Tukey Order, Calibres and the Rationals

Abstract

One partially ordered set, $Q$, is a Tukey quotient of another, $P$, denoted $P \geq_T Q$, if there is a map $\phi : P \to Q$ carrying cofinal sets of $P$ to cofinal sets of $Q$. Let $X$ be a space and denote by $\mathcal{K}(X)$ the set of compact subsets of $X$, ordered by inclusion. For certain separable metrizable spaces $M$, Tukey upper and lower bounds of $\mathcal{K}(M)$ are calculated. Results on invariants of $\mathcal{K}(M)$'s are deduced. The structure of all $\mathcal{K}(M)$'s under $\le_T$ is investigated. Particular emphasis is placed on the position of $\mathcal{K}(M)$ when $M$ is: completely metrizable, the rationals $\mathbb{Q}$, co-analytic or analytic.