Chain conditions, elementary amenable groups, and descriptive set theory

29 Pages·2015·0.41 MB·English

CHAIN CONDITIONS, ELEMENTARY AMENABLE GROUPS,
AND DESCRIPTIVE SET THEORY
PHILLIPWESOLEKANDJAYWILLIAMS
Abstract. Wefirstconsiderthreewell-knownchainconditionsinthespace
ofmarkedgroups: theminimalconditiononcentralizers,themaximalcondi-
tiononsubgroups,andthemaximalconditiononnormalsubgroups. Foreach
condition,weproduceacharacterizationintermsofwell-foundeddescriptive-
set-theoretic trees. Using these characterizations, we demonstrate that the
sets given by these conditions are co-analytic and not Borel in the space of
markedgroups. Wethenadaptourtechniquestoshowelementaryamenable
markedgroupsmaybecharacterizedbywell-foundeddescriptive-set-theoretic
trees,andtherefore,elementaryamenabilityisequivalenttoachaincondition.
Ourcharacterizationagainimpliesthesetofelementaryamenablegroupsis
co-analytic and non-Borel. As corollary, we obtain a new, non-constructive,
proof of the existence of finitely generated amenable groups that are not
elementaryamenable.
1. Introduction
Chain conditions appear frequently in the study of countable groups. These
are finiteness conditions that forbid certain infinite sequences of subgroups. An
elementary but interesting example of such a condition is the property of being
polycyclic. From a geometric group theory perspective, these finiteness condi-
tions ought to restrict the complexity of the groups, as in the case of polycyclic
groups. From a descriptive set theory perspective, however, the chain conditions
arenon-Borelco-analyticstatementsand,therefore,eitheradmit“nice”non-chain-
condition characterizations - e.g. polycyclic groups are soluble with each term of
the derived series finitely generated - or describe large and wild classes. In this
work, we explore this tension in four chain conditions in the space of marked
groups.
In the space of marked groups, denoted G, we first consider three well-known
chain conditions: the minimal condition on centralizers, the maximal condition
on subgroups, and the maximal condition on normal subgroups. We characterize
each of these in terms of well-founded descriptive-set-theoretic trees. This charac-
terization implies the classes in question are large and wild, whereby they do not
admit “nice” characterizations.
Date:February2015.
2010 Mathematics Subject Classification. Primary20E34,Secondary54H05.
Key words and phrases. Chain conditions, elementary amenable groups, descriptive set
theory.
1
2 PHILLIPWESOLEKANDJAYWILLIAMS
Theorem 1.1. Each of the subsets of G defined by the minimal condition on
centralizers, the maximal condition on subgroups, and the maximal condition on
normal subgroups are co-analytic and not Borel. This remains true when restrict-
ing to finitely generated groups.
Ourtechniquesadditionallygivenewordinal-valuedisomorphisminvariantsun-
bounded below the first uncountable ordinal in the cases of the minimal condition
on centralizers and the maximal condition on subgroups. The ordinal-valued iso-
morphism invariant we obtain in the case of the maximal condition on normal
subgroups is not new and has been considered in the literature; cf. [3]. However,
ourapproachisnew, andweshowthatthisinvariantisunboundedbelowthefirst
uncountable ordinal.
We next consider the set of elementary amenable marked groups. We like-
wise characterize these in terms of descriptive-set-theoretic trees. It follows that
elementary amenability is indeed a chain condition.
Theorem 1.2. A countable group G is elementary amenable if and only if there
is no infinite descending sequence of the form
G=G ≥G ≥...≥G ≥...
0 1 n
such that for all n ≥ 0, G (cid:54)= {e} and there is a finitely generated subgroup
n
K ≤ G with G = [K ,K ]∩H , where H is the intersection of all index-
n n n+1 n n n n
(≤(n+1)) normal subgroups of K .
n
WeremarkthatitcanbethecasethatG =G foralli∈N;thisphenomenon
i i+1
occurs for infinite finitely generated simple groups.
