Thin ultrafilters, P-hierarchu and MArtin Axiom

0.13 MB·English

by  Michał Machura

Thin ultrafilters, P-hierarchy and Martin
Axiom
Micha l Machura and Andrzej Starosolski
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1
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January 10, 2012
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n
a
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Abstract

]
O Under MA we prove that for the ideal I of thin sets on ω and for
L any ordinal γ ≤ ω there is an I-ultrafilter (in the sense of Baum-
1
. gartner), which belongs to the class P of P-hierarchy of ultrafilters.
h γ
t Since the class of P ultrafilters coincides with a class of P-points,
a 2
m out result generalize theorem of Flaˇskov´a, which states that there are
I-ultrafilters which are not P-points. It is also related to theorem
[

which states that under CH for any tall P-ideal I on ω there is an
1
v I-ultrafilter, however the ideal of thin sets is not P-ideal.
5
2
7
1
1 Introduction
.
1
0
2 Baumgartner in the article Ultrafilters on ω ([1]) introduced a notion of I-
1
ultrafilters:
:
v
i
X A filter on ω is an I-ultrafilters, if and only if, for every function f ∈ ωω
r there is a set U ∈ u such that f[U] ∈ I.
a
This kind of ultrafilters was studied by large group of mathematician.
We shall mention only the most important papers in this subject from our
point of view: J. Brendle [2], C. Laflamme [15], Shelah ([19]). The theory
of I-ultrafilters was developted by Flaˇskova´ in a series of articles and in her
Ph.D thesis [8].
Key words: P-hierarchy, CH, P-points, monotone sequential contour; 2000 MSC:
54F65, 54C99
1
In this paper we restrict our attention to thin ultrafilters i.e. I-ultrafilters
where ideal I is ideal generated by thin sets (thin ideal):
A set A ⊂ ω with enumeration A = {a : n < ω} is called thin if
m
a
n
lim = 0.
n→∞ an+1
The thin ideal is an example of tall ideal but it is not P-ideal.
In[8]Flaˇskova´provedunderMartinAxiomthattherearethinultrafilters,
which are not a P-point.
Ultrafilters on ω may be classified with respect to sequential contours of
different ranks, that is, iterations of the Fr´echet filter by contour operations.
This wayanω -sequence {P } ofpairwisedisjoint classes ofultrafilters
1 α 1≤α≤ω1
- the P-hierarchy - is obtained, where P-points correspond to the class P ,
2
allowing us to look at the P-hierarchy as the extension of notion of P-point.
The following theorem was proved by Starosolski, see [21] Proposition 2.1:
Proposition 1.1. An ultrafilter u is a P-point if and only if u belongs to the
class P in P-hierarchy.
2
Many inmportant information about P-hierarchy may be found in [21].
For additional information regarding sequential cascades and contours one
can look at [6], [7], [5], [20]. However the most important definitions and
conventions shall be repeated below.
Since P-point correspond to P ultrafilter in P-hierarchy of ultrafilters
2
(more about P-hierarchy one can find below), it would interesting to know
to which classes of P-hierarchy can belong I-ultrafilters. In [17] it was shown
that under CH in every class P there are I-ultrafilters. In this paper we
α
improve thisresult replacing CHbyMAforthinultrafilters. Let usintroduce
all necessary definitions and tools.
The set of natural numbers (finite ordinal numbers) we denote ω. The
filter considered in this paper will be defined on infinite countable set (except
one indicated case). This will be usually a set maxV of maximal elements
of a cascade V (see definition of cascade below) and we will often identify it
with ω without indication. The following convention we be applied without
mentioning it:
If u is a filter on A ⊂ B, then we identify u with the filter on B for
which u is a filter-base. If F is a filter base, then by hFi we denote a filter
generated by F.
2
The cascade is a tree V without infinite branches and with a least element
∅ . A cascade is sequential if for each non-maximal element of V (v ∈
V
V \ maxV) the set v+V of immediate successors of v (in V) is countably
infinite. We write v+ instead of v+W if it is known in which cascade the
successors of v are considered. If v ∈ V \maxV, then the set v+ (as infinite)
may be endowed with an order of the type ω, and then by (v ) we denote
n n∈ω
the sequence of elements of v+, and by v - the n-th element of v+W.
nW
The rank of v ∈ V (r (v) or r(v)) is defined inductively as follows: r(v) =
V
0 if v ∈ maxV, and otherwise r(v) is the least ordinal greater than the
ranks of all immediate successors of v. The rank r(V) of the cascade V
is, by definition, the rank of ∅ . If it is possible to order all sets v+ (for
V
v ∈ V \ maxV) so that for each v ∈ V \ maxV the sequence (r(v ) ) is
n n<ω
non-decreasing, then the cascade V is monotone, and we fix such an order
on V without indication.
