tomitagt.dvi


@
Applied General Topology

c© Universidad Politécnica de Valencia

Volume 5, No. 1, 2004

pp. 97- 101

A countably compact free Abelian group whose
size has countable cofinality

I. Castro Pereira and A. H. Tomita ∗

Abstract. Based on some set-theoretical observations, compact-
ness results are given for general hit-and-miss hyperspaces. Compact-

ness here is sometimes viewed splitting into ”κ-Lindelöfness” and ”κ-

compactness” for cardinals κ. To focus only hit-and-miss structures,

could look quite old-fashioned, but some importance, at least for the

techniques, is given by a recent result of Som Naimpally, to who this

article is hearty dedicated.

2000 AMS Classification: Primary 54H15: Secondary 22B99, 54D30.

Keywords: forcing, countably compact group, convergence, Continuum Hypoth-
esis, countable cofinality, size

1. Introduction

In 1990, Tkachenko showed that there exists a countably compact group topology
on the free Abelian group of size c, assuming the Continuum Hypothesis (CH).
Dikranjan and Shakmatov (see [1]) proved that there is no countably compact group
topology on any free group and asked the following question:

For which cardinals κ can the free Abelian group of size κ be endowed with a
countably compact group topology?.

In [3], it was shown that it is consistent that 2c is such a cardinal, and as a
consequence, any infinite cardinal κ ≤ 2c with κ = κω. For cardinals that do not
satisfy κ = κω, a natural question due to van Douwen [2] needs to be addressed:

1.5 Question. If X is an infinite group (or homogeneous space) which is count-
ably compact, is |X|ω = |X|? Is at least cf(|X|) 6= ω?

It was shown in [2] that the answer to van Douwen’s question is yes under GCH;
recently in [6], question 1.5 above was answered in the negative. However, the
example contains convergent sequences and all its elements have order 2.

In this note, we obtain the following:

Example 1.1. It is consistent that 2c is ‘arbitrarily large’ and that there exists
a countably compact group topology on the free Abelian group of size λ for any
λ ∈ [c,2c]. In particular, it is consistent that there are countably compact group
topologies on a free Abelian group whose size has countable cofinality.

∗The research in this paper was partially conducted while the second author was visiting Pro-
fessor T. Nogura at Ehime University with the financial support of the Ministry of Education of
Japan.



98 I. Castro Pereira and A. H. Tomita

2. Preliminaries

We will construct via forcing a free set X = {xβ : β < κ} such that for every
γ ∈ [c,κ), the group generated by {xβ : β < γ} is countably compact. An element
of the group generated is given by

∑
ξ∈dom F F(ξ)xξ, where F is a function whose

domain is a finite subset of κ and the range is Z.
A sequence {gn : n ∈ ω} in the group G generated by X can be coded as {F(n) :

n ∈ ω}, where dom F(n) ∈ [κ]<ω and rng F(n) ⊆ Z. So gn =
∑
ξ∈domF(n) F(n)(ξ)xξ =

xF(n).
For the properties of denseness that will be required later on, it will be useful to

work with particular sequences (see [5]):

Definition 2.1. Let F : ω −→
⋃
E∈[κ]<ω\{∅}(Z \ {0})

E, where F is 1 − 1.

We say that F is a sequence of type I if {dom F(n) : n ∈ ω} is faithfully indexed.
We say that F is a sequence of type II if there exists E such that dom F(n) = E

for every n ∈ ω and there exists µ ∈ E such that {F(n)(µ) : n ∈ ω} is faithfully
indexed.

Lemma 2.2. Let {an : n ∈ ω} be a sequence in G = 〈{xβ : β < κ}〉 that does not
contain a constant subsequence. Then there exists a sequence F of type I or type
II such that {xF(n) : n ∈ ω} ⊆ {an : n ∈ ω}

Proof. Easy exercise �

Thus, from the lemma above, it suffices to show that every sequence in G coded
by a sequence of type I or type II has an accumulation point.

