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For every natural number n we introduce a new weak choice principle $\mathrm {nRC_{fin}}$:
Given any infinite set x, there is an infinite subset$y\subseteq x$and a selection function f that chooses an n-element subset from every finite$z\subseteq y$containing at least n elements.
By constructing new permutation models built on a set of atoms obtained as Fraïssé limits, we will study the relation of $\mathrm {nRC_{fin}}$ to the weak choice principles $\mathrm {RC_m}$ (that has already been studied in [3] and [6]):
Given any infinite set x, there is an infinite subset$y\subseteq x$with a choice function f on the family of all m-element subsets of y.
Moreover, we prove a stronger analogue of the results in [6] when we study the relation between $\mathrm {nRC_{fin}}$ and $\mathrm {kC_{fin}^-}$ which is defined by
Given any infinite family$\mathcal {F}$of finite sets of cardinality greater than k, there is an infinite subfamily$\mathcal {A}\subseteq \mathcal {F}$with a selection function f that chooses a k-element subset from each$A\in \mathcal {A}$.
The chapter gives several consistency results related to the dWPHP problem. It also considers the hardness conjecture for feasibly infinite NP sets. It relates witnessing of dWPHP to various computational complexity conjectures.
For every $n\in \omega \setminus \{0,1\}$ we introduce the following weak choice principle:
$\operatorname {nC}_{<\aleph _0}^-:$For every infinite family$\mathcal {F}$of finite sets of size at least n there is an infinite subfamily$\mathcal {G}\subseteq \mathcal {F}$with a selection function$f:\mathcal {G}\to \left [\bigcup \mathcal {G}\right ]^n$such that$f(F)\in [F]^n$for all$F\in \mathcal {G}$.
Moreover, we consider the following choice principle:
$\operatorname {KWF}^-:$For every infinite family$\mathcal {F}$of finite sets of size at least$2$there is an infinite subfamily$\mathcal {G}\subseteq \mathcal {F}$with a Kinna–Wagner selection function. That is, there is a function$g\colon \mathcal {G}\to \mathcal {P}\left (\bigcup \mathcal {G}\right )$with$\emptyset \not =f(F)\subsetneq F$for every$F\in \mathcal {G}$.
We will discuss the relations between these two choice principles and their relations to other well-known weak choice principles. Moreover, we will discuss what happens when we replace $\mathcal {F}$ by a linearly ordered or a well-ordered family.
We show that MRP + MA implies that ITP(λ,ω2) holds for all cardinal λ ≥ ω2. This generalizes a result by Weiβ who showed that PFA implies that ITP(λ, ω2) holds for all cardinal λ ≥ ω2. Consequently any of the known methods to prove MRP + MA consistent relative to some large cardinal hypothesis requires the existence of a strongly compact cardinal. Moreover if one wants to force MRP + MA with a proper forcing, it requires at least a supercompact cardinal. We also study the relationship between MRP and some weak versions of square. We show that MRP implies the failure of □(λ, ω) for all λ ≥ ω2 and we give a direct proof that MRP + MA implies the failure of □(λ, ω1) for all λ ≥ ω2.
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