2 Tamara Hummel And

نویسنده

  • CARL G. JOCKUSCH
چکیده

We study some generalized notions of cohesiveness which arise naturally in connection with eeective versions of Ram-sey's Theorem. An innnite 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 arith-metical deenability levels of n{cohesive and n{r{cohesive sets. For example, we show that for all n 2, there exists a 0 n+1 n{cohesive set. We improve this result for n = 2 by showing that there is a 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 2. In addition, for n 2 we characterize the jumps of n{cohesive degrees as exactly the degrees 0 (n+1) and show that each n{r{cohesive degree has jump > 0 (n) .

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تاریخ انتشار 1997