Let T be the set of opens subarcs in T. We say that Φ : Dp −→ T has finite
support if {ξ ∈ Dp : Φ(ξ) 6= T} is finite. Given two such functions Φ and Φ

∗, we

say that Φ ≤ Φ∗ if Φ(ξ) ⊆ Φ∗(ξ) for each ξ ∈ Dp.

Lemma 2.3. If F is of type I or type II and φ∗ is a function of finite support as
above then given an open set W of T there exists m ∈ dom F and Φ ≤ Φ∗ of finite
support such that

∑
ξ∈dom F(m) F(m)(ξ)Φ(ξ) ⊆ W.

Proof. See [5]. �

Definition 2.4. Let {Fβ : β < κ} be an enumeration of all sequences of type I or
type II in definition 2.1.

3. The partial order

All the background information on forcing required in this paper can be found
in [4].

Definition 3.1. Let P be the family of all element p = (αp,{xp,ξ : ξ ∈ Dp},{Ap,ζ :
ζ ∈ Ep) satisfying the following conditions:
i) αp ∈ ω1;
ii) Dp ∈ [κ]

ω;

iii) xp,ξ ∈ T
αp;

iv) Ep ∈ [κ]
ω;

v) Ap,ζ ⊆ Dp ∩ c;
vi) dom Fζ(n) ⊆ Dp for each n ∈ ω, ζ ∈ Ep;
vii) xp,ξ is an accumulation point of {xp,Fζ(n) : n ∈ ω} for each ξ ∈ Ap,ζ and

each ζ ∈ Ep;
viii) {xp,ξ : ξ ∈ Dp} is a faithfully indexed free set.
Given p,q ∈ P, p extends q if
a) αp ≥ αq;



Free Abelian groups whose size has countable cofinality 99

b) Dp ⊇ Dq;
c) xp,ξ|αq = xq,ξ for all ξ ∈ Dq;
d) Ep ⊇ Eq;
e) Ap,ζ ⊇ Aq,ζ for all ζ ∈ Eq.

The following results will be proven in the next section:

Lemma 3.2. (CH) The partial order P is countably closed and ω2−cc.

Lemma 3.3. The set Dα,ξ,ζ,µ = {p ∈ P : αp ≥ α,ξ ∈ Dp,ζ ∈ Ep ∧ Ap,ζ \ µ 6= ∅}
is dense in P, for each α,µ < ω1 and ξ,ζ < κ.

We are ready to prove the main result:

Proof. (of Example 1.1) Start with a model of GCH and let G be a generic set
for the partial order P. The forcing is cardinal preserving by Lemma 3.2, no new
countable subsets of the ground model are added, CH holds and κ = 2c.

For each ξ,ζ ∈ κ, let xξ =
⋃
p∈G∧ξ∈Dp

xp,ξ and Aζ =
⋃
p∈G∧ζ∈Ep

Ap,ζ. By

denseness of the sets in Lemma 3.3, each xξ is defined and is a function in 2
c and

the set {xβ : β < κ} is free. Also, each Aζ is defined and has size c. Clearly, xµ is
an accumulation point of {xFζ(n) : n ∈ ω} for each µ ∈ Aζ.

Fix λ ∈ [c,κ]. Clearly the group generated by {xβ : β < λ} is free Abelian and
from Lemma 2.2 it will be countably compact as well. �

4. Some proofs

We start by proving an auxiliary lemma which will be used in the proofs of
Lemmas 3.2 and 3.3.

Lemma 4.1. Let r = (αr,{xr,ξ : ξ ∈ Dr},{Ar,ζ : ζ ∈ Er}) satisfying all conditions
in Definition 3.1 with the exception of condition viii). Then there exists a condition
p ∈ P such that p ‘extends’ r, that is, conditions a) − e) are satisfied.

Proof. We shall define p such that αp = αr + ω, Dp = Dr, Ep = Er, Ap,ζ = Ar,ζ
for all ζ ∈ Ep and xp,ξ|αr = xr,ξ for all ξ ∈ Dp.