For v ∈ V we denote by v↑ or v↑V a subcascade of V built by v and all
successors of v.
If F = {F : s ∈ S} is a family of filters on X and if G is a filter on S,
s
then the contour of {F } along G is defined by
s
F = {F : s ∈ S} = F .
s s
Z Z
G G G[∈Gs\∈G
Such a construction has been used by many authors ([9], [10], [11]) and
is also known as a sum (or as a limit) of filters.
Operation of sum of filters we apply in definition of contour of cascade:
Fix a cascade V. Let G(v) be a filter on v+ for every v ∈ V \ maxV. For
v ∈ maxV let G(v) be a trivial ultrafilter on a singleton {v} (we can treat it
as principal ultrafilter on maxV according to convention we assumed). This
way we have defined a function v 7−→ G(v). Contour of each sub-cascade v↑
is defined inductively with respect to rank of v:
G
v↑ = {{v}}
Z
for v ∈ maxV (i.e. G v↑ is just a trivial ultrafilter on singleton {v}) ;
R
G G
v↑ = w↑ : w ∈ v+
Z Z (cid:26)Z (cid:27)
G(v)
3
for v ∈ V \maxv. Eventually we put
G G
V = ∅ .
V
Z Z
Usually we shall assume that all the filters G(v) are Frechet (for v ∈ V \
G
maxV). In that case we shall write V instead of V.
Similar filters were considered inR [12], [13], [4].R Let V be a monotone
sequential cascade and let u = V. Then a rank r(u) of u is, by definition,
the rank of V. R
It was shown in [7] that if V = W, then r(V) = r(W).
R R
Weshallsaythataset F meshes acontourV (F#V)ifandonlyifV∪{F}
has finite intersection property and can be extended to a filter. If ω\F ∈ V,
then we say that F is residual with respect to V .
Let us define P for 1 ≤ α < ω on βω (see [21]) as follows: u ∈ P if
α 1 α
there is no monotone sequential contour C of rank α such that C ⊂ u, and
α α
for each β in the range 1 ≤ β < α there exists a monotone sequential contour
C of rank β such that C ⊂ u. Moreover, if for each α < ω there exists a
β β 1
monotone sequential contour C of rank α such that C ⊂ u, then we write
α α
u ∈ P .
ω1
Let us consider a monotonecascade V anda monotonesequential cascade
W. We will say that W is a sequential extension of V if:
1) V is a subcascade of cascade W,
2) if v+V is infinite, then v+V = v+W,
3) r (v) = r (v) for each v ∈ V.
V W
Obviously, a monotone cascade may have many sequential extensions.
Notice that if W is a sequential extension of V and U ⊂ maxV, then U
is residual for V if and only if U is residual for W.
The following theorem was proved in [21] Theorem 2.8:
Theorem 1.2. The following statements are equivalent:
1. P-points exist,
2. P classes are non-empty for each countable successor α,
α
3. There exists a countable successor α > 1 such that the class P is
α
non-empty.
4
Starosolski has proved in [23] Theorem 2.6 that under CH every class P
α
is nonempty.
2 Lemmas
The following lemmas will be used in the prove of a main theorem.
The first lemma is one of lemmas proved in [?] (see: Lemma 6.3 ):
Lemma 2.1. Let α < ω be a limit ordinal and let (V : n < ω) be a sequence
1 n
of monotone sequential contours such that r(V ) < r(V ) < α for every n
n n+1
and that V has finite intersection property. Then there is no monotone
n<ω n
sequentiaSl contour W of rank α such that W ⊂ h n<ωVni.
S
As a corollary we get:
Lemma 2.2. Assume Martin Axiom. Let α < ω be a limit ordinal, let
1
(V ) be an increasing (”⊂”) sequence of monotone sequential contours,
n n<ω
such that r(V ) < α and let F be a family of sets of cardinality less than c
n
such that V ∪F has finite intersection property. Then h V ∪F)i
n<ω n n<ω n
does not cSontain any monotone sequential contour of rank α. S
Proof: One may assume that F is closed under finite intersection and
contains Frechet filter. We consider partially ordered set:
P = {hS,Ni : S ⊂ ω is finite, N ∈ F and maxS < minN}.
To simplyfize notion for p = hS,Ni ∈ P we put S = S and N = N. An
p p
order on P is given by formula:
q ≤ p ⇔ S ⊇ S , N ⊆ N and S \S ⊂ N .
q p q p q p p
For a given cascade V, finite set S and natural number k we define property
Φ(V,S,k). The definition is inductive: for any v ∈ maxV we put
Φ(v,V,S,k) ⇔ v ∈ S.
