Carl Jockusch | Searcharxiv

arXiv2014

Asymptotic density and the Ershov hierarchy

We classify the asymptotic densities of the $Δ^0_2$ sets according to their level in the Ershov hierarchy. In particular, it is shown that for $n \geq 2$, a real $r \in [0,1]$ is the density of an $n$-c.e.\ set if and only if it is a difference of left-$Π_2^0$ reals. Further, we show that the densities of the $ω$-c.e.\ sets coincide with the densities of the $Δ^0_2$ sets, and there are $ω$-c.e.\ sets whose density is not the density of an $n$-c.e. set for any $n \in ω$.

math.LO↗

arXiv1997

Generalized cohesiveness

We study some generalized notions of cohesiveness which arise naturally in connection with effective versions of Ramsey's Theorem. An infinite set $A$ of natural numbers is $n$--cohesive (respectively, $n$--r--cohesive) if $A$ is almost homogeneous for every computably enumerable (respectively, computable) $2$--coloring of the $n$--element sets of natural numbers. (Thus the $1$--cohesive and $1$--r--cohesive sets coincide with the cohesive and r--cohesive sets, respectively.) We consider the degrees of unsolvability and arithmetical definability levels of $n$--cohesive and $n$--r--cohesive sets. For example, we show that for all $n \ge 2$, there exists a $Δ^0_{n+1}$ $n$--cohesive set. We improve this result for $n = 2$ by showing that there is a $Π^0_2$ $2$--cohesive set. We show that the $n$--cohesive and $n$--r--cohesive degrees together form a linear, non--collapsing hierarchy of degrees for $n \geq 2$. In addition, for $n \geq 2$ we characterize the jumps of $n$--cohesive degrees as exactly the degrees ${\bf \geq \jump{0}{(n+1)}}$ and show that each $n$--r--cohesive degree has jump ${\bf > \jump{0}{(n)}}$.

math.LO↗