Our characterization gives two new invariants of elementary amenable groups:
the decomposition rank and decomposition degree. We further obtain
Theorem 1.3. The sets of elementary amenable groups and finitely generated
elementary amenable groups are co-analytic and non-Borel in the space of marked
groups.
Itiswell-knownthatthesetofamenablegroupsisBorelinthespaceofmarked
groups. Our theorem thus gives a non-constructive answer to an old question of
M. Day [4], which was open until R. I. Grigorchuk [6] constructed groups of inter-
mediategrowth: Are all finitely generated amenable groups elementary amenable?
Corollary1.4. Thereisafinitelygeneratedamenablegroupthatisnotelementary
amenable.
The paper is organized as follows. In Section 2, we discuss the basic properties
of G and introduce concepts from descriptive set theory. In Sections 3,4, and 5,
we analyze sets of groups satisfying various chain conditions. This introduces our
use of descriptive-set-theoretic trees to study the structure of groups as well as
the ordinal-valued invariants arising from those trees. In Section 6, we use those
same techniques to analyze elementary amenable groups. In Section 7, we prove
the maps used throughout the paper are indeed Borel. Those who are content
to believe that our constructions are Borel can safely skip this section without
missing any group-theoretic content. Finally, Section 8 discusses some questions
arising from this paper not touched upon in earlier sections.
CHAIN CONDITIONS AND DESCRIPTIVE SET THEORY 3
2. Preliminaries
2.1. The space of marked groups. Inordertoapplythetechniquesofdescrip-
tive set theory to groups, we need an appropriate space of groups. Let F be the
ω
free group on the letters {ai}i∈N; so Fω is a free group on countably many genera-
tors with a distinguished set of generators. The power set of F may be naturally
ω
identified with the Cantor space {0,1}Fω =:2Fω. It is easy to check the collection
of normal subgroups of Fω, denoted G, is a closed subset of 2Fω and, hence, a
compact Polish space. Each N ∈ G is identified with a marked group. That is
thegroupG=Fω/N alongwithadistinguishedgeneratingset{fN(ai)}i∈N where
f :F →G is the usual projection; we always denote this projection by f . For
N ω N
amarkedgroupG,weabusenotationandsayG∈G; ofcourse,weformallymean
G = F /N for some N ∈ G. Since every countable group is a quotient of F , G
ω ω
givesacompactPolishspaceofallcountablegroups. Asub-basisforthistopology
is given by sets of the form
O :={N ∈G |γ ∈N},
γ
where γ ∈F along with their complements.
ω
Similarreasoningleadsustodefinethespaceof m-generatedmarkedgroups
as
(cid:92)
G := {N (cid:69)F |a ∈N}.
m ω i
i≥m
This is a closed subset of G and so is a compact Polish space in its own right. We
further let G := ∪ G be the space of finitely generated marked groups. As
fg m≥1 m
this is an F subset of G, it is a standard Borel space, with Borel sets precisely
σ
those sets of the form G ∩B with B Borel in G; a standard Borel space is a
fg
Borel space which admits a Polish topology that induces the Borel structure. We
can thus also talk about Borel functions with domain G .
fg
It is convenient to give the marked groups G=F /N a preferred enumeration.
ω
To this end, we fix an enumeration γ := (γi)i∈N of Fω. Each G is thus taken
to come with an enumeration fN(γ) := (fN(γi))i∈N; note the enumeration of G
may have many repetitions. When we write G as G={g ,g ,...}, we will always
0 1
mean this enumeration. Later in the paper we will work with N<N, i.e. the set of
finite sequences of natural numbers. If (s ,...,s )=:s∈N<N, we will write {g }
0 n s
for the set {g ,...,g }. Note that this set may have fewer than n+1 elements,
s0 sn
e.g. if s = s = ... = s , or even if the s are distinct but enumerate the same
0 1 n i
element.
Wewilloftendiscussquotientsofgroupsorparticularsubgroupsofgroups,and
of course we wish to view these as elements of G. A quotient of a marked group
is obviously again a marked group. However, subgroups of marked groups do not
have an obvious marking. The enumeration gives us a preferred way to select
markings for subgroups. If H ≤F /N =G∈G, let π : F →F be induced by
ω H ω ω
mapping the generators (ai)i∈N of Fω as follows:
(cid:40)
γ , if f (γ )∈H
π (a ):= j N j
H j
e, else.
