论文标题

关于线性订单的凝聚力

On cohesive powers of linear orders

论文作者

Dimitrov, Rumen, Harizanov, Valentina, Morozov, Andrey, Shafer, Paul, Soskova, Alexandra A., Vatev, Stefan V.

论文摘要

可计算结构的凝聚力是超级能力的有效类似物,其中粘性集扮演了超滤器的作用。令$ω$,$ζ$和$η$表示自然数,整数和理性的相应订单类型时,当被认为是线性订单时。我们研究了可计算线性订单的凝聚力,并特别强调了$ω$的可计算副本。如果$ \ MATHCAL {L} $是$ω$的可计算副本,则可以计算在$ω$的情况下同构,则$ \ MATHCAL {l} $的每个凝聚力都具有订单型$ω+ω+ζη$。但是,有可计算的副本为$ω$,一定不计算在通常的呈现中,具有凝聚力不等于$ω+ζη$。例如,我们表明有一个可计算的$ω$具有订单类型$ω+η$的粘性功率的可计算副本。 Our most general result is that if $X \subseteq \mathbb{N} \setminus \{0\}$ is a Boolean combination of $Σ_2$ sets, thought of as a set of finite order-types, then there is a computable copy of $ω$ with a cohesive power of order-type $ω+ σ(X \cup \{ω+ ζη+ ω^*\})$,其中$σ(x \ cup \ {ω+ζη+ω^*\})$表示订单类型的$ x $和订单类型$ type $ω+ζη+ω+ω^*$。此外,如果$ x $是有限的且非空的,则有一个可计算的$ω$具有订单类型$ω+σ(x)$的凝聚力的可计算副本。

Cohesive powers of computable structures are effective analogs of ultrapowers, where cohesive sets play the role of ultrafilters. Let $ω$, $ζ$, and $η$ denote the respective order-types of the natural numbers, the integers, and the rationals when thought of as linear orders. We investigate the cohesive powers of computable linear orders, with special emphasis on computable copies of $ω$. If $\mathcal{L}$ is a computable copy of $ω$ that is computably isomorphic to the usual presentation of $ω$, then every cohesive power of $\mathcal{L}$ has order-type $ω+ ζη$. However, there are computable copies of $ω$, necessarily not computably isomorphic to the usual presentation, having cohesive powers not elementarily equivalent to $ω+ ζη$. For example, we show that there is a computable copy of $ω$ with a cohesive power of order-type $ω+ η$. Our most general result is that if $X \subseteq \mathbb{N} \setminus \{0\}$ is a Boolean combination of $Σ_2$ sets, thought of as a set of finite order-types, then there is a computable copy of $ω$ with a cohesive power of order-type $ω+ σ(X \cup \{ω+ ζη+ ω^*\})$, where $σ(X \cup \{ω+ ζη+ ω^*\})$ denotes the shuffle of the order-types in $X$ and the order-type $ω+ ζη+ ω^*$. Furthermore, if $X$ is finite and non-empty, then there is a computable copy of $ω$ with a cohesive power of order-type $ω+ σ(X)$.

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