English

On effective sigma-boundedness and sigma-compactness

Logic 2018-08-16 v1

Abstract

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.

Keywords

Cite

@article{arxiv.1110.0919,
  title  = {On effective sigma-boundedness and sigma-compactness},
  author = {Vladimir Kanovei},
  journal= {arXiv preprint arXiv:1110.0919},
  year   = {2018}
}

Comments

arXiv admin note: substantial text overlap with arXiv:1103.1060

R2 v1 2026-06-21T19:15:22.515Z