4 PHILLIPWESOLEKANDJAYWILLIAMS
WethenidentifyH withF /ker(f ◦π ). InthecaseH hasadistinguishedfinite
ω N H
generating set {g ,...,g }, we instead define π (a ) = γ and π (a ) = e for
i0 in H ij ij H j
j (cid:54)= i ; this streamlines our proofs later. We often appeal to this convention
k
implicitly.
We will consider maps from and on G. A slogan from descriptive set theory
is “Borel = explicit” meaning if you describe a map “explicitly”, i.e. without an
appeal to something like the axiom of choice, it should be Borel. All of the maps
we discuss in the next few sections will be “explicit” in this sense, so we will not
provetheyareBorelwhenwedefinethem,inordertokeepthefocusonthegroup-
theoretic aspects of our constructions. We will often use enumerations of groups
in our constructions, but this will not require choice since every marked group
comes with a preferred enumeration. For those who are interested in the details,
we discuss the descriptive-set-theoretic aspects of our constructions in Section 7.
2.2. Descriptive set theory. We are interested in certain types of non-Borel
subsets of G. The following definitions and theorems are all fundamental in de-
scriptive set theory; a standard reference is [11].
Definition 2.1. Let X,Y be uncountable standard Borel spaces. Then A⊆Y is
analytic(denotedΣ1)ifthereisaBorelsetB ⊆X×Y suchthatproj (B)=A.
1 Y
A set C ⊆Y is co-analytic (denoted Π1) if Y \C is analytic.
1
Every Borel set is analytic, but any uncountable standard Borel space contains
non-Borel analytic sets. It follows that there are non-Borel co-analytic sets. The
collectionofanalyticsetsisclosedundercountableunions,countableintersections,
and Borel preimages. It follows the collection of co-analytic sets is closed under
countable unions, countable intersections, and Borel preimages. We remark that
sets defined using a single existential quantifier which ranges over an uncountable
standard Borel space are often analytic as such quantification can typically be
rewritten as a projection of a Borel set. Thus sets defined by using a universal
quantifier over an uncountable set are often co-analytic.
Definition 2.2. Let X,Y be standard Borel spaces, and A ⊆ X, B ⊆ Y. We
say that A Borel reduces to B if there is a Borel map f: X → Y such that
f−1(B)=A.
If A Borel reduces to B and B is Borel, analytic, or co-analytic, then so is A.
This gives us a method for proving that sets are, for example, co-analytic simply
by showing they Borel reduce to a co-analytic set. One important example comes
from the space of (descriptive-set-theoretic) trees.
Definition 2.3. A set T ⊆ N<N of finite sequences of natural numbers is a tree
if it is closed under initial segments. A sequence x∈NN is a branch of T if for all
n ∈ N, x (cid:22) n ∈ T. For s ∈ T, T := {r ∈ N<N | s(cid:97)r ∈ T} where “(cid:97)” indicates
s
concatenation of finite sequences.
As with groups, we may identify X ⊆ N<N with an element f ∈ 2N<N. We
X
define
N<N
Tr :={x∈2 |x is a tree }.
CHAIN CONDITIONS AND DESCRIPTIVE SET THEORY 5
ThesetTrisaclosedsubsetof2N<N andsoisacompactPolishspace. Asub-basis
for the topology on Tr is given by sets of the form
O :={T ∈Tr |t∈T},
t
where t∈N<N along with their complements.
There are two subsets of Tr of particular interest to us:
IF :={T ∈Tr |T has a branch }
and WF :=Tr\IF. We call WF the set of well-founded trees and IF the set
of ill-founded trees. One can check that IF is analytic, so WF is co-analytic.
The importance of these sets comes from the following fact.
Theorem 2.4. [11,Theorem27.1] Every analytic set Borel reduces to IF. There-
fore, every co-analytic set Borel reduces to WF.
