TY - JOUR
T1 - Large strongly anti-Urysohn spaces exist
AU - Juhász, István
AU - Shelah, Saharon
AU - Soukup, Lajos
AU - Szentmiklóssy, Zoltán
N1 - Publisher Copyright:
© 2022 Elsevier B.V.
PY - 2023/1/1
Y1 - 2023/1/1
N2 - As defined in [3], a Hausdorff space is strongly anti-Urysohn (in short: SAU) if it has at least two non-isolated points and any two infinite closed subsets of it intersect. Our main result answers the two main questions of [3] by providing a ZFC construction of a locally countable SAU space of cardinality 2c. The construction hinges on the existence of 2c weak P-points in ω⁎, a very deep result of Ken Kunen. It remains open if SAU spaces of cardinality >2c could exist, while it was shown in [3] that 22c is an upper bound. Also, we do not know if crowded SAU spaces, i.e. ones without any isolated points, exist in ZFC but we obtained the following consistency results concerning such spaces. (1) It is consistent that c is as large as you wish and there is a locally countable and crowded SAU space of cardinality c+. (2) It is consistent that both c and 2c are as large as you wish and there is a crowded SAU space of cardinality 2c. (3) For any uncountable cardinal κ the following statements are equivalent: (i) κ=cof([κ]ω,⊆). (ii) There is a locally countable and crowded SAU space of size κ in the generic extension obtained by adding κ Cohen reals. (iii) There is a locally countable and countably compact T1-space of size κ in some CCC generic extension.
AB - As defined in [3], a Hausdorff space is strongly anti-Urysohn (in short: SAU) if it has at least two non-isolated points and any two infinite closed subsets of it intersect. Our main result answers the two main questions of [3] by providing a ZFC construction of a locally countable SAU space of cardinality 2c. The construction hinges on the existence of 2c weak P-points in ω⁎, a very deep result of Ken Kunen. It remains open if SAU spaces of cardinality >2c could exist, while it was shown in [3] that 22c is an upper bound. Also, we do not know if crowded SAU spaces, i.e. ones without any isolated points, exist in ZFC but we obtained the following consistency results concerning such spaces. (1) It is consistent that c is as large as you wish and there is a locally countable and crowded SAU space of cardinality c+. (2) It is consistent that both c and 2c are as large as you wish and there is a crowded SAU space of cardinality 2c. (3) For any uncountable cardinal κ the following statements are equivalent: (i) κ=cof([κ]ω,⊆). (ii) There is a locally countable and crowded SAU space of size κ in the generic extension obtained by adding κ Cohen reals. (iii) There is a locally countable and countably compact T1-space of size κ in some CCC generic extension.
KW - Cohen model
KW - Crowded
KW - Free sequence
KW - Hausdorff space
KW - Urysohn space
UR - http://www.scopus.com/inward/record.url?scp=85140306540&partnerID=8YFLogxK
U2 - 10.1016/j.topol.2022.108288
DO - 10.1016/j.topol.2022.108288
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AN - SCOPUS:85140306540
SN - 0166-8641
VL - 323
JO - Topology and its Applications
JF - Topology and its Applications
M1 - 108288
ER -