1,617 research outputs found
On existential declarations of independence in IF Logic
We analyze the behaviour of declarations of independence between existential
quantifiers in quantifier prefixes of IF sentences; we give a syntactical
criterion for deciding whether a sentence beginning with such prefix exists
such that its truth values may be affected by removal of the declaration of
independence. We extend the result also to equilibrium semantics values for
undetermined IF sentences.
The main theorem allows us to describe the behaviour of various particular
classes of quantifier prefixes, and to prove as a remarkable corollary that all
existential IF sentences are equivalent to first-order sentences.
As a further consequence, we prove that the fragment of IF sentences with
knowledge memory has only first-order expressive power (up to truth
equivalence)
Topological Representation of Geometric Theories
Using Butz and Moerdijk's topological groupoid representation of a topos with
enough points, a `syntax-semantics' duality for geometric theories is
constructed. The emphasis is on a logical presentation, starting with a
description of the semantical topological groupoid of models and isomorphisms
of a theory and a direct proof that this groupoid represents its classifying
topos. Using this representation, a contravariant adjunction is constructed
between theories and topological groupoids. The restriction of this adjunction
yields a contravariant equivalence between theories with enough models and
semantical groupoids. Technically a variant of the syntax-semantics duality
constructed in [Awodey and Forssell, arXiv:1008.3145v1] for first-order logic,
the construction here works for arbitrary geometric theories and uses a slice
construction on the side of groupoids---reflecting the use of `indexed' models
in the representation theorem---which in several respects simplifies the
construction and allows for an intrinsic characterization of the semantic side.Comment: 32 pages. This is the first pre-print version, the final revised
version can be found at
http://onlinelibrary.wiley.com/doi/10.1002/malq.201100080/abstract (posting
of which is not allowed by Wiley). Changes in v2: updated comment
Logic Programming as Constructivism
The features of logic programming that
seem unconventional from the viewpoint of classical logic
can be explained in terms of constructivistic logic. We
motivate and propose a constructivistic proof theory of
non-Horn logic programming. Then, we apply this formalization
for establishing results of practical interest.
First, we show that 'stratification can be motivated in a
simple and intuitive way. Relying on similar motivations,
we introduce the larger classes of 'loosely stratified' and
'constructively consistent' programs. Second, we give a
formal basis for introducing quantifiers into queries and
logic programs by defining 'constructively domain
independent* formulas. Third, we extend the Generalized
Magic Sets procedure to loosely stratified and constructively
consistent programs, by relying on a 'conditional
fixpoini procedure
First-Order Logical Duality
From a logical point of view, Stone duality for Boolean algebras relates
theories in classical propositional logic and their collections of models. The
theories can be seen as presentations of Boolean algebras, and the collections
of models can be topologized in such a way that the theory can be recovered
from its space of models. The situation can be cast as a formal duality
relating two categories of syntax and semantics, mediated by homming into a
common dualizing object, in this case 2. In the present work, we generalize the
entire arrangement from propositional to first-order logic. Boolean algebras
are replaced by Boolean categories presented by theories in first-order logic,
and spaces of models are replaced by topological groupoids of models and their
isomorphisms. A duality between the resulting categories of syntax and
semantics, expressed first in the form of a contravariant adjunction, is
established by homming into a common dualizing object, now \Sets, regarded
once as a boolean category, and once as a groupoid equipped with an intrinsic
topology. The overall framework of our investigation is provided by topos
theory. Direct proofs of the main results are given, but the specialist will
recognize toposophical ideas in the background. Indeed, the duality between
syntax and semantics is really a manifestation of that between algebra and
geometry in the two directions of the geometric morphisms that lurk behind our
formal theory. Along the way, we construct the classifying topos of a decidable
coherent theory out of its groupoid of models via a simplified covering theorem
for coherent toposes.Comment: Final pre-print version. 62 page
Logic in the Tractatus
I present a reconstruction of the logical system of the Tractatus, which differs from classical logic in two ways. It includes an account of Wittgensteinâs âform-seriesâ device, which suffices to express some effectively generated countably infinite disjunctions. And its attendant notion of structure is relativized to the fixed underlying universe of what is named.
There follow three results. First, the class of concepts definable in the system is closed under finitary induction. Second, if the universe of objects is countably infinite, then the property of being a tautology is \Pi^1_1-complete. But third, it is only granted the assumption of countability that the class of tautologies is \Sigma_1-definable in set theory.
Wittgenstein famously urges that logical relationships must show themselves in the structure of signs. He also urges that the size of the universe cannot be prejudged. The results of this paper indicate that there is no single way in which logical relationships could be held to make themselves manifest in signs, which does not prejudge the number of objects
Conditionals and modularity in general logics
In this work in progress, we discuss independence and interpolation and
related topics for classical, modal, and non-monotonic logics
- âŠ