Thus a set A is co-analytic if and only if it Borel reduces to WF.
We are interested in WF for a second reason. Let ORD denote the class of
ordinals. For any T ∈ WF, we can define a function ρ : T → ORD inductively
T
as follows: If t ∈ T has no extensions in T, let ρ (t) = 0. Otherwise let ρ (t) =
T T
sup{ρ (s)+1|t(cid:40)s}. We may then define a rank function ρ: Tr →ORD by
T
(cid:40)
ρ (∅)+1, if T ∈WF
ρ(T)= T
ω , else.
1
For T = ∅, we define ρ(T) = 0. The function ρ is bounded above by ω , the first
1
uncountable ordinal. Furthermore, this rank function has a special property:
Definition 2.5. Let X be a standard Borel space and A ⊆ X. A function
φ: A → ORD is a Π1-rank if there are relations ≤Π, ≤Σ⊆ X ×X such that
1 φ φ
≤Π is co-analytic, ≤Σ is analytic, and for all y ∈A,
φ φ
x∈A∧φ(x)≤φ(y)⇔x≤Σ y
φ
⇔x≤Π y.
φ
Given any rank function on A, one may use it to define an order ≤ on A. The
φ
idea of the above definition is that if φ is a Π1-rank, then the initial segments of
1
≤ are Borel, and this is witnessed in a uniform way.
φ
Theorem 2.6. [11, Exercise 34.6] The function ρ: WF →ORD is a Π1-rank.
1
WemayusethisfacttocreateotherΠ1-ranksinaneasyway: LetX beastandard
1
Borel space. If A ⊆ X Borel reduces to WF via f, then the map x (cid:55)→ ρ(f(x)) is
a Π1-rank.
1
The most important fact about Π1-ranks for this paper is the following ([11,
1
Theorem 35.23]):
Theorem 2.7 (The Boundedness Theorem for Π1-ranks). Let X be a standard
1
Borel space, A⊆X co-analytic, and φ: A→ω a Π1-rank. Then
1 1
A is Borel ⇐⇒ sup{φ(x)|x∈A}<ω .
1
6 PHILLIPWESOLEKANDJAYWILLIAMS
We will use the Boundedness Theorem to show that certain Π1 sets are not
1
Borel, by showing that they come with Π1-ranks with images unbounded below
1
ω . Tothisend,wewilloftenusethefollowingfactabouttheranksoftrees,which
1
follows immediately from the definition.
Lemma 2.8. Suppose S,T are trees and φ: S → T is a map such that s (cid:40) t ⇒
φ(s) (cid:40) φ(t). (We call such a map monotone.) Then ρ (s) ≤ ρ (φ(s)) for all
S T
s∈S. In particular ρ(S)≤ρ(T).
3. The minimal condition on centralizers
We wish to show certain chain conditions give rise to sets of marked groups
which are Π1 and not Borel in G. We begin by looking at the following chain
1
condition.
Definition 3.1. AsubgroupH ofGisacentralizerinGifH =C (A)forsome
G
A⊆G.
Definition 3.2. A group G satisfies the minimal condition on centralizers
if there is no strictly decreasing infinite chain C > C > ... of centralizers in
0 1
G. We denote the class of countable groups satisfying the minimal condition on
centralizers by M .
C
The class M is large, containing abelian groups, linear groups, and finitely
C
generatedabelian-by-nilpotentgroups;see[1]forfurtherdiscussion. Itisnothard
to check that a group G satisfies the minimal condition on centralizers if and only
if it satisfies the maximal condition on centralizers, but our analysis is easier if we
think about the minimal version of the chain condition.
Given a group G ∈ G, we construct a tree T ⊆ N<N and associated groups
G
G ∈G for each s∈T . Each G will be a centralizer in G.
s G s
• Put ∅∈T and let G :=G=C (∅).
G ∅ G
• Supposethats∈T andG =C ({g })hasalreadybeendefined. IfC ({g }∪
G s G s G s
{g })(cid:54)=C ({g }), then put s(cid:97)i∈T and G :=C ({g }∪{g }).
i G s G s(cid:97)i G s i
Lemma 3.3. The map Φ : G →Tr given by G(cid:55)→T is Borel.
