When querying incomplete databases, the possibility of optionally obtaining information -when availableis a distinctive feature. In particular, this feature is present in SPARQL, the language recommended by the W3C for querying Semantic Web data. Unfortunately, its implementation has been shown to have adverse consequences. Two of the most notable are an increase in evaluation complexity and th...

Theoretical foundations for querying databases based on bags are studied in this paper. We fully determine the strength of many polynomial-time bag operators relative to an ambient query language. Then we obtain BQL, a query language for bags, by picking the strongest combination of these operators. The relationship between the nested relational algebra and various fragments of BQL is investiga...

In this paper we address the question of the expressibility of discrete event speciied systems (DEVS), i.e., we characterize the subclass of dynamical systems which can be homomorphically represented by DEVS models. We show that causal dynamical systems with piecewise constant input and output segment spaces are DEVS-representable. Moreover, DEVS-representable dynamical systems are closed under...

Two graphs with adjacency matrices A and B are isomorphic if there exists a permutation matrix P for which the identity PAP = B holds. Multiplying through by P and relaxing the permutation matrix to a doubly stochastic matrix leads to the notion of fractional isomorphism. We show that the levels of the Sherali-Adams hierarchy of linear programming relaxations applied to fractional isomorphism i...

PURPOSE The aim of this study was to explore the relationship between chronic cigarette smoking and meibomian gland dysfunction (MGD). METHODS This study enrolled 322 smokers with MGD and 2067 non-smokers with MGD. All enrolled subjects were tested in the following sequence: Ocular Surface Disease Index (OSDI), tear film breakup time (TBUT), corneal fluorescein staining (CFS), Schirmer I test...

The aim of the study was to evaluate the morphological changes of meibomian glands (MGs) in primary blepharospasm (PBS) by in vivo laser scanning confocal microscopy (LSCM) and to investigate the correlations between clinical data of PBS and LSCM parameters of MGs. This prospective and case-control study recruited 30 consecutive PBS patients and 30 age- and gender-matched healthy controls. Afte...

Real-time systems are amongst the most safety critical systems involving computer software and the incorrect functioning of this software can cause great damage, up to and including the loss of life. If seems sensible therefore to write real-time software in a way that gives us the best chance of correctly implementing specifications. Because of the high level of functional programming language...

This paper continues our work on innnite, recursive structures. We investigate the descriptive complexity of several logics over recursive structures, including rst-order, second-order, and xpoint logic, exhibiting connections between expressibility of a property and its computational complexity. We then address 0{1 laws, proposing a version that applies to recursive structures, and using it to...

Consistent query answering is a standard approach for producing meaningful query answers when data is inconsistent. Recent work on consistent query answering in the presence of ontologies has shown this problem to be intractable in data complexity even for ontologies expressed in lightweight description logics. In order to better understand the source of this intractability, we investigate the ...