On the Full Transitivity of a Cotorsion Hull of the Pierce Group ()
1. Introduction
The groups discussed in the paper are abelian and the operation is written in additive terms. We use here the notation and terminology of the monographs [1] [2] .
The symbol
denotes a fixed prime number.
and
are respectively the groups of integer and rational numbers. A subgroup
of the group
is called fully invariant if it is self-mapped for any endomorphism of the group
.
The knowledge of the construction of fully invariant subgroups of an abelian group and their lattice is essentially helpful in the study of the properties of the group itself and also in the investigation of the properties of its rings of endomorphisms and quasi-endomorphisms, the group of automorphisms and other algebraic systems connected with the initial group.
For a sufficiently wide class of
-groups these topics were studied by R. Baer, I. Kaplansky, P. Linton, R. Pierce, D. Moore, E. Hewett and others. The works of A. Mader, R. Göbel, P. A. Krylov, S. Ya. Grinshpon, A. I. Moskalenko and other authors are dedicated to the investigation of these topics in torsion-free and mixed groups.
However little is known about the results obtained in this area for the class of cotorsion groups. A group
is called a cotorsion group if its extension by means of any torsion-free group
splits as follows:
. The importance of the class of cotorsion groups in the theory of abelian groups is due to two factors: for any groups
,
, the group
is a cotorsion one and any reduced group
is isomorphically embeddable in the group
called the cotorsion hull of the group
. If the torsion part of the group
is denoted by
, then

where
. Thus the study of cotorsion groups essentially reduces to the study of groups of the form
, where
is a
-primary group.
It is noteworthy that endomorpohisms in cotorsion groups are completely defined by their action on the torsion part and, as shown by W. May and E. Toubassi [3] , for a mixed group
the ring of endomorphisms
is isomorphic to
if and only if
is a fully invariant subgroup of the cotorsion hull
.
The notion of full transitivity of a group plays an essential role in describing the lattice of fully invariant subgroups.
By the
-indicator of an element
of the group
we mean an increasing sequence of ordinal numbers

where
is the generalized
-height of an element, i.e. for
if
and
. Now for the set of indicators we can introduce the order

A reduced
-group is called fully transitive if for arbitrary elements
and
, when
there exists an endomorphism
of the group such that
. The class of fully transitive groups includes such important groups as separable
-groups, algebraically compact groups and quasi-pure injective groups.
Using the indicators of fully transitive groups we can describe the lattice of fully invariant subgroups (see [4] -[11] ).
For a module over a commutative ring, A. Mader formulated a general scheme that can be used to describe the lattice of fully invariant submodules of the module (see [10] , Theorem 2.1 or [12] , Theorem 1.1).
In the same way as we did for a
-group we define the notion of full transitivity for the group
. According to A. Mader [10] , an algebraically compact group is fully transitive and described with the aid of indicators the lattice of fully invariant subgroups of this group. This means to describe the lattice of fully invariant subgroups of the group
when
is a torsion-complete group. When
is the direct sum of cyclic
-groups, A. Moskalenko [11] proved that
is also fully transitive and described by means of indicators the lattice of fully invariant subgroups of the group
. In general, for the separable primary group
, the cotorsion hull
is not fully transitive. In particular if
is an infinite direct sum of torsion-complete groups, then, as shown by the author [13] , the group
is not fully transitive and in that case the lattice of fully invariant subgroups of the group
cannot be described by means of indicators (see [12] ).
R. Pierce [14] considered the primary group
, a ring of whose endomorphisms has the form
(1.1)
where
is the ring of small endomorphisms of the group
which is the ideal of the ring of endomorphisms
of the group
, whereas
is the ring of integer
-adic numbers. A small endomorphism of the group
is defined as follows (see [14] ).
For all
there exists an integer
such that
. (1.2)
The Pierce group
is important when studying the ring of endomorphisms of abelian groups (see [15] ). The aim of the present paper consists in elucidating the full transitivity of the cotorsion hull
and also in finding the conditions, under which the cotorsion hull is not fully transitive.
2. Full Transitivity of the Cotorsion Hull of the Pierce Group
As mentioned above, R. Pierce [14] considered the separable primary group
with a standard basic subgroup
,
,
,
, where
is a torsion-complete group, i.e. the torsion part of a
-adic completion of the group
. The cardinality is
and the ring of endomorphisms of the group
has form (1.1).
To study the full transitivity of the group
, we use the following representation of elements of the cotorsion hull of
given by A. Moskalenko [11] for the separable
-group 
. (2.1)
Representation of elements in this form makes it easy to calculate the height and the indicator. In particular, if
, then
(2.2)
where
is the smallest infinite ordinal number.
Let
be a basic subgroup of the reduced separable
-group
lying between
and
. Elements
,
,
. As is know, an endomorphism
of the group
extends uniquely to an endomorphism of
.
The following lemma is true.
Lemma 2.1. If
and there exists no endomorphism
of the group
for which
, then a cotorsion hull
is not fully transitive.
Proof. Consider two elements