C G
Intuitively,Lemma3.3holdssinceourconstructionisexplicit; wedelayarigorous
proof until Section 7.
Lemma 3.4. T is well-founded if and only if G∈M .
G C
Proof. If G ∈ M , then T contains no infinite branches by definition. If G ∈/
C G
M , then there is some infinite A ⊆ G such that for all finite B ⊆ A, C (A) (cid:54)=
C G
C (B). Let a < a < a < ... be such that A = {g ,g ,...}. By mov-
G 0 1 2 a0 a1
ing to a subsequence if necessary, we may assume that C ({g ,...,g }) (cid:13)
G a0 an
C ({g ,...,g }) for all n ∈ N. Then (a ,...,a ) ∈ T for all n ∈ N, so
G a0 an+1 0 n G
T has an infinite branch. (cid:3)
G
Lemma 3.5. Let H,G∈G. If H (cid:44)→G, then ρ(T )≤ρ(T ).
H G
CHAIN CONDITIONS AND DESCRIPTIVE SET THEORY 7
Proof. Let α: H (cid:44)→ G and let ψ: N → N be such that α(h ) = g . We now
k ψ(k)
define a map φ : T → N<N: Let φ(∅) = ∅. If s ∈ T and s = (s ,...,s ),
H H 0 n
let φ(s) = (ψ(s ),...,ψ(s )). Clearly φ is monotone. Further, if s ∈ T , then
0 n H
H = C ({h }) (cid:44)→ C ({g }). Since C ({g })∩α(H) ∼= C ({h }), we have
s H s G φ(s) G φ(s) H s
that CG({gφ(s(cid:22)k)})(cid:54)=CG({gφ(s(cid:22)(k+1))}) for all k <|s|. Thus φ(s)∈TG. It follows
φ(T )⊆T , and by Lemma 2.8, ρ(T )≤ρ(T ). (cid:3)
H G H G
Corollary 3.6. If G,G(cid:48) ∈G and G∼=G(cid:48), then ρ(T )=ρ(T ).
G G(cid:48)
We thus see that ρ(T ) is an isomorphism invariant, so it makes sense to talk
G
abouttherankofagroupGwiththeminimalconditiononcentralizers,evenwhen
not considering a specific marking.
Definition 3.7. If G has the minimal condition on centralizers, then ρ(T ) for
G
some (any) marking of G is called the centralizer rank of G.
We also mention that the above results, except for Lemma 3.3, work with
arbitrary enumerations of the group G, not just those that can arise from viewing
G as a marked group. Certain enumerations may be easier to use to calculate
ρ(T ), and Corollary 3.6 assures us that using these enumerations will not affect
G
the answer. The same will be true of our later constructions. Of course, in this
paper Lemma 3.3 and analogous results are of central importance, so we will
continue to work with groups as elements of G.
We now argue the centralizer rank is unbounded below ω .
1
Lemma 3.8. For A,B ∈ M with A nonabelian, A×B ∈ M and ρ(T ) <
C C B
ρ(T ).
A×B
Proof. It is easy to see A×B ∈M . Let a∈A be noncentral. Then
C
C ({(a,e)})=(A×B)
A×B i
for some i∈T since the centralizer is not all of A×B. Further,
A×B
B ∼={e}×B ≤C ({(a,e)}),
A×B
so by Lemma 3.5, ρ((T ) )=ρ(T )≥ρ(T ). The result now follows. (cid:3)
A×B i (A×B)i B
Lemma 3.9. Let {Ai}i∈N be countable groups. If Ai ∈ MC for all i ∈ N, then
there is a group A∈M such that ρ(T )≥ρ(T ) for all i∈N.
C A Ai
Proof. Let A = ∗i∈NAi. By [12, Corollary 4.1.6], which says that centralizers in
free products are cyclic or centralizers of a conjugate of a free factor, we infer
that A ∈ M . Lemma 3.5 now implies that ρ(T ) ≥ ρ(T ) for all i ∈ N, as
C A Ai
desired. (cid:3)
Lemma 3.10. For all α<ω , there is G∈M such that ρ(T )≥α.
