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## Fundamenta Mathematicae

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## Monotone normality and nabla products

### Volume 254 / 2021

Fundamenta Mathematicae 254 (2021), 99-120 MSC: Primary 03E75, 54A35, 54B10, 54D15, 54D20; Secondary 54A25, 54B99, 54G20, 54G99. DOI: 10.4064/fm926-10-2020 Published online: 23 December 2020

#### Abstract

Roitman’s combinatorial principle $\Delta$ is equivalent to monotone normality of the nabla product, $\nabla (\omega +1)^\omega$. If $\{ X_n : n\in \omega \}$ is a family of metrizable spaces and $\nabla _n X_n$ is monotonically normal, then $\nabla _n X_n$ is hereditarily paracompact. Hence, if $\Delta$ holds then the box product $\square (\omega +1)^\omega$ is paracompact. Large fragments of $\Delta$ hold in $\mathsf {ZFC}$, yielding large subspaces of $\nabla (\omega +1)^\omega$ that are ‘really’ monotonically normal. Countable nabla products of metrizable spaces which are respectively: arbitrary, of size $\le \mathfrak {c}$, or separable, are monotonically normal under respectively: $\mathfrak {b}=\mathfrak {d}$, $\mathfrak {d}=\mathfrak {c}$ or the Model Hypothesis.

It is consistent and independent that $\nabla A(\omega _1)^\omega$ and $\nabla (\omega _1+1)^\omega$ are hereditarily normal (or hereditarily paracompact, or monotonically normal). In $\mathsf {ZFC}$ neither $\nabla A(\omega _2)^\omega$ nor $\nabla (\omega _2+1)^\omega$ is hereditarily normal.

#### Authors

• Hector A. Barriga-AcostaPosgrado Conjunto en Ciencias Matemáticas
UMSNH-UNAM
Morelia, Mexico
e-mail
• Paul M. GartsideDepartment of Mathematics
University of Pittsburgh
Pittsburgh, PA, U.S.A.
e-mail

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