61,786 research outputs found
On relating CTL to Datalog
CTL is the dominant temporal specification language in practice mainly due to
the fact that it admits model checking in linear time. Logic programming and
the database query language Datalog are often used as an implementation
platform for logic languages. In this paper we present the exact relation
between CTL and Datalog and moreover we build on this relation and known
efficient algorithms for CTL to obtain efficient algorithms for fragments of
stratified Datalog. The contributions of this paper are: a) We embed CTL into
STD which is a proper fragment of stratified Datalog. Moreover we show that STD
expresses exactly CTL -- we prove that by embedding STD into CTL. Both
embeddings are linear. b) CTL can also be embedded to fragments of Datalog
without negation. We define a fragment of Datalog with the successor build-in
predicate that we call TDS and we embed CTL into TDS in linear time. We build
on the above relations to answer open problems of stratified Datalog. We prove
that query evaluation is linear and that containment and satisfiability
problems are both decidable. The results presented in this paper are the first
for fragments of stratified Datalog that are more general than those containing
only unary EDBs.Comment: 34 pages, 1 figure (file .eps
Queries with Guarded Negation (full version)
A well-established and fundamental insight in database theory is that
negation (also known as complementation) tends to make queries difficult to
process and difficult to reason about. Many basic problems are decidable and
admit practical algorithms in the case of unions of conjunctive queries, but
become difficult or even undecidable when queries are allowed to contain
negation. Inspired by recent results in finite model theory, we consider a
restricted form of negation, guarded negation. We introduce a fragment of SQL,
called GN-SQL, as well as a fragment of Datalog with stratified negation,
called GN-Datalog, that allow only guarded negation, and we show that these
query languages are computationally well behaved, in terms of testing query
containment, query evaluation, open-world query answering, and boundedness.
GN-SQL and GN-Datalog subsume a number of well known query languages and
constraint languages, such as unions of conjunctive queries, monadic Datalog,
and frontier-guarded tgds. In addition, an analysis of standard benchmark
workloads shows that most usage of negation in SQL in practice is guarded
negation
Worldwide, Many See Belief in God as Essential to Morality
This report examines whether people think it is necessary to believe in God in order to be a moral person. The results are based on surveys in 40 countries taken by the Pew Research Center in Spring 2011, Spring 2013, and Winter 2013-2014
Groups with poly-context-free word problem
We consider the class of groups whose word problem is poly-context-free; that
is, an intersection of finitely many context-free languages. We show that any
group which is virtually a finitely generated subgroup of a direct product of
free groups has poly-context-free word problem, and conjecture that the
converse also holds. We prove our conjecture for several classes of soluble
groups, including metabelian groups and torsion-free soluble groups, and
present progress towards resolving the conjecture for soluble groups in
general. Some of the techniques introduced for proving languages not to be
poly-context-free may be of independent interest.Comment: 38 pages, no figure
Global Digital Communication: Texting, Social Networking Popular Worldwide
Presents survey findings about trends in the use of cell phone technologies and social networking sites in twenty-one countries by age, gender, and education and in relation to GDP per capita
A Global "No" to a Nuclear-Armed Iran
Presents survey findings from twenty-one countries about views on Iran's nuclear weapons program, economic sanctions, military action, and Mahmoud Ahmadinejad, as well as trends in favorability ratings
Global Publics Embrace Social Networking
Analyzes survey results on trends in the use of social networking sites, cell phone and computer ownership, and use of Internet and e-mail in twenty-two countries by age, gender, and education. Compares generational and gender gaps among countries
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