1 C G
Proof. We prove this inductively. Clearly the lemma holds for α = 0. Suppose
α=β+1 and the lemma holds for β. Let G∈M be such that ρ(T )≥β and
C G
A∈M be nonabelian. Applying Lemma 3.8, we see ρ(T )≥β+1.
C A×G
Supposeαisalimitordinal. Sinceαiscountable,thereisacountableincreasing
sequence of α <α such that sup α =α. Let G ∈M be such that ρ(T )>
i i∈N i i C Gi
α . Applying Lemma 3.9, there is some G ∈ M such that ρ(T ) > α for all
i C G i
i∈N. It now follows that ρ(T )≥α. (cid:3)
G
8 PHILLIPWESOLEKANDJAYWILLIAMS
Lemma 3.11. For all α < ω , there is a finitely generated G ∈ M such that
1 C
ρ(T )≥α.
G
Proof. Let H ∈M be a group such that ρ(T )≥α. Then [10, Corollary on pg.
C H
949] implies that H embeds into a 3-generated group G ∈ M . By Lemma 3.5,
C
ρ(T )≥ρ(T )≥α verifying the lemma. (cid:3)
G H
We remark that the proof of the result cited in the previous uses nothing more
complicated than free products with amalgamation and is similar to the classical
Higman-Neumann-Neumann embedding result [9].
Theorem 3.12. M is Π1 and not Borel in G, and M ∩G is Π1 and not
C 1 C fg 1
Borel in G .
fg
Proof. Let Φ be the Borel map from Lemma 3.3. By Lemma 3.4, Φ−1(WF) =
C C
M ,andsinceΦ isBorel,M isΠ1. Lemma3.10impliestheranksofthetreesin
C C C 1
Φ (M )areunboundedbelowω ,sotheΠ1-rankonM givenbyG(cid:55)→ρ(Φ (G))
C C 1 1 C C
is unbounded below ω . By Theorem 2.7, we conclude that M is not Borel.
1 C
Lemma 3.11 implies the ranks of the trees in Φ (M ∩G ) are also unbounded
C C fg
below ω , and by Theorem 2.7, we conclude that M ∩G is also not Borel. (cid:3)
1 C fg
4. The maximal condition on subgroups
Wenextconsideramorebasicchaincondition. ProvingtheanalogueofLemma
3.9 in this context is more complicated, which is why we present it after the
previous section.
Definition 4.1. A group G satisfies the maximal condition on subgroups,
abbreviatedbysayingagroupsatisfiesmax, ifthereisnostrictlyincreasingchain
H < H < H < ... of subgroups of G. Equivalently, a group G satisfies max if
0 1 2
allofitssubgroupsarefinitelygenerated. Wedenotetheclassofgroupssatisfying
max as M .
max
Given a group G ∈ G, we construct a tree T ⊆ N<N and associated groups
G
G ∈G for each s∈T .
s G
• Put ∅∈T and let G :={e}.
G ∅
• Supposethats∈T andG =(cid:104){g }(cid:105)hasalreadybeendefined. If(cid:104){g }∪{g }(cid:105)=(cid:54)
G s s s i
(cid:104){g }(cid:105), then put s(cid:97)i∈T and G :=(cid:104){g }∪{g }(cid:105).
s G s(cid:97)i s i
Lemma 4.2. The map Φ : G →Tr given by G(cid:55)→T is Borel.
M G
We will prove Lemma 4.2 in Section 7.
Lemma 4.3. T is well-founded if and only if G∈M .
G max
Proof. If G ∈ M , then T contains no infinite branches by definition. If
max G
G ∈/ M , then there is some infinitely generated subgroup H ≤ G. There
max
is some increasing sequence a < a < ... of natural numbers such that H =
0 1
(cid:104)g ,g ,...(cid:105). We may assume that (cid:104)g ,...,g (cid:105)(cid:12)(cid:104)g ,...,g (cid:105) for all n∈N.
a0 a1 a0 an a0 an+1
Then (a ,...,a )∈T for all n∈N, so T has an infinite branch. (cid:3)
0 n G G
Lemma 4.4. Let H,G∈G. If H (cid:44)→G, then ρ(T )≤ρ(T ).