List
⋃
E∈[Dp]<ω

(Z \ {0})E in length ω as {Fn : n ∈ ω}. We shall define by

induction xp,ξ(α + n) for each ξ ∈ Dp and each n ∈ ω so that conditions vii) is
satisfied (all other conditions are trivially satisfied) and xp,Fn (α + n) 6= 0 ∈ T.
Suppose that condition vii) is satisfied for {xp,ξ|αr+n : ξ ∈ Dp}. For each ζ ∈ Ep
and µ ∈ Ap,ζ let Bζ,µ be an infinite subset of ω such that {xp,Fζ(m)|αr+n : m ∈ Bζ,µ}
converges to xp,µ|αr+n. Partition each Bζ,µ into {Bζ,µ,k : k ∈ ω} each of infinite
size and let {Wl : l ∈ ω} be a basis for T. Enumerate all possible pairs (Bζ,µ,k,Wl)
as {(Bζt,µt,kt,Wlt) : k ∈ ω}. We will define by induction a decreasing family
Φt : ω −→ T for t ∈ ω of finite support as defined prior to Lemma 2.3.

Start with Φ0 such that the support of Φ0 contains domFn and 0 /∈
∑

ξ∈domFn
Fn(ξ)Φ0(ξ).

Since Fζ|Bζ0,µ0,k0 is of type I or II, from Lemma 2.3, there exists m0 ∈ Bζ0,µ0,k0
such that Φ1 ≤ Φ0 of finite support such that

∑
ξ∈dom Fζ0 (m0)

Fζ(m0)(ξ)Φ1(ξ) ⊆

W0.
By induction, using Lemma 2.3, define {Φt : t ∈ ω} and {mt : t ∈ ω} such that

Φt+1 ≤ Φt and
∑

ξ∈dom Fζt (mt)
Fζt(mt)(ξ)Φt+1(ξ) ⊆ Wt. For each ξ ∈ Dp define

xp,ξ(α + n) ∈
⋂
t∈ω Φt(ξ). Then condition vii) is satisfied by {xp,ξ|α+n+1 : ξ ∈ Dp}

and
∑

ξ∈dom Fn
Fn(ξ)xp,ξ(α + n) 6= 0. �



100 I. Castro Pereira and A. H. Tomita

Proof. (of Lemma 3.2) It is straightforward to see that the partial order is countably
closed. Indeed, if pn+1 ≤ pn for each n ∈ ω, define pω = (αpω,{xpω,ξ : ξ ∈
Dpω },{Apω,ζ : ζ ∈ Epω }), where αω = sup{αpn : n ∈ ω}, Dpω =

⋃
n∈ω Dpn,

xpω,ξ =
⋃
ξ∈Dpn

xpn,ξ, Epω =
⋃
n∈ω Epn, Apω,ζ =

⋃
Apn,ζ. Then pω ≤ pn for each

n ∈ ω.
We will now check that P is ω2-cc. Let {pβ : β < ω2} be a subset of P. From CH

and the ∆-system lemma, we conclude that there exists I ∈ [ω2]
ω2 and D ∈ [κ]≤ω

such that Dpβ ∩ Dpγ = D for any pair {β,γ} ∈ [I]
2. Without loss of generality, we

can assume that there exists α < ω1 such that αpβ = α for every β ∈ I. Also we
can assume that |{xpβ,ξ : β ∈ I}| = 1 for each ξ ∈ D.

Fix β,γ ∈ I. Let Dr = Dpβ ∪ Dpγ . Define xr,ξ as xpβ,ξ if ξ ∈ Dpβ or xpγ,ξ if
ξ ∈ Dpγ \ D. Set Er = Epβ ∪ Epγ and define Ar,ζ as Apβ,ζ if ζ ∈ Epβ \ Epγ ; Apγ,ζ
if ζ ∈ Epγ \ Epβ or Apβ,ζ ∪ Apγ,ζ if ζ ∈ Epβ ∩ Epγ . Note that r = (α,{xr,ξ : ξ ∈
Dr},{Ar,ζ : ζ ∈ Er}) may not be a condition in P but it can be extended to a
condition p by applying Lemma 4.1. The condition p extends pγ and pβ. Therefore,
there are no antichains of size ω2. �

