Generalized Cohesiveness
Hummel, Tamara ; Jockusch, Carl G.
J. Symbolic Logic, Tome 64 (1999) no. 1, p. 489-516 / Harvested from Project Euclid
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 $n \geq 2$, there exists a $\Delta^0_{n+1}$ n-cohesive set. We improve this result for n = 2 by showing that there is a $\Pi^0_2$ 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 $\geq 0^{(n+1)}$ and also characterize the jumps of the n-r-cohesive degrees.
Publié le : 1999-06-14
Classification: 
@article{1183745791,
     author = {Hummel, Tamara and Jockusch, Carl G.},
     title = {Generalized Cohesiveness},
     journal = {J. Symbolic Logic},
     volume = {64},
     number = {1},
     year = {1999},
     pages = { 489-516},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1183745791}
}
Hummel, Tamara; Jockusch, Carl G. Generalized Cohesiveness. J. Symbolic Logic, Tome 64 (1999) no. 1, pp.  489-516. http://gdmltest.u-ga.fr/item/1183745791/