846 research outputs found
Mass problems and intuitionistic higher-order logic
In this paper we study a model of intuitionistic higher-order logic which we
call \emph{the Muchnik topos}. The Muchnik topos may be defined briefly as the
category of sheaves of sets over the topological space consisting of the Turing
degrees, where the Turing cones form a base for the topology. We note that our
Muchnik topos interpretation of intuitionistic mathematics is an extension of
the well known Kolmogorov/Muchnik interpretation of intuitionistic
propositional calculus via Muchnik degrees, i.e., mass problems under weak
reducibility. We introduce a new sheaf representation of the intuitionistic
real numbers, \emph{the Muchnik reals}, which are different from the Cauchy
reals and the Dedekind reals. Within the Muchnik topos we obtain a \emph{choice
principle} and a \emph{bounding principle} where range over Muchnik
reals, ranges over functions from Muchnik reals to Muchnik reals, and
is a formula not containing or . For the convenience of the
reader, we explain all of the essential background material on intuitionism,
sheaf theory, intuitionistic higher-order logic, Turing degrees, mass problems,
Muchnik degrees, and Kolmogorov's calculus of problems. We also provide an
English translation of Muchnik's 1963 paper on Muchnik degrees.Comment: 44 page
Generalized cohesiveness
We study some generalized notions of cohesiveness which arise naturally in
connection with effective versions of Ramsey's Theorem. An infinite set of
natural numbers is --cohesive (respectively, --r--cohesive) if is
almost homogeneous for every computably enumerable (respectively, computable)
--coloring of the --element sets of natural numbers. (Thus the
--cohesive and --r--cohesive sets coincide with the cohesive and
r--cohesive sets, respectively.) We consider the degrees of unsolvability and
arithmetical definability levels of --cohesive and --r--cohesive sets.
For example, we show that for all , there exists a
--cohesive set. We improve this result for by showing that there is
a --cohesive set. We show that the --cohesive and
--r--cohesive degrees together form a linear, non--collapsing hierarchy of
degrees for . In addition, for we characterize the jumps
of --cohesive degrees as exactly the degrees {\bf \geq \jump{0}{(n+1)}}
and show that each --r--cohesive degree has jump {\bf > \jump{0}{(n)}}
The Complexity of Orbits of Computably Enumerable Sets
The goal of this paper is to announce there is a single orbit of the c.e.
sets with inclusion, \E, such that the question of membership in this orbit
is -complete. This result and proof have a number of nice
corollaries: the Scott rank of \E is \wock +1; not all orbits are
elementarily definable; there is no arithmetic description of all orbits of
\E; for all finite , there is a properly
orbit (from the proof).
A few small corrections made in this versionComment: To appear in the Bulletion of Symbolic Logi
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