of the group
. Then by the condition of the theorem and (2.2) we have
. As is known, each endomorphism of the group
extends uniquely to an endomorphism of the group
. We will show that if for an endomorphism
,
, then
. Let
(2.3)
be the element of the group
defined by the sequence
. For an endomorphism
of the group
let us show that
. According to ([1] , Section 50), the extension of
is defined from the commutative diagram
(2.4)
where
is the identical inclusion,

The commutativity of diagram (2.4) immediately follows from the definition of these homomorphisms.
To extension (2.3) there corresponds the sequence
, where elements are defined as follows: fix a system of generators
of the group
,
,
. Let
,
, be a system of representatives of the adjacent classes
of the group
,
,
,
. Denote
.
Then for each
,
.
For an endomorphism
of the group
we have
,
, and can define
(2.5)
It is obvious that the right-hand part of equality (2.5) defines the extension of an endomorphism
on
and if
is some other endomorphism of the group
, which induces
on
, then
contains
and
([1] , Proposition 34.1). From (2.5) we have
.
Now we can consider an element
(2.6)
of the group
and with its aid define the corresponding short exact sequence.
Let
be the group defined by a system of generators
which are defined by the relations of the group
and the equalities
,
,
. Then
(2.7)
where
is the identical inclusion and, for each element
,
,
,
is a short exact sequence. To extension (2.7) there corresponds sequence (2.6) (see [11] , Proof of Theorem 1). Let us show that by using extensions (2.3) and (2.7) we can compose the commutative diagram
(2.8)
where
is the above-mentioned endomorphism and
,
,
,
. Indeed, from the definition of a triple
we immediately conclude that (2.8) is a commutative diagram.
Thus we have shown that (2.4) and (2.8) are commutative diagrams. Then, according to ([1] , Section 50),
and
are equivalent extensions and thereby define one and the same sequence from
. But, by virtue of our construction,
is the sequence corresponding to the extension
; therefore it corresponds to the extension
, too. Thus
. Therefore if the endomorphism
maps the element
into
, then
, i.e. we have proved more than what has been mentioned at the beginning of the proof of the lemma. Thus it obviously follows that if there exists no endomorphism
of the group
for which
, then there exists no endomorphism
of the group
which maps the element
into
, i.e.
is not fully transitive. The lemma is proved.
For the Pierce group
the following statement is true.
Theorem 2.1. The cotorsion hull
of the group
is not fully transitive.
Proof. We use representation (2.1) of cotorsion hull elements and assume that
and
are elements of the group
, where
,
. By virtue of (11, Item 2), elements
and
can be written in the form

where
,
. Since
and
is infinite, taking into account ([14] : Lemma 15.1, Theorem 15.4) we can assume that
(2.9)
By (2.2) we have
, i.e. the following condition is fulfilled
.
Let
be an endomorphism of the group
. Using (1.1) we have
, where
is a small endomorphism of the group
and
is the
-adic number
. As is known ([1] , Section 39), the endomorphism
uniquely extends to the endomorphism
of the group 

Since
(2.10)
and
is a small endomorphism of the group
(see (1.2)), starting with some
we have
.
Therefore

On the other hand, from (2.10) we obtain

Therefore

But
since
,
, and in that case the equality
would contradict condition (2.9). Therefore
.
Thus there exists no endomorphism
of the group
which extends to the endomorphism
of the group
and
. Then from Lemma 1.1 it follows that Theorem 2.1 is valid.
Note that one more example of a separable primary group, the cotorsion hull of which is not fully transitive, can be found in ([11] , item 3).
As mentioned above, if the separable primary group
is a direct sum of cyclic
-groups or a cotorsioncomplete group, then the cotorsion hull
is fully transitive. In 1993, at Professor A. Fomin’s seminar A. Moskalenko made a conjecture that
is fully transitive only in these two cases. The proved lemma and theorem may serve as a positive argument in favor of this conjecture.
Acknowledgements
This study was supported by the grant (ATSU-2013/44) of Akaki Tsereteli University.