On the Ranked Points of A $\Pi^0_1$ Set
Cenzer, Douglas ; Smith, Rick L.
J. Symbolic Logic, Tome 54 (1989) no. 1, p. 975-991 / Harvested from Project Euclid
This paper continues joint work of the authors with P. Clote, R. Soare and S. Wainer (Annals of Pure and Applied Logic, vol. 31 (1986), pp. 145--163). An element $x$ of the Cantor space $2^\omega$ is said have rank $\alpha$ in the closed set $P$ if $x$ is in $D^\alpha(P)\backslash D^{\alpha + 1}(P)$, where $D^\alpha$ is the iterated Cantor-Bendixson derivative. The rank of $x$ is defined to be the least $\alpha$ such that $x$ has rank $\alpha$ in some $\Pi^0_1$ set. The main result of the five-author paper is that for any recursive ordinal $\lambda + n$ (where $\lambda$ is a limit and $n$ is finite), there is a point with rank $\lambda + n$ which is Turing equivalent to $O^{(\lambda + 2n)}$. All ranked points constructed in that paper are $\Pi^0_2$ singletons. We now construct a ranked point which is not a $\Pi^0_2$ singleton. In the previous paper the points of high rank were also of high hyperarithmetic degree. We now construct $\triangle^0_2$ points with arbitrarily high rank. We also show that every nonrecursive RE point is Turing equivalent to an RE point of rank one and that every nonrecursive $\triangle^0_2$ point is Turing equivalent to a hyperimmune point of rank one. We relate Clote's notion of the height of a $\Pi^0_1$ singleton in the Baire space with the notion of rank. Finally, we show that every hyperimmune point $x$ is Turing equivalent to a point which is not ranked.
Publié le : 1989-09-14
Classification: 
@article{1183743032,
     author = {Cenzer, Douglas and Smith, Rick L.},
     title = {On the Ranked Points of A $\Pi^0\_1$ Set},
     journal = {J. Symbolic Logic},
     volume = {54},
     number = {1},
     year = {1989},
     pages = { 975-991},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1183743032}
}
Cenzer, Douglas; Smith, Rick L. On the Ranked Points of A $\Pi^0_1$ Set. J. Symbolic Logic, Tome 54 (1989) no. 1, pp.  975-991. http://gdmltest.u-ga.fr/item/1183743032/