949 research outputs found
A Decidable Confluence Test for Cognitive Models in ACT-R
Computational cognitive modeling investigates human cognition by building
detailed computational models for cognitive processes. Adaptive Control of
Thought - Rational (ACT-R) is a rule-based cognitive architecture that offers a
widely employed framework to build such models. There is a sound and complete
embedding of ACT-R in Constraint Handling Rules (CHR). Therefore analysis
techniques from CHR can be used to reason about computational properties of
ACT-R models. For example, confluence is the property that a program yields the
same result for the same input regardless of the rules that are applied.
In ACT-R models, there are often cognitive processes that should always yield
the same result while others e.g. implement strategies to solve a problem that
could yield different results. In this paper, a decidable confluence criterion
for ACT-R is presented. It allows to identify ACT-R rules that are not
confluent. Thereby, the modeler can check if his model has the desired
behavior.
The sound and complete translation of ACT-R to CHR from prior work is used to
come up with a suitable invariant-based confluence criterion from the CHR
literature. Proper invariants for translated ACT-R models are identified and
proven to be decidable. The presented method coincides with confluence of the
original ACT-R models.Comment: To appear in Stefania Costantini, Enrico Franconi, William Van
Woensel, Roman Kontchakov, Fariba Sadri, and Dumitru Roman: "Proceedings of
RuleML+RR 2017". Springer LNC
Rings and spirals in barred galaxies. I Building blocks
In this paper we present building blocks which can explain the formation and
properties both of spirals and of inner and outer rings in barred galaxies. We
first briefly summarise the main results of the full theoretical description we
have given elsewhere, presenting them in a more physical way, aimed to an
understanding without the requirement of extended knowledge of dynamical
systems or of orbital structure. We introduce in this manner the notion of
manifolds, which can be thought of as tubes guiding the orbits. The dynamics of
these manifolds can govern the properties of spirals and of inner and outer
rings in barred galaxies. We find that the bar strength affects how unstable
the L1 and L2 Lagrangian points are, the motion within the 5A5A5Amanifold tubes
and the time necessary for particles in a manifold to make a complete turn
around the galactic centre. We also show that the strength of the bar, or, to
be more precise, of the non-axisymmetric forcing at and somewhat beyond the
corotation region, determines the resulting morphology. Thus, less strong bars
give rise to R1 rings or pseudorings, while stronger bars drive R2, R1R2 and
spiral morphologies. We examine the morphology as a function of the main
parameters of the bar and present descriptive two dimensional plots to that
avail. We also derive how the manifold morphologies and properties are modified
if the L1 and L2 Lagrangian points become stable. Finally, we discuss how
dissipation affects the manifold properties and compare the manifolds in
gas-like and in stellar cases. Comparison with observations, as well as clear
predictions to be tested by observations will be given in an accompanying
paper.Comment: Typos corrected to match the version in press in MNRA
Evaluation of a wave-vector-frequency-domain method for nonlinear wave propagation
A wave-vector-frequency-domain method is presented to describe one-directional forward or backward acoustic wave propagation in a nonlinear homogeneous medium. Starting from a frequency-domain representation of the second-order nonlinear acoustic wave equation, an implicit solution for
the nonlinear term is proposed by employing the Green’s function. Its approximation, which is more suitable for numerical implementation, is used. An error study is carried out to test the efficiency of the model by comparing the results with the Fubini solution. It is shown that the error grows as the propagation distance and step-size increase. However, for the specific case tested, even at a step size as large as one wavelength, sufficient accuracy for plane-wave propagation is observed. A two-dimensional steered transducer problem is explored to verify the nonlinear acoustic field directional independence
of the model. A three-dimensional single-element transducer problem is solved to verify the forward model by comparing it with an existing nonlinear wave propagation code. Finally, backward-projection behavior is examined. The sound field over a plane in an absorptive medium is backward
projected to the source and compared with the initial field, where good agreement is observed
EffectiveSan: Type and Memory Error Detection using Dynamically Typed C/C++
Low-level programming languages with weak/static type systems, such as C and
C++, are vulnerable to errors relating to the misuse of memory at runtime, such
as (sub-)object bounds overflows, (re)use-after-free, and type confusion. Such
errors account for many security and other undefined behavior bugs for programs
written in these languages. In this paper, we introduce the notion of
dynamically typed C/C++, which aims to detect such errors by dynamically
checking the "effective type" of each object before use at runtime. We also
present an implementation of dynamically typed C/C++ in the form of the
Effective Type Sanitizer (EffectiveSan). EffectiveSan enforces type and memory
safety using a combination of low-fat pointers, type meta data and type/bounds
check instrumentation. We evaluate EffectiveSan against the SPEC2006 benchmark
suite and the Firefox web browser, and detect several new type and memory
errors. We also show that EffectiveSan achieves high compatibility and
reasonable overheads for the given error coverage. Finally, we highlight that
EffectiveSan is one of only a few tools that can detect sub-object bounds
errors, and uses a novel approach (dynamic type checking) to do so.Comment: To appear in the Proceedings of 39th ACM SIGPLAN Conference on
Programming Language Design and Implementation (PLDI2018
Evaluation of the IMPACTS Computer Science Presentations
Recent computer science enrollments have shown positive trends. However, these trends are not evenly distributed by gender and race. Efforts to recruit underrepresented students should focus on providing information that demystifies the field of computer science. This paper reports on such an effort to inform underrepresented high school students about the field and its diversity. The results suggest that increasing awareness in an enjoyable format can increase student interest in pursuing computer science. These results can provide guidance about ways to encourage students to take high school computer science classes as motivation and preparation for college-level computer science
Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction
Superconductivity in iron selenides has experienced a rapid growth, but not
without major inconsistencies in the reported properties. For
alkali-intercalated iron selenides, even the structure of the superconducting
phase is a subject of debate, in part because the onset of superconductivity is
affected much more delicately by stoichiometry and preparation than in cuprate
or pnictide superconductors. If high-quality, pure, superconducting
intercalated iron selenides are ever to be made, the intertwined physics and
chemistry must be explained by systematic studies of how these materials form
and by and identifying the many coexisting phases. To that end, we prepared
pure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, and
examined differences in their structures by high-resolution synchrotron and
single-crystal x-ray diffraction. We found four distinct phases: semiconducting
K_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from
0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and an
oxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure upon
exposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 does
not become superconducting by doping, but the distinct iron-rich minority phase
K_xFe_2Se_2 precipitates from single crystals upon cooling from above the
vacancy ordering temperature. This coexistence of metallic and semiconducting
phases explains a broad maximum in resistivity around 100 K. Further studies to
understand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure will
shed light on the maximum fraction of superconducting K_xFe_2Se_2 that can be
obtained by solid state synthesis.Comment: 12 pages, 16 figures, supplemental materia
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