25,491 research outputs found
Enumeration Reducibility in Closure Spaces with Applications to Logic and Algebra
In many instances in first order logic or computable algebra, classical
theorems show that many problems are undecidable for general structures, but
become decidable if some rigidity is imposed on the structure. For example, the
set of theorems in many finitely axiomatisable theories is nonrecursive, but
the set of theorems for any finitely axiomatisable complete theory is
recursive. Finitely presented groups might have an nonrecursive word problem,
but finitely presented simple groups have a recursive word problem. In this
article we introduce a topological framework based on closure spaces to show
that many of these proofs can be obtained in a similar setting. We will show in
particular that these statements can be generalized to cover arbitrary
structures, with no finite or recursive presentation/axiomatization. This
generalizes in particular work by Kuznetsov and others. Examples from first
order logic and symbolic dynamics will be discussed at length
Satisfaction classes in nonstandard models of first-order arithmetic
A satisfaction class is a set of nonstandard sentences respecting Tarski's
truth definition. We are mainly interested in full satisfaction classes, i.e.,
satisfaction classes which decides all nonstandard sentences. Kotlarski,
Krajewski and Lachlan proved in 1981 that a countable model of PA admits a
satisfaction class if and only if it is recursively saturated. A proof of this
fact is presented in detail in such a way that it is adaptable to a language
with function symbols. The idea that a satisfaction class can only see finitely
deep in a formula is extended to terms. The definition gives rise to new
notions of valuations of nonstandard terms; these are investigated. The notion
of a free satisfaction class is introduced, it is a satisfaction class free of
existential assumptions on nonstandard terms.
It is well known that pathologies arise in some satisfaction classes. Ideas
of how to remove those are presented in the last chapter. This is done mainly
by adding inference rules to M-logic. The consistency of many of these
extensions is left as an open question.Comment: Thesis for the degree of licentiate of philosophy, 74 pages, 4
figure
Unsolvability Cores in Classification Problems
Classification problems have been introduced by M. Ziegler as a
generalization of promise problems. In this paper we are concerned with
solvability and unsolvability questions with respect to a given set or language
family, especially with cores of unsolvability. We generalize the results about
unsolvability cores in promise problems to classification problems. Our main
results are a characterization of unsolvability cores via cohesiveness and
existence theorems for such cores in unsolvable classification problems. In
contrast to promise problems we have to strengthen the conditions to assert the
existence of such cores. In general unsolvable classification problems with
more than two components exist, which possess no cores, even if the set family
under consideration satisfies the assumptions which are necessary to prove the
existence of cores in unsolvable promise problems. But, if one of the
components is fixed we can use the results on unsolvability cores in promise
problems, to assert the existence of such cores in general. In this case we
speak of conditional classification problems and conditional cores. The
existence of conditional cores can be related to complexity cores. Using this
connection we can prove for language families, that conditional cores with
recursive components exist, provided that this family admits an uniform
solution for the word problem
A Second Step Towards Complexity-Theoretic Analogs of Rice's Theorem
Rice's Theorem states that every nontrivial language property of the
recursively enumerable sets is undecidable. Borchert and Stephan initiated the
search for complexity-theoretic analogs of Rice's Theorem. In particular, they
proved that every nontrivial counting property of circuits is UP-hard, and that
a number of closely related problems are SPP-hard.
The present paper studies whether their UP-hardness result itself can be
improved to SPP-hardness. We show that their UP-hardness result cannot be
strengthened to SPP-hardness unless unlikely complexity class containments
hold. Nonetheless, we prove that every P-constructibly bi-infinite counting
property of circuits is SPP-hard. We also raise their general lower bound from
unambiguous nondeterminism to constant-ambiguity nondeterminism.Comment: 14 pages. To appear in Theoretical Computer Scienc
On the isolated points in the space of groups
We investigate the isolated points in the space of finitely generated groups.
We give a workable characterization of isolated groups and study their
hereditary properties. Various examples of groups are shown to yield isolated
groups. We also discuss a connection between isolated groups and solvability of
the word problem.Comment: 30 pages, no figure. v2: minor changes, published version from March
200
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