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On butterfly-points in β X , Tychonoff products and weak Lindelöf numbers

Sergei Logunov (2022)

Commentationes Mathematicae Universitatis Carolinae

Let X be the Tychonoff product α < τ X α of τ -many Tychonoff non-single point spaces X α . Let p X * be a point in the closure of some G X whose weak Lindelöf number is strictly less than the cofinality of τ . Then we show that β X { p } is not normal. Under some additional assumptions, p is a butterfly-point in β X . In particular, this is true if either X = ω τ or X = R τ and τ is infinite and not countably cofinal.

On certain porous sets in the Orlicz space of a locally compact group

Ibrahim Akbarbaglu, Saeid Maghsoudi (2012)

Colloquium Mathematicae

Let G be a locally compact group with a fixed left Haar measure. Given Young functions φ and ψ, we consider the Orlicz spaces L φ ( G ) and L ψ ( G ) on a non-unimodular group G, and, among other things, we prove that under mild conditions on φ and ψ, the set ( f , g ) L φ ( G ) × L ψ ( G ) : f * g is well defined on G is σ-c-lower porous in L φ ( G ) × L ψ ( G ) . This answers a question raised by Głąb and Strobin in 2010 in a more general setting of Orlicz spaces. We also prove a similar result for non-compact locally compact groups.

On certain types of convergences

Ján Borsík (1992)

Mathematica Bohemica

Mappings preserving Cauchy sequences and certain types of convergences connected with these mappings are investigated.

On character of points in the Higson corona of a metric space

Taras O. Banakh, Ostap Chervak, Lubomyr Zdomskyy (2013)

Commentationes Mathematicae Universitatis Carolinae

We prove that for an unbounded metric space X , the minimal character 𝗆 χ ( X ˇ ) of a point of the Higson corona X ˇ of X is equal to 𝔲 if X has asymptotically isolated balls and to max { 𝔲 , 𝔡 } otherwise. This implies that under 𝔲 < 𝔡 a metric space X of bounded geometry is coarsely equivalent to the Cantor macro-cube 2 < if and only if dim ( X ˇ ) = 0 and 𝗆 χ ( X ˇ ) = 𝔡 . This contrasts with a result of Protasov saying that under CH the coronas of any two asymptotically zero-dimensional unbounded metric separable spaces are homeomorphic.

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