44 research outputs found
Natural Factors of the Medvedev Lattice Capturing IPC
Skvortsova showed that there is a factor of the Medvedev lattice which
captures intuitionistic propositional logic (IPC). However, her factor is
unnatural in the sense that it is constructed in an ad hoc manner. We present a
more natural example of such a factor. We also show that for every non-trivial
factor of the Medvedev lattice its theory is contained in Jankov's logic, the
deductive closure of IPC plus the weak law of the excluded middle. This answers
a question by Sorbi and Terwijn
The degree structure of Weihrauch-reducibility
We answer a question by Vasco Brattka and Guido Gherardi by proving that the
Weihrauch-lattice is not a Brouwer algebra. The computable Weihrauch-lattice is
also not a Heyting algebra, but the continuous Weihrauch-lattice is. We further
investigate the existence of infinite infima and suprema, as well as embeddings
of the Medvedev-degrees into the Weihrauch-degrees
Instance reducibility and Weihrauch degrees
We identify a notion of reducibility between predicates, called instance
reducibility, which commonly appears in reverse constructive mathematics. The
notion can be generally used to compare and classify various principles studied
in reverse constructive mathematics (formal Church's thesis, Brouwer's
Continuity principle and Fan theorem, Excluded middle, Limited principle,
Function choice, Markov's principle, etc.).
We show that the instance degrees form a frame, i.e., a complete lattice in
which finite infima distribute over set-indexed suprema. They turn out to be
equivalent to the frame of upper sets of truth values, ordered by the reverse
Smyth partial order. We study the overall structure of the lattice: the
subobject classifier embeds into the lattice in two different ways, one
monotone and the other antimonotone, and the -dense degrees
coincide with those that are reducible to the degree of Excluded middle.
We give an explicit formulation of instance degrees in a relative
realizability topos, and call these extended Weihrauch degrees, because in
Kleene-Vesley realizability the -dense modest instance degrees
correspond precisely to Weihrauch degrees. The extended degrees improve the
structure of Weihrauch degrees by equipping them with computable infima and
suprema, an implication, the ability to control access to parameters and
computation of results, and by generally widening the scope of Weihrauch
reducibility
Computability Theory
Computability is one of the fundamental notions of mathematics, trying to capture the effective content of mathematics. Starting from Gödel’s Incompleteness Theorem, it has now blossomed into a rich area with strong connections with other areas of mathematical logic as well as algebra and theoretical computer science
Use-Bounded Strong Reducibilities
We study the degree structures of the strong reducibilities and , as well as and . We show that any noncomputable c.e. set is part of a uniformly c.e. copy of (\BQ,\leq) in the c.e. cl-degrees within a single wtt-degree; that there exist uncountable chains in each of the degree structures in question; and that any countable partially-ordered set can be embedded into the cl-degrees, and any finite partially-ordered set can be embedded into the ibT-degrees. We also offer new proofs of results of Barmpalias and Lewis-Barmpalias concerning the non-existence of cl-maximal sets