Proof. (of Lemma 3.3) Let q be an arbitrary element of P.
If ζ ∈ Eq, define Er = Eq. Let θ ∈ (µ,c) such that θ /∈ Dq ∪ {ξ}, set Dr = Dq ∪

{θ}∪{ξ} and define xq,θ as an accumulation point of the sequence {xq,Fζ(n) : n ∈ ω}.
If ξ ∈ Dr \ Dq define xq,ξ = 0 ∈ T

αq .
If ζ /∈ Eq, choose θ ∈ (µ,c) \ (Dq ∪

⋃
n∈ω dom Fζ(n) ∪ {ξ}) and set Dr = Dq ∪

(
⋃
n∈ω dom Fζ(n)) ∪ {θ,ξ}. If ψ ∈ Dr \ (Dq ∪ {θ}), define xq,ψ = 0 ∈ T

αq . Define
xq,θ as an accumulation point of {xq,Fζ(n) : n ∈ ω}.

In either case, define Ar,ρ = Aq,ρ for each ρ ∈ Er \ {ζ} and Ar,ζ = Aq,ζ ∪ {θ}. If
αq ≥ α, let xr,η = xq,η for each η ∈ Dr; otherwise, let xr,η = xq,η ∪ {(β,0) : αq ≤
β < α} for each η ∈ Dr. Set αr = max{α,αq}.

The set r = (αr,{xr,η : η ∈ Dr},{Ar,ρ : ρ ∈ Er}) satisfies all the conditions
to be an element of P with the possible exception of condition viii). Applying the
Lemma 4.1, there exists a condition p ‘below’ r. Such condition p will be an element
of Dα,ξ,ζ,µ and below q. �

Note: Independently from this note, D. Dikranjan and D. Shakmatov produced
a model of ZFC + CH with 2c ”arbitrarily large” and, in this model they obtained
a characterization of Abelian groups of size κ with κ ≤ 2c wich admit a countably
compact group topology without non–trivial convergent sequences. Using forcing
they constructed a group monomorphism π : Q(κ) ⊕(Q/Z)(κ) −→ Tω1 such that for
every almost n– torsion subset E from Q(κ) ⊕ (Q/Z)(κ), π(E) is HFD (hereditarily
finally dense) in T[n]ω1 and has a cluster point in π(Q(κ) ⊕ (Q/Z)(κ)).

References

[1] W. W. Comfort, K. H. Hofmann and D. Remus, Topological groups and semigroups, Recent
progress in general topology (Prague, 1991), 57–144, North-Holland, Amsterdam, 1992.

[2] E. K. van Douwen, The weight of a pseudocompact (homogeneous) space whose cardinality has
countable cofinality, Proc. Amer. Math. Soc. 80 (1980), 678–682.

[3] P. B. Koszmider, A. H. Tomita and S. Watson, Forcing countably compact group topologies on a
larger free Abelian group, Topology Proc. 25 (Summer 2000), 563–574.

[4] K. Kunen, Set theory. An introduction to independence proofs, Studies in Logic and the Founda-
tions of Mathematics, 102. North-Holland Publishing Co., Amsterdam, 1980. xvi+313.

[5] M. G. Tkachenko, Countably compact and pseudocompact topologies on free abelian groups, Izves-
tia VUZ. Matematika 34 (1990), 68–75.

[6] A. H. Tomita, Two countably compact groups: one of size ℵω and the other of weight ℵω without
non-trivial convergent sequences, to appear in Proc. Amer. Math. Soc.



Free Abelian groups whose size has countable cofinality 101

Received December 2002

Accepted February 2003

I. Castro Pereira, A. H. Tomita (castro@ime.usp.br, tomita@ime.usp.br)
Departamento de Matemática, Instituto de Matemática e Estat́ıstica, Universidade
de São Paulo, Caixa Postal 66281, CEP 05315-970, São Paulo, Brasil