7,067 research outputs found
Cascades: A view from Audience
Cascades on online networks have been a popular subject of study in the past
decade, and there is a considerable literature on phenomena such as diffusion
mechanisms, virality, cascade prediction, and peer network effects. However, a
basic question has received comparatively little attention: how desirable are
cascades on a social media platform from the point of view of users? While
versions of this question have been considered from the perspective of the
producers of cascades, any answer to this question must also take into account
the effect of cascades on their audience. In this work, we seek to fill this
gap by providing a consumer perspective of cascade.
Users on online networks play the dual role of producers and consumers.
First, we perform an empirical study of the interaction of Twitter users with
retweet cascades. We measure how often users observe retweets in their home
timeline, and observe a phenomenon that we term the "Impressions Paradox": the
share of impressions for cascades of size k decays much slower than frequency
of cascades of size k. Thus, the audience for cascades can be quite large even
for rare large cascades. We also measure audience engagement with retweet
cascades in comparison to non-retweeted content. Our results show that cascades
often rival or exceed organic content in engagement received per impression.
This result is perhaps surprising in that consumers didn't opt in to see tweets
from these authors. Furthermore, although cascading content is widely popular,
one would expect it to eventually reach parts of the audience that may not be
interested in the content. Motivated by our findings, we posit a theoretical
model that focuses on the effect of cascades on the audience. Our results on
this model highlight the balance between retweeting as a high-quality content
selection mechanism and the role of network users in filtering irrelevant
content
The glass transition and crystallization kinetic studies on BaNaB9O15 glasses
Transparent glasses of BaNaB9O15 (BNBO) were fabricated via the conventional
melt-quenching technique. The amorphous and the glassy nature of the
as-quenched samples were respectively, confirmed by X-ray powder diffraction
(XRD) and differential scanning calorimetry (DSC). The glass transition and
crystallization parameters were evaluated under non-isothermal conditions using
DSC. The correlation between the heating rate dependent glass transition and
the crystallization temperatures was discussed and deduced the Kauzmann
temperature for BNBO glass-plates and powdered samples. The values of the
Kauzmann temperature for the plates and powdered samples were 776 K and 768 K,
respectively. Approximation-free method was used to evaluate the
crystallization kinetic parameters for the BNBO glass samples. The effect of
the sample thickness on the crystallization kinetics of BNBO glasses was also
investigated.Comment: 23 pages, 12 figure
Evolution of excitation wavelength dependent photoluminescence in nano-CeO2 dispersed ferroelectric liquid crystals
The optical properties of nano-ceria (nano-CeO2) dispersed ferroelectric liquid crystals (FLCs) have been investigated by excitation wavelength dependent photoluminescence (PL) spectroscopy. The PL spectra of nano-ceria exhibited a strong excitation wavelength dependence in the 255-370 nm range. The red shift in the violet emission band of ceria i.e. from 368 nm to 396 nm with increasing excitation wavelength, has been attributed to the recombination of electrons trapped in the defect band and the deeply trapped holes in oxygen vacancies. This excitation wavelength dependence of ceria has noticeably been manifested in the PL response of FLC-CeO2 nanocomposites as well. PL emission recorded at an excitation wavelength where host and guest materials show intense emission, i.e. 340 nm, exhibits a quenching effect connected to the overlapping of emission and absorption bands of the host FLC and guest ceria NPs respectively. No blue/red shift in the spectral energy band was observed at 310 and 340 nm excitations. On the other hand, emission spectra at a lower excitation wavelength followed a reverse trend: an increase in the emission intensity, with a large blue shift in spectral energy band. The mechanisms involved in the changes of the PL spectrum of FLC-ceria nanocomposites with varying ceria concentration and excitation wavelengths are discussed in detail
Undetected locked-joint failures in kinematically redundant manipulators: a workspace analysis
Includes bibliographical references.Robots are frequently used for operations in hostile environments. The very nature of these environments, however, increases the likelihood of robot failures. Common failure tolerance techniques rely on effective failure detection. Since a failure may not always be successfully detected, or even if detected, may not be detected soon enough, it becomes important to consider the behavior of manipulators with undetected