141 research outputs found
On effective sigma-boundedness and sigma-compactness
We prove several theorems on sigma-bounded and sigma-compact pointsets. We
start with a known theorem by Kechris, saying that any lightface \Sigma^1_1 set
of the Baire space either is effectively sigma-bounded (that is, covered by a
countable union of compact lightface \Delta^1_1 sets), or contains a
superperfect subset (and then the set is not sigma-bounded, of course). We add
different generalizations of this result, in particular, 1) such that the
boundedness property involved includes covering by compact sets and equivalence
classes of a given finite collection of lightface \Delta^1_1 equivalence
relations, 2) generalizations to lightface \Sigma^1_2 sets, 3) generalizations
true in the Solovay model.
As for effective sigma-compactness, we start with a theorem by Louveau,
saying that any lightface \Delta^1_1 set of the Baire space either is
effectively sigma-compact (that is, is equal to a countable union of compact
lightface \Delta^1_1 sets), or it contains a relatively closed superperfect
subset. Then we prove a generalization of this result to lightface \Sigma^1_1
sets.Comment: arXiv admin note: substantial text overlap with arXiv:1103.106
Non-uniformizable sets with countable cross-sections on a given level of the projective hierarchy
We present a model of set theory, in which, for a given , there exists
a non-ROD-uniformizable planar lightface set in , whose all vertical cross-sections are countable sets (and in
fact Vitali classes), while all planar boldface sets with
countable cross-sections are -uniformizable. Thus it is true
in this model, that the ROD-uniformization principle for sets with countable
cross-sections first fails precisely at a given projective level.Comment: A revised version of the originally submitted preprin
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