H G
CHAIN CONDITIONS AND DESCRIPTIVE SET THEORY 9
Proof. Let ψ: N → N be such that h = g . We now define a map φ : T →
k ψ(k) H
N<N: Letφ(∅)=∅. Ifs∈N<N ands=(s ,...,s ), letφ(s)=(ψ(s ),...,ψ(s )).
0 n 0 n
Clearly φ is monotone. Furthermore, if s∈T , then H ∼=G , hence φ(T )⊆
H s φ(s) H
T . Lemma 2.8 now implies ρ(T )≤ρ(T ). (cid:3)
G H G
The previous lemma implies ρ(T ) is a group invariant.
G
Corollary 4.5. If G,G(cid:48) ∈G and G∼=G(cid:48), then ρ(T )=ρ(T ).
G G(cid:48)
Definition 4.6. If G has the maximal condition on subgroups, then ρ(T ) for
G
some (any) marking of G is called the subgroup rank of G.
Lemma 4.7. For all groups G∈Mmax, G×Z∈Mmax and ρ(TG)<ρ(TG×Z).
Proof. It is easy to see G×Z satisfies max. For the latter condition, let G =
{g ,g ,...} and G×Z={a ,a ,...}. There is some k ∈N such that a =(e ,z)
0 1 0 1 k G
where Z = (cid:104)z(cid:105). Let ψ: N → N be defined such that aψ(m) = (gm,eZ). The map
φ:T →N<N givenby(s ,...,s )(cid:55)→(ψ(s ),...,ψ(s ))isclearlymonotone,and
G 0 n 0 n
further, φ(TG)⊆(TG×Z)k. By Lemma 2.8, ρ(TG)≤ρ((TG×Z)k)<ρ(TG×Z). (cid:3)
Lemma 4.8. Let {Ai}i∈N be countable groups. If Ai ∈ Mmax for each i ∈ N,
then there is a group A∈M such that ρ(T )≥ρ(T ) for all i∈N.
max A Ai
Proof. This is a consequence of [14, Theorem 2] due to A. Y. Olshanskii. This
resultgivesa2-generatedgroupAcontainingeachoftheA suchthateveryproper
i
subgroup of A is either contained in a conjugate of some A , is infinite cyclic, or
i
is infinite dihedral. Thus if every subgroup of each A is finitely generated, then
i
every subgroup of A is finitely generated, and so A ∈ M . Since each A is a
max i
subgroupofA,Lemma4.4impliesthatρ(T )≥ρ(T )foralli∈Nasdesired. (cid:3)
A Ai
Lemma 4.9. For all α<ω , there is G∈M such that ρ(T )≥α.
1 max G
Proof. The proof is the same as that of Lemma 3.10, with Lemmas 4.7 and 4.8
referenced at the appropriate places. (cid:3)
Theorem 4.10. M is Π1 and not Borel in G and G .
max 1 fg
Proof. The proof is the same as that of Theorem 3.12 using Lemmas 4.2 and 4.9
whereappropriate. ThestatementistrueforG simplybecauseM ⊆G . (cid:3)
fg max fg
5. The maximal condition on normal subgroups
GivenagroupGandasetS ⊆G,wewrite(cid:104)(cid:104)S(cid:105)(cid:105) todenotethenormalclosure
G
of S in G. We suppress the subscript G when the group is clear from context.
Definition 5.1. A group G satisfies the maximal condition on normal sub-
groups, abbreviated by saying a group satisfies max-n, if there is no infinite
strictly increasing chain of normal subgroups of G. Equivalently, a group G satis-
fies max-n if each of its normal subgroups is the normal closure of finitely many
elements of G. We denote the class of groups satisfying max-n as M .
n
Given G∈G, we construct a tree T ⊆N<N and associated groups G ∈G for
G s
each s∈T .
