ⊕ W n](/img/revistas/ruma/v48n2/2a04536x.png)
Proof of the Corollary. It is well-known that the existence of exact sequences
![W](/img/revistas/ruma/v48n2/2a04539x.png)
![′ W](/img/revistas/ruma/v48n2/2a04540x.png)
![Ext1(W, W ) = 0](/img/revistas/ruma/v48n2/2a04541x.png)
![Ext1 (W ′,W ′) = 0](/img/revistas/ruma/v48n2/2a04542x.png)
![n](/img/revistas/ruma/v48n2/2a04543x.png)
![Un+1](/img/revistas/ruma/v48n2/2a04545x.png)
![Un ⊕ W](/img/revistas/ruma/v48n2/2a04546x.png)
![′ Un ⊕ W](/img/revistas/ruma/v48n2/2a04547x.png)
![W](/img/revistas/ruma/v48n2/2a04548x.png)
![′ W](/img/revistas/ruma/v48n2/2a04549x.png)
Remark. Assume that are monomorphisms with cokernels
and
, respectively, and that
and
Then
splits if and only if
splits.
Proof: According to the corollary, we can assume . Assume that
splits, thus
is isomorphic to
. Look at the exact sequence
. If it does not split, then
, but
is isomorphic to
As we have mentioned, the lemma is well-known; an equivalent assertion was used for example by Roiter in his proof of the first Brauer-Thrall conjecture, a corresponding proof can be found in [R4]. We include here a slightly different proof:
Applying the functor to the short exact sequence
we obtain the exact sequence
![U0 → U1 → ⋅⋅⋅ → Ui → ⋅⋅⋅ .](/img/revistas/ruma/v48n2/2a04569x.png)
![Ext](/img/revistas/ruma/v48n2/2a04570x.png)
![k](/img/revistas/ruma/v48n2/2a04571x.png)
![Ext1 (W, U0)](/img/revistas/ruma/v48n2/2a04572x.png)
![k](/img/revistas/ruma/v48n2/2a04573x.png)
![n0](/img/revistas/ruma/v48n2/2a04574x.png)
![Un → Un+1](/img/revistas/ruma/v48n2/2a04575x.png)
![n ≥ n0.](/img/revistas/ruma/v48n2/2a04577x.png)
![Hom](/img/revistas/ruma/v48n2/2a04578x.png)
![Hom (W, Un+1 ) → Hom (W, W )](/img/revistas/ruma/v48n2/2a04580x.png)
![p : Un+1 → W](/img/revistas/ruma/v48n2/2a04581x.png)
![h ∈ Hom (W, Un+1 )](/img/revistas/ruma/v48n2/2a04582x.png)
![ph = 1W](/img/revistas/ruma/v48n2/2a04583x.png)
![p : Un+1 → W](/img/revistas/ruma/v48n2/2a04584x.png)
![Un → Un+1](/img/revistas/ruma/v48n2/2a04585x.png)
Remark. In general, there is no actual bound on the number . However, in case of dealing with the chain of inclusions
![Ext1 (W, U0 )](/img/revistas/ruma/v48n2/2a04588x.png)
![k](/img/revistas/ruma/v48n2/2a04589x.png)
![Ext1(W, U ) 0](/img/revistas/ruma/v48n2/2a04590x.png)
![E](/img/revistas/ruma/v48n2/2a04591x.png)
![E = End (W ).](/img/revistas/ruma/v48n2/2a04592x.png)
Proof: Look at the surjective maps
being induced by the maps (and these maps are not only
-linear, but even
-linear). Assume that
is bijective, for some
. As we have seen above, this implies that the sequence
0 → Un ![]() | (*) |
splits. Now the map is obtained from
as the induced exact sequence using the map
. With
also any induced exact sequence will split. Thus
is a split monomorphism (and
will be bijective, again). Thus, as soon as we get a bijection
for some
, then also all the following maps
with
are bijective.
Example. Consider the -quiver with subspace orientation:
![Λ](/img/revistas/ruma/v48n2/2a04612x.png)
![k](/img/revistas/ruma/v48n2/2a04613x.png)
![Λ](/img/revistas/ruma/v48n2/2a04614x.png)
Note that a map with cokernel
exists only in case the base-field
has at least 3 elements; of course, there is always a map
with cokernel
We have and it turns out that the module
is the following:
The pushout diagram involving the modules (twice) and
is constructed as follows: denote by
monomorphisms
which factor through the indecomposable projective modules
, respectively. We can assume that
, so that a mesh relation is satisfied. Denote the 3 summands of
by
, with non-zero maps
such that
There is the following commutative square, for any
we are interested when
:
(the only calculation which has to be done concerns the third entries: ). Note that
(as well as
) does not split.
But now we deal with a module such that
This implies that
is isomorphic to
. Thus the next pushout construction yields an exact sequence of the form
Acknowledgement. The author is indebted to Dieter Vossieck for a careful reading of the final version of the paper.
[BP] Bautista, R. and Pérez, E.: On modules and complexes without self-extensions. Communications in Algebra 34 (2006), 3139-3152. [ Links ]
[R1] Ringel, C.M.: Representations of k-species and bimodules. J.Algebra 41 (1976), 269-302. [ Links ]
[R2] Ringel, C.M.: Bricks in hereditary length categories. Resultate der Math. 6 (1983), 64-70. [ Links ]
[R3] Ringel, C.M.: Some algebraically compact modules I. In: Abelian Groups and Modules (ed. A. Facchini and C. Menini). Kluwer (1995), 419-439. [ Links ]
[R4] Ringel, C.M.: The Gabriel-Roiter measure. Bull. Sci. math. 129 (2005). 726-748. [ Links ]
[R5] Ringel, C.M.: Prüfer modules which are not of finite type. In preparation. [ Links ]
[S] Smalø, S.: Lectures on Algebras. Mar del Plata, Argentina March 2006. Revista Unión Matemática Argentina 48-2 (2007), 21-45. [ Links ]
[Z1] Zwara, G.: A degeneration-like order for modules. Arch. Math. 71 (1998), 437-444 [ Links ]
[Z2] Zwara, G.: Degenerations of finite-dimensional modules are given by extensions. Compositio Mathematica 121 (2001), 205-218. [ Links ]
Claus Michael Ringel
Fakultät für Mathematik, Universität Bielefeld,
POBox 100 131,
D-33 501 Bielefeld, Germany
ringel@math.uni-bielefeld.de
Recibido: 5 de marzo de 2007
Aceptado: 16 de abril de 2007