Answering Conjunctive Queries under Updates

We consider the task of enumerating and counting answers to $k$-ary conjunctive queries against relational databases that may be updated by inserting or deleting tuples. We exhibit a new notion of q-hierarchical conjunctive queries and show that these can be maintained efficiently in the following sense. During a linear time preprocessing phase, we can build a data structure that enables constant delay enumeration of the query results; and when the database is updated, we can update the data structure and restart the enumeration phase within constant time. For the special case of self-join free conjunctive queries we obtain a dichotomy: if a query is not q-hierarchical, then query enumeration with sublinear$^\ast$ delay and sublinear update time (and arbitrary preprocessing time) is impossible. For answering Boolean conjunctive queries and for the more general problem of counting the number of solutions of k-ary queries we obtain complete dichotomies: if the query's homomorphic core is q-hierarchical, then size of the the query result can be computed in linear time and maintained with constant update time. Otherwise, the size of the query result cannot be maintained with sublinear update time. All our lower bounds rely on the OMv-conjecture, a conjecture on the hardness of online matrix-vector multiplication that has recently emerged in the field of fine-grained complexity to characterise the hardness of dynamic problems. The lower bound for the counting problem additionally relies on the orthogonal vectors conjecture, which in turn is implied by the strong exponential time hypothesis. $^\ast)$ By sublinear we mean $O(n^{1-\varepsilon})$ for some $\varepsilon>0$, where $n$ is the size of the active domain of the current database

CT angiography, MR angiography and rotational digital subtraction angiography for volumetric assessment of intracranial aneurysms. An experimental study

The purpose of our experimental study was to assess the accuracy and precision of CT angiography (CTA), MR angiography (MRA) and rotational digital subtraction angiography (DSA) for measuring the volume of an in vitro aneurysm model. A rigid model of the anterior cerebral circulation harbouring an anterior communicating aneurysm was connected to a pulsatile circuit. It was studied using unenhanced 3D time-of-flight MRA, contrast-enhanced CTA and rotational DSA angiography. The source images were then postprocessed on dedicated workstations to calculate the volume of the aneurysm. CTA was more accurate than MRA (P=0.0019). Rotational DSA was more accurate than CTA, although the difference did not reach statistical significance (P=0.1605), and significantly more accurate than MRA (P<0.00001). CTA was more precise than MRA (P=0.12), although this did not reach statistical significance. Rotational DSA can be part of the diagnosis, treatment planning and support endovascular treatment of intracranial aneurysms. The emerging endovascular treatment techniques which consist of using liquid polymers as implants to exclude aneurysms from arterial circulation would certainly benefit from this precise measurement of the volume of aneurysm

Dynamical system representation of open address hash functions

This paper demonstrates how a broad collection of hash function families can be expressed as dynamical systems. We then show that this representation can be useful for analysis. In particular, we provide an analysis which proves that a widely-used family of double hash functions will transform any initial distribution of keys into the uniform distribution over the table space

A saddle coil system for TCV and RMP spectrum optimisation

A multi-purpose saddle coil system is proposed as part of a future upgrade of TCV (Tokamak à Configuration Variable). This system consists of 3 rows of 8 coils, each coil having independent power supplies, and provides simultaneously error field correction (EFC), resonant magnetic perturbation (RMP) and fast vertical control (VC). Other applications, like resistive wall mode (RWM) control and controlled plasma rotation, are also considered. Edge localized modes (ELM), related to the high confinement regime (H-mode), lead to a degradation of the plasma confinement and a release of energetic particles towards the vessel walls. Scaling the current experimental data to ITER predicts that the power flux related to ELMs will cause an intolerable erosion and heat load on the plasma facing components. Experiments on DIII-D and JET have demonstrated that the application of RMP is able to mitigate or suppress ELMs while keeping sufficient confinement properties. The description of the mechanism responsible for this phenomenon is still incomplete. The limits of the process, in terms of operation domain, are not yet accurately known. Experiments in different Tokamaks reveal opposite results for similar conditions. With that respect, TCV unique plasma shaping and positioning capability could extend the range of accessible magnetic perturbation modes for a given coil system geometry. Feeding the coil system with independent power supplies allows an optimisation of the coil current distribution, both toroidally and poloidally. A Lagrange method has been developed and appears to be an efficient way to minimize parasitic modes and current requirements while imposing the amplitude of a number of target modes. For example, the relative amplitude of edge modes can be increased at the cost of lower absolute amplitudes, demonstrating a degree of controllability on the localisation of the magnetic perturbation. In order to apply the Lagrange method, the characterization of the system in terms of spectral degeneracy is crucial to determine the set of simultaneously achievable target modes

Enhancement of the Binding Energy of Charged Excitons in Disordered Quantum Wires

Negatively and positively charged excitons are identified in the spatially-resolved photoluminescence spectra of quantum wires. We demonstrate that charged excitons are weakly localized in disordered quantum wires. As a consequence, the enhancement of the "binding energy" of a charged exciton is caused, for a significant part, by the recoil energy transferred to the remaining charged carrier during its radiative recombination. We discover that the Coulomb correlation energy is not the sole origin of the "binding energy", in contrast to charged excitons confined in quantum dots.Comment: 4 Fig

Understanding herbivore-plant-soil feedbacks to improve grazing management on Mediterranean mountain grasslands

The surface of many European mountain grasslands is decreasing due to global change and extensive grazing stands out as a key tool for their conservation. Sound knowledge of grassland ecosystem functioning and its feedback processes is required to implement sustainable grazing management. This study aimed to understand the effect of different grazing intensities on herbivore-plant-soil feedbacks in Mediterranean mountain grasslands. We estimated spatial distribution of sheep grazing intensity using GPS technology in order to assess the effect of grazing pressure on vegetation and soil properties measured throughout the study area. Our results showed that grazing intensity ranged from 0.06 to 2.85 livestock units / ha, corresponding to a gradient of pasture utilisation rates varying from 2.38% to 45.60% of annual productivity from pasture. Increasing grazing pressure was associated with smaller relative cover and species richness of non-leguminous forbs, while the opposite trends were observed for graminoids. Forage had a greater concentration of N and smaller C:N ratio in more heavily grazed areas. Increasing grazing intensity was also associated with higher values of total soil N, NO3-, NH4+, soil organic carbon, microbial biomass C and activity of ß-glucosidase. Higher litter quality was the main factor explaining greater content of soil organic matter, which favoured both soil microbes and plant productivity. Grazing induced changes in the plant community triggered positive hervibore-plant-soil feedbacks, as they ultimately improved forage quality and productivity, which significantly influenced the pasture preference of free-ranging domestic grazers. Our work showed that grazing management aiming pasture utilisation rates of around 45% is critical in sustaining positive herbivore-plant-soil feedbacks and preserving or enhancing the whole ecosystem functioning in the Mediterranean mountain grasslands studied. © 2021 The Author