G
10 PHILLIPWESOLEKANDJAYWILLIAMS
• Put ∅∈T and let G :=G.
G ∅
• Supposethats∈T andG =G/(cid:104)(cid:104){g }(cid:105)(cid:105)hasalreadybeendefined. If(cid:104)(cid:104){g }∪
G s s s
{g }(cid:105)(cid:105)=(cid:54) (cid:104)(cid:104){g }(cid:105)(cid:105), then put s(cid:97)i∈T and G :=G/(cid:104)(cid:104){g }∪{g }(cid:105)(cid:105).
i s G s(cid:97)i s i
Lemma 5.2. The map Φ : G →Tr given by G(cid:55)→T is Borel.
Mn G
We prove Lemma 5.2 in Section 7.
Lemma 5.3. T is well-founded if and only if G∈M .
G n
Proof. IfG∈M ,thenT containsnoinfinitebranchesbydefinition. IfG∈/ M ,
n G n
then there is a normal subgroup N (cid:69)G such that N =(cid:104)(cid:104)g ,g ,...(cid:105)(cid:105) and
a0 a1
(cid:104)(cid:104)g ,...,g (cid:105)(cid:105)(cid:12)(cid:104)(cid:104)g ,...,g (cid:105)(cid:105)
a0 an a0 an+1
for all n ∈ N. Thus the sequence (a ,...,a ) is an element of T for any n ∈ N,
0 n G
so T has an infinite branch. (cid:3)
G
Lemma 5.4. If G ∈ M and f : G (cid:16) G(cid:48), then ρ(T ) ≥ ρ(T ) with equality if
n G G(cid:48)
and only if f is injective.
Proof. Since G is max-n, ker(f) = (cid:104)(cid:104)S(cid:105)(cid:105) for some finite S = {g ,...,g }. We
s0 sn
may assume that n is minimal, so no element of S is in the normal closure of the
others. Setting(s ,...,s )=:s∈N<N,theminimalityofS impliess∈T ;inthe
0 n G
caseker(f)={1},wetakes=∅. Letψ: N→Nbeamapsuchthatg(cid:48) =f(g ).
k ψ(k)
Then for all i ,...,i ∈N,
0 k
G(cid:48)/(cid:104)(cid:104)g(cid:48) ,...,g(cid:48) (cid:105)(cid:105) ∼=G/(cid:104)(cid:104)g ,...,g ,S(cid:105)(cid:105) ,
i0 ik G(cid:48) ψ(i0) ψ(ik) G
and the monotone map φ:T →N<N given by (r ,...,r )(cid:55)→(ψ(r ),...,ψ(r ))
G(cid:48) 0 n 0 n
sends T into (T ) . By Lemma 2.8, ρ(T ) ≤ ρ((T ) ) ≤ ρ(T ), and the right-
G(cid:48) G s G(cid:48) G s G
most inequality is strict if and only if s(cid:54)=∅. (cid:3)
We conclude this rank is also isomorphism invariant.
Corollary 5.5. If G,G(cid:48) ∈G and G∼=G(cid:48), then ρ(T )=ρ(T ).
G G(cid:48)
Recall that a group is hopfian if it is not isomorphic to any of its proper
quotients. The following corollary is easy enough to prove directly, but it follows
immediately from Lemma 5.4 and Corollary 5.5.
Corollary 5.6. If G∈M , then G is hopfian.
n
Unlike the previous invariants, this rank has appeared before in the literature;
cf. [3].
Definition 5.7. IfGhasthemaximalconditiononnormalsubgroups,thenρ(T )
G
for some (any) marking of G is called the length of G.
If we were to follow our template from previous sections, we would move on to
analogues of Lemmas 3.8 and 3.9. However, we were unable to prove an analogue
of Lemma 3.9 which would take advantage of Lemma 5.4. Such a result would be
a sort of dual version of the result of Olshanskii cited in the proof of Lemma 4.8.
Specifically, the following question is open to the best of the authors’ knowledge: