321 research outputs found
Using SPARQL – the practitioners’ viewpoint
A number of studies have analyzed SPARQL log data to draw conclusions about how SPARQL is being used. To complement this work, a survey of SPARQL users has been undertaken. Whilst confirming some of the conclusions of the previous studies, the current work is able to provide additional insight into how users create SPARQL queries, the difficulties they encounter, and the features they would like to see included in the language. Based on this insight, a number of recommendations are presented to the community. These relate to predicting and avoiding computationally expensive queries; extensions to the language; and extending the search paradigm
Dynamical phase transition in one-dimensional kinetic Ising model with nonuniform coupling constants
An extension of the Kinetic Ising model with nonuniform coupling constants on
a one-dimensional lattice with boundaries is investigated, and the relaxation
of such a system towards its equilibrium is studied. Using a transfer matrix
method, it is shown that there are cases where the system exhibits a dynamical
phase transition. There may be two phases, the fast phase and the slow phase.
For some region of the parameter space, the relaxation time is independent of
the reaction rates at the boundaries. Changing continuously the reaction rates
at the boundaries, however, there is a point where the relaxation times begins
changing, as a continuous (nonconstant) function of the reaction rates at the
boundaries, so that at this point there is a jump in the derivative of the
relaxation time with respect to the reaction rates at the boundaries.Comment: 17 page
Spontaneous magnetization of the Ising model on the Sierpinski carpet fractal, a rigorous result
We give a rigorous proof of the existence of spontaneous magnetization at
finite temperature for the Ising spin model defined on the Sierpinski carpet
fractal. The theorem is inspired by the classical Peierls argument for the two
dimensional lattice. Therefore, this exact result proves the existence of
spontaneous magnetization for the Ising model in low dimensional structures,
i.e. structures with dimension smaller than 2.Comment: 14 pages, 8 figure
The random field ising model: algorithmic complexity and phase transition
The random field Ising model (RFIM) is investigated from the complexity point of view. We prove that finding a ground state of the ferromagnetic RFIM is a polynomial (P) optimization problem in any dimension d. A new rigidity algorithm for the search of the ground state morphology is also given. In contrast, the problem associated to the antiferromagnetic RFIM is shown to be an NP-complete optimization problem. The absence of any sensivity to d contrasts sharply with the known results previously obtained for the frustration model of spin glasses. Our results show, in particular, the absence of a simple one to one correspondence between finite Tc phase transition and NP-completeness properties in statistical mechanics models with competing interactions
Context-Free Path Queries on RDF Graphs
Navigational graph queries are an important class of queries that canextract
implicit binary relations over the nodes of input graphs. Most of the
navigational query languages used in the RDF community, e.g. property paths in
W3C SPARQL 1.1 and nested regular expressions in nSPARQL, are based on the
regular expressions. It is known that regular expressions have limited
expressivity; for instance, some natural queries, like same generation-queries,
are not expressible with regular expressions. To overcome this limitation, in
this paper, we present cfSPARQL, an extension of SPARQL query language equipped
with context-free grammars. The cfSPARQL language is strictly more expressive
than property paths and nested expressions. The additional expressivity can be
used for modelling graph similarities, graph summarization and ontology
alignment. Despite the increasing expressivity, we show that cfSPARQL still
enjoys a low computational complexity and can be evaluated efficiently.Comment: 25 page
On large deviation properties of Erdos-Renyi random graphs
We show that large deviation properties of Erd\"os-R\'enyi random graphs can
be derived from the free energy of the -state Potts model of statistical
mechanics. More precisely the Legendre transform of the Potts free energy with
respect to is related to the component generating function of the graph
ensemble. This generalizes the well-known mapping between typical properties of
random graphs and the limit of the Potts free energy. For
exponentially rare graphs we explicitly calculate the number of components, the
size of the giant component, the degree distributions inside and outside the
giant component, and the distribution of small component sizes. We also perform
numerical simulations which are in very good agreement with our analytical
work. Finally we demonstrate how the same results can be derived by studying
the evolution of random graphs under the insertion of new vertices and edges,
without recourse to the thermodynamics of the Potts model.Comment: 38 pages, 9 figures, Latex2e, corrected and extended version
including numerical simulation result
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