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Sarka Stejskalova

Publications and source records attributed to Sarka Stejskalova.

4 recordsLinked to original sources

Generalized cardinal invariants for an inaccessible $κ$ with compactness at $κ^{++}$

We show that if the existence of a supercompact cardinal $κ$ with a weakly compact cardinal $λ$ above $κ$ is consistent, then the following are consistent as well (where $\mathfrak{t}(κ)$ and $\mathfrak{u}(κ)$ are the tower number and the ultrafilter number, respectively): (i) There is an inaccessible cardinal $κ$ such that $κ^+ < \mathfrak{t}(κ)= \mathfrak{u}(κ)< 2^κ$ and $SR(κ^{++})$ hold, and (ii) There is an inaccessible cardinal $κ$ such that $κ^+ = \mathfrak{t}(κ) < \mathfrak{u}(κ)< 2^κ$ and $SR(κ^{++}), TP(κ^{++})$ and $\neg wKH(κ^+)$ hold. The cardinals $\mathfrak{u}(κ)$ and $2^κ$ can have any reasonable values in these models. We obtain these results by combining the forcing construction from Brooke-Taylor, Fischer, Friedman and Montoya with the Mitchell forcing and with (new and old) indestructibility results for compactness principles. Apart from $\mathfrak{u}(κ)$ and $\mathfrak{t}(κ)$ we also compute the values of $\mathfrak{b}(κ)$, $\mathfrak{d}(κ)$, $\mathfrak{s}(κ)$, $\mathfrak{r}(κ)$, $\mathfrak{a}(κ)$, $\mathrm{cov}(M_κ)$, $\mathrm{add}(M_κ)$, $\mathrm{non}(M_κ)$, $\mathrm{cof}(M_κ)$ which will all be equal to $\mathfrak{u}(κ)$. In (ii), we compute $\mathfrak{p}(κ) = \mathfrak{t}(κ) = κ^+$ by observing that the $κ^+$-distributive quotient of the Mitchell forcing adds a tower of size $κ^+$. Finally, we observe that (i) and (ii) hold also for the traditional invariants on $κ= ω$, using Mitchell forcing up to a weakly compact cardinal; in this case we also obtain the disjoint stationary sequence property $DSS(ω_2)$, which implies the negation of the approachability property $\neg AP(ω_2)$.

math.LO↗

Small u(kappa) at singular kappa with compactness at kappa++

We show that the tree property, stationary reflection and the failure of approachability at $κ^{++}$ are consistent with $\mathfrak{u}(κ) = κ^+ < 2^κ$, where $κ$ is a singular strong limit cardinal with the countable or uncountable cofinality. As a by-product, we show that if $λ$ is a regular cardinal, then stationary reflection at $λ^+$ is indestructible under all $λ$-cc forcings (out of general interest, we also state a related result for the preservation of club stationary reflection).

math.LO↗

Easton's theorem for the tree property below aleph_omega

Starting with infinitely many supercompact cardinals, we show that the tree property at every cardinal $\aleph_n$, $1 < n <ω$, is consistent with an arbitrary continuum function below $\aleph_ω$ which satisfies $2^{\aleph_n} > \aleph_{n+1}$, $n<ω$. Thus the tree property has no provable effect on the continuum function below $\aleph_ω$ except for the restriction that the tree property at $κ^{++}$ implies $2^κ>κ^+$ for every infinite $κ$.

math.LO↗

Indestructibility of the tree property

In the first part of the paper, we show that if $ω\le κ< λ$ are cardinals, $κ^{<κ} = κ$, and $λ$ is weakly compact, then in $V[\M(κ,λ)]$ the tree property at $λ= κ^{++V[\M(κ,λ)]}$ is indestructible under all $κ^+$-cc forcing notions which live in $V[\Add(κ,λ)]$, where $\Add(κ,λ)$ is the Cohen forcing for adding $λ$-many subsets of $κ$ and $\M(κ,λ)$ is the standard Mitchell forcing for obtaining the tree property at $λ= (κ^{++})^{V[\M(κ,λ)]}$. This result has direct applications to Prikry-type forcing notions and generalized cardinal invariants. In the second part, we assume that $λ$ is supercompact and generalize the construction and obtain a model $V^*$, a generic extension of $V$, in which the tree property at $(κ^{++})^{V^*}$ is indestructible under all $κ^+$-cc forcing notions living in $V[\Add(κ,λ)]$, and in addition by all forcing notions living in $V^*$ which are $κ^+$-closed and ``liftable'' in a prescribed sense (such as $κ^{++}$-directed closed forcings or well-met forcings which are $κ^{++}$-closed with the greatest lower bounds).

math.LO↗