Luzin and anti-Luzin almost disjoint families
Under MA_{omega_1} every uncountable almost disjoint family is either anti-Luzin or has an uncountable Luzin subfamily. This fails under CH. Related properties are also investigated.
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arXiv subjects
Lajos Soukup
Publications and source records attributed to Lajos Soukup.
Smooth graphs
A graph G on omega_1 is called <omega-smooth if for each uncountable subset W of omega_1, G is isomorphic to G[W-W'] for some finite W'. We show that in various models of ZFC if a graph G is <omega-smooth then G is necessarily trivial, i.e, either complete or empty. On the other hand, we prove that the existence of a non-trivial, <omega-smooth graph is also consistent with ZFC.
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Combinatorial principles from adding Cohen reals
In this paper we first formulate several ``combinatorial principles'' concerning kappa \times omega matrices of subsets of omega and prove that they are valid in the generic extension obtained by adding any number of Cohen reals to any ground model V, provided that the parameter kappa is an omega-inaccessible regular cardinal in V. Then we present a large number of applications of these principles, mainly to topology. Some of these consequences had been established earlier in generic extensions obtained by adding omega_2 Cohen reals to ground models satisfying CH, mostly for the case kappa=omega_2.
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Intersection properties of open sets, II.
A topological space is called P_2 ( P_3, P_{<omega} ) if and only if it does not contain two (three, finitely many) uncountable open sets with empty intersection. We show that (i) there are 0-dimensional P_{<omega} spaces of size 2^omega, (ii) there are compact P_{<omega} spaces of size omega_1, (iii) the existence of a Psi-like examples for a compact P_{<omega} space of size omega_1 is independent of ZFC, (iv) it is consistent that 2^omega is as large as you wish but every first countable (and so every compact) P_2 space has cardinality<=omega_1.
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More Set-theory around the weak Freese-Nation property
In this paper, we introduce a very weak square principle which is even weaker than the similar principle introduced by Foreman and Magidor. A characterization of this principle is given in term of sequences of elementary submodels of H(χ). This is used in turn to prove a characterization of kappa-Freese-Nation property under the very weak square principle and a weak variant of the Singular Cardinals Hypothesis. A typical application of this characterization shows that under 2^{\aleph_0}<\aleph_ωand our very weak square for \aleph_ω, the partial ordering [omega_ω]^{<ω} (ordered by inclusion) has the aleph_1-Freese-Nation property. On the other hand we show that, under Chang's Conjecture for \aleph_ωthe partial ordering above does not have the aleph_1-Freese-Nation property. Hence we obtain the independence of our characterization of the kappa-Freese-Nation property and also of the very weak square principle from ZFC.
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How to force a countably tight, initially ω_1-compact and non-compact space?
Improving a result of M. Rabus we force a normal, locally compact, 0-dimensional,Frechet-Uryson, initially omega_1-compact and non-compact space X of size omega_2 having the following property: for every open (or closed) set A in X we have |A|<omega_2 or |X\A|<omega_2.
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Forcing countable networks for spaces satisfying R(X^omega)=omega.
We show that all finite powers of a Hausdorff space X do not contain uncountable weakly separated subspaces iff there is a c.c.c poset P such that 1_P forces that ``X is a countable union of 0-dimensional subspaces of countable weight.'' We also show that this theorem is sharp in two different senses: (i) we can't get rid of using generic extensions, (ii) we have to consider all finite powers of $X$.
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On a theorem of Shapiro
We show that a theorem of Leonid B. Shapiro which was proved under MA, is actually independent from ZFC. We also give a direct proof of the Boolean algebra version of the theorem under MA(Cohen).
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On the number of non-isomorphic subgraphs
Let K be the family of graphs on omega_1 without cliques or independent subsets of size omega_1 . We prove that: 1) it is consistent with CH that every G in K has 2^{omega_1} many pairwise non-isomorphic subgraphs, 2) the following proposition holds in L: (*) there is a G in K such that for each partition (A,B) of omega_1 either G cong G[A] or G cong G[B], 3) the failure of (*) is consistent with ZFC.
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What makes a space have large weight?
We formulate several conditions (two of them are necessary and sufficient) which imply that a space of small character has large weight. In section 3 we construct a ZFC example of a first countable 0-dimensional space X of size 2^omega with w(X)=2^omega and nw(X)=omega, we show that CH implies the existence of a 0-dimensional space Y of size omega_1 with w(Y)=nw(Y)=omega_1 and chi(Y)=R(Y)=omega, and we prove that it is consistent that 2^omega is as large as you wish and there is a 0-dimensional space Z of size 2^omega such that w(Z)=nw(Z)=2^omega but chi(Z)=R(Z^omega)=omega.
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