588 research outputs found
On the Herbrand content of LK
We present a structural representation of the Herbrand content of LK-proofs
with cuts of complexity prenex Sigma-2/Pi-2. The representation takes the form
of a typed non-deterministic tree grammar of order 2 which generates a finite
language of first-order terms that appear in the Herbrand expansions obtained
through cut-elimination. In particular, for every Gentzen-style reduction
between LK-proofs we study the induced grammars and classify the cases in which
language equality and inclusion hold.Comment: In Proceedings CL&C 2016, arXiv:1606.0582
Pushdown Compression
The pressing need for eficient compression schemes for XML documents has
recently been focused on stack computation [6, 9], and in particular calls for
a formulation of information-lossless stack or pushdown compressors that allows
a formal analysis of their performance and a more ambitious use of the stack in
XML compression, where so far it is mainly connected to parsing mechanisms. In
this paper we introduce the model of pushdown compressor, based on pushdown
transducers that compute a single injective function while keeping the widest
generality regarding stack computation. The celebrated Lempel-Ziv algorithm
LZ78 [10] was introduced as a general purpose compression algorithm that
outperforms finite-state compressors on all sequences. We compare the
performance of the Lempel-Ziv algorithm with that of the pushdown compressors,
or compression algorithms that can be implemented with a pushdown transducer.
This comparison is made without any a priori assumption on the data's source
and considering the asymptotic compression ratio for infinite sequences. We
prove that Lempel-Ziv is incomparable with pushdown compressors
Singular and Plural Functions for Functional Logic Programming
Functional logic programming (FLP) languages use non-terminating and
non-confluent constructor systems (CS's) as programs in order to define
non-strict non-determi-nistic functions. Two semantic alternatives have been
usually considered for parameter passing with this kind of functions: call-time
choice and run-time choice. While the former is the standard choice of modern
FLP languages, the latter lacks some properties---mainly
compositionality---that have prevented its use in practical FLP systems.
Traditionally it has been considered that call-time choice induces a singular
denotational semantics, while run-time choice induces a plural semantics. We
have discovered that this latter identification is wrong when pattern matching
is involved, and thus we propose two novel compositional plural semantics for
CS's that are different from run-time choice.
We study the basic properties of our plural semantics---compositionality,
polarity, monotonicity for substitutions, and a restricted form of the bubbling
property for constructor systems---and the relation between them and to
previous proposals, concluding that these semantics form a hierarchy in the
sense of set inclusion of the set of computed values. We have also identified a
class of programs characterized by a syntactic criterion for which the proposed
plural semantics behave the same, and a program transformation that can be used
to simulate one of them by term rewriting. At the practical level, we study how
to use the expressive capabilities of these semantics for improving the
declarative flavour of programs. We also propose a language which combines
call-time choice and our plural semantics, that we have implemented in Maude.
The resulting interpreter is employed to test several significant examples
showing the capabilities of the combined semantics.
To appear in Theory and Practice of Logic Programming (TPLP)Comment: 53 pages, 5 figure
Project Presentation: Algorithmic Structuring and Compression of Proofs (ASCOP)
International audienceComputer-generated proofs are typically analytic, i.e. they essentially consist only of formulas which are present in the theorem that is shown. In contrast, mathematical proofs written by humans almost never are: they are highly structured due to the use of lemmas. The ASCOP-project aims at developing algorithms and software which structure and abbreviate analytic proofs by computing useful lemmas. These algorithms will be based on recent groundbreaking results establishing a new connection between proof theory and formal language theory. This connection allows the application of e cient algorithms based on formal grammars to structure and compress proofs
Average-Case Complexity
We survey the average-case complexity of problems in NP.
We discuss various notions of good-on-average algorithms, and present
completeness results due to Impagliazzo and Levin. Such completeness results
establish the fact that if a certain specific (but somewhat artificial) NP
problem is easy-on-average with respect to the uniform distribution, then all
problems in NP are easy-on-average with respect to all samplable distributions.
Applying the theory to natural distributional problems remain an outstanding
open question. We review some natural distributional problems whose
average-case complexity is of particular interest and that do not yet fit into
this theory.
A major open question whether the existence of hard-on-average problems in NP
can be based on the PNP assumption or on related worst-case assumptions.
We review negative results showing that certain proof techniques cannot prove
such a result. While the relation between worst-case and average-case
complexity for general NP problems remains open, there has been progress in
understanding the relation between different ``degrees'' of average-case
complexity. We discuss some of these ``hardness amplification'' results
Tree Grammars for the Elimination of Non-prenex Cuts
Recently a new connection between proof theory and formal language theory was introduced. It was shown that the operation of cut elimination for proofs with prenex Pi_1-cuts in classical first-order logic corresponds to computing the language of a particular type of tree grammars. The present paper extends this connection to arbitrary (i.e. non-prenex) cuts without quantifier alternations. The key to treating non-prenex cuts lies in using a new class of tree grammars, constraint grammars, which describe the relationship of the applicability of its productions by a propositional formula
- …