Article contents
WAYS OF DESTRUCTION
Published online by Cambridge University Press: 08 October 2021
Abstract
We study the following natural strong variant of destroying Borel ideals:
$\mathbb {P}$
$+$
-destroys
$\mathcal {I}$
if
$\mathbb {P}$
adds an
$\mathcal {I}$
-positive set which has finite intersection with every
$A\in \mathcal {I}\cap V$
. Also, we discuss the associated variants
$$ \begin{align*} \mathrm{non}^*(\mathcal{I},+)=&\min\big\{|\mathcal{Y}|:\mathcal{Y}\subseteq\mathcal{I}^+,\; \forall\;A\in\mathcal{I}\;\exists\;Y\in\mathcal{Y}\;|A\cap Y|<\omega\big\},\\ \mathrm{cov}^*(\mathcal{I},+)=&\min\big\{|\mathcal{C}|:\mathcal{C}\subseteq\mathcal{I},\; \forall\;Y\in\mathcal{I}^+\;\exists\;C\in\mathcal{C}\;|Y\cap C|=\omega\big\} \end{align*} $$
Among other results, (1) we give a simple combinatorial characterisation when a real forcing
$\mathbb {P}_I$
can
$+$
-destroy a Borel ideal
$\mathcal {J}$
; (2) we discuss many classical examples of Borel ideals, their
$+$
-destructibility, and cardinal invariants; (3) we show that the Mathias–Prikry,
$\mathbb {M}(\mathcal {I}^*)$
-generic real
$+$
-destroys
$\mathcal {I}$
iff
$\mathbb {M}(\mathcal {I}^*)\ +$
-destroys
$\mathcal {I}$
iff
$\mathcal {I}$
can be
$+$
-destroyed iff
$\mathrm {cov}^*(\mathcal {I},+)>\omega $
; (4) we characterise when the Laver–Prikry,
$\mathbb {L}(\mathcal {I}^*)$
-generic real
$+$
-destroys
$\mathcal {I}$
, and in the case of P-ideals, when exactly
$\mathbb {L}(\mathcal {I}^*)$
$+$
-destroys
$\mathcal {I}$
; and (5) we briefly discuss an even stronger form of destroying ideals closely related to the additivity of the null ideal.
Keywords
MSC classification
- Type
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- Information
- Copyright
- © The Author(s), 2021. Published by Cambridge University Press on behalf of The Association for Symbolic Logic
References
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