Next we define Φ(v,V,S,k) for v ∈ V \maxV:
Φ(v,V,S,k) ⇔ card{w ∈ v+ : Φ(w,V,S,k)} ≥ k.
Eventually we put Φ(V,S,k) = Φ(∅ ,V,S,k). Let V be cascades such that
V n
5
V = V , for any n. We define dense sets for every V and k < ω:
n n n
R
D = {p ∈ P : Φ(V ,S ,k)};
n,k n p
and for every F ∈ F:
D = {p ∈ P : N ⊂ F};
F p
Let G be a generic filter and let G = {S : p ∈ G}. It is easy to see that G
p
is infinite, G\F is finite for every F S∈ F and n<ωVn ∪F ∪{G} has finite
intersection property. S
Let W = {U ∩ G : U ∈ V } for every n. It is easy to see that W is
n n n
monotone sequential contour of the same rank as V . Consider a sequence
n
(W : n < ω). By Lemma 2.1 the union W do not contains contour
n n<ω n
of rank α. S
Similarly but easer one can prove:
Lemma 2.3. Assume Martin Axiom. Let α = δ+1 be a succesor ordinal, let
V be sequence of monotone sequential contour of rank r(V) = δ and let F be a
family of sets of cardinality less than c such that V∪F has finite intersection
property. Then hV ∪F)i does not contain any monotone sequential contour
of rank α.
3 Main result
In this section we shall present main result of the paper.
Theorem 3.1. Assume Martin Axiom. Let I be thin ideal, and let 0 < γ <
ω be an ordinal. Then there exists an I-ultrafilter u which belongs to P .
1 γ
Proof. We shall split proof into four cases: γ = 1, γ = 2, γ > 2 is a succesor
ordinal (the main step), γ is limit ordinal.
Step 0: γ = 1 -trivial.
Step 1: γ = 2. Let {W ,α < ω } be an enumeration of all monotone
α 1
sequential contours of rank 2. Let ωω = {f : α < ω }.
α 1
By transfinite induction, for α < ω we build filter bases F , such that:
1 α
1. F is a Frechet filter;
0
2. for each α < ω card(F ) = α·ω;
1 α
6
3. F ⊂ F for α < β;
α β
4. F = F for α limit ordinal;
α β<α β
S
5. for each α < ω there is F ∈ F such that f [F] ∈ I;
1 α+1 α
6. for each α < ω there is F ∈ F such that the complement of F
1 α+1
belongs to W .
α
Suppose that F is already define, we will show how to build F . First
α α+1
we shall add to F a set G which should take care on conditions 5. Next
α α
we will take care on condition 6 by adding set A to the list of generators of
α
F .
α+1
If there is U such that f [U] is finite and F ∪{U} has finite intersection
α α
property, then we put G = U and we are done. Assume the opposite. In
α
this case we shall use Martin’s Axiom. Consider a poset:
P = {K ⊂ ω : K is finine and [u,v]∩K = {u,v} ⇒ v > u2}.
For every F and k < ω we define a dense sets
D = {K : card(K ∩f [F]) ≥ k}.
F,k α
There is a generic filter G which intersect each D Let G = f−1( {K :
F,k α α
K ∈ G}). S
Notice that F ∪ {G } has finite intersection property and cardinality
α α
less than c. A subbase of any sequential contour of rank 2 has cardinality at
least d > ℵ , but d = c under MA, thus none of them one, in partucular W ,
0 α
is contained in F ∪{G }. This means that there is a set A such that its
α α α
complement belongs to W and a family F = F ∪ {G ,A } has finite
α α+1 α α α
intersection property.
Step 3: for limit γ. The proof in this case is base on the same idea as
step 1, but it is more sophisticated and technical.
Let (V ) be an increasing (”⊂”) sequence of monotone sequential con-
n n<ω
tours, such that their ranks r(V ) are smaller than γ but converging to γ.
n
For each n < ω denote by V a (fixed) monotone sequential cascade such that
n
V = V . Let {W ,α < c} be an enumeration of all monotone sequential
n n α
Rcontours of rank γ. Let ωω = {f : α < c}.
α
By transfinite induction, for α < c we build filter bases F , such that:
α
1. F is a Frechet filter;
0
7
2. for each α < c, card(F ) = α·ω;
α
3. F ⊂ F for α < β;
α β
4. F = F for α limit ordinal;
α β<α β
S
5. V ∪ F has finite intersection property;
i<ω i α<ω1 α
S S
6. for each α < ω there is F ∈ F such that f [F] ∈ I;
1 α+1 α
7. for each α < ω there is F ∈ F such that the complement of F
1 α+1
belongs to W .
α
Suppose that F is already define, we will show how to build F . This
α α+1
shall be done in 2 substeps. First step is to fint by one set G such that
α
V ∪F ∪{G } has finite intersection property and f [G ] ∈ I. For
m<ω n α α α α
Sthis purposewe useMartin’s Axiom. Theset Gα takecareonallthecontours
V . Adding it as generator to F will ensure preservation of conditions 5
n α+1
and 6. On the last step will take care on condition 7 by adding set A to the
α
list of generators of F .
α+1
Substep i) Let us introduce an axillary definition.
Definition: Fix a monotone sequential cascade V, a set F and a function
f ∈ ωω. For each v ∈ V, we write U ∈ S(v) if
1. U ⊂ maxv↑;
2. (U ∩F)# v↑;
R
3. card(f[U ∩F]) = 1.
We following two claims are crucial, for proof see [17] Proposition 3.2:
Proposition 3.2. One that one of the following possibilities holds:
A) S(∅ ) 6= ∅;
V
B) there is an antichain (with respect to the order of a cascade) A ⊂ V
such that:
1. S(v) = ∅ for all v ∈ A,
2. {maxw↑ : w ∈ v+,S(w) 6= ∅} # v↑ for all v ∈ A,
(cid:0)S (cid:1) R
3. {maxv↑ : v ∈ A} # V.
(cid:0)S (cid:1) R
8
For a given cascade V, base of a filter F and a function f ∈ ωω. Put
JV{v : card(f[maxv↑ ∩F]) = ω}.
F
If a cascade is fixed we write J instead of JV.
F F
Proposition 3.3. Let V be a cascade. Either for every F ∈ F
{maxv↑ : v ∈ J }# V
F
Z
[
either there is H such that f[H] finite and H# V.
R
Proof of the proposition:
Assume that for some F
¬( {maxv↑ : v ∈ J }# V)
F
Z
[
We use previous proposition lemma . There are two possibilities:
A) There is U such that (U ∩F)# V and f[U ∩F] is finite. Then put
H = U ∩F. R
B) There is an antichain A such that:
• {maxv↑ : v ∈ A}# V
S R
• {maxw↑ : w ∈ v+}# v↑ for any v ∈ A.
S R
• For every v ∈ A there is NO T
v
– T ⊂ maxv↑
v
– (T ∩F)# v↑
v
R
– f[T ∩F] finite
v
• For every v ∈ A and infinite many w ∈ v+ there is U
w
– U ⊂ maxw↑
w
– (U ∩F)# w↑
w
R
– f[U ∩F] finite
w
9
Put
T = U .
v w
w[∈v+
Observe that f[T ∩ F] has to be infinte. But (T ∩ F)# V Thus
v v∈A v
v∈Amaxv↑# V. On the other hand A ⊂ JF, soS v∈JF maxv↑#R V.
S ContradictiRon! S R
Fix cascades V such that V = V , a function f = f . There are two
n n n α
possibilities: R
Case A: There are F and infinite many n for which there is U such that
n
(U ∩F)# V and f[U ∩F] is finite. Without lost of genereality one may
n n n
assume thaRt for every n there is such U and by shrinking it that f[U ∩F]
n n
is one-point.
We put H = U ∩F and consider two subcases:
n n
If f[H ] is finite then put G = f−1[ f[H ]]. It is obvoius that
n n α n n
f[Gα]Sis thin as finite and nVn∪Fα∪{Gα} hSas finite intersection property.
S
Assume that f[H ] is infinite. Let f[H ] = {x }. We can select x
n n n n n
suchthattheyareSfarawayonefromeachotherandgetset{xn : n ∈ M} ∈ I.
Put G = f−1[ f[H ]] = f−1[{x : n ∈ M}]. It is easy to see that
α n∈M n n
nVn∪Fα∪{GSα} has finite intersection property, because the union nVn
Sis increasing. S
Case B: For every V and every F we have
n
{maxv↑ : v ∈ JVn}# V
F Z n
[
In the case B we shall use Martin’s Axiom. Consider again the poset:
P = {K ⊂ ω : K is finite and [u,v]∩K = {u,v} ⇒ v > u2},
ordered by inclusion.
For every V and F let AVn be an antichain defined as in proposition 3.2
n F
. For a given F and v ∈ AVn let
F
CVn,F = w ∈ v+Vn : (∃U )(∃x ) (U ∩F)# v↑Vn and f(U ∩F) = {x } .
v (cid:26) w w w Z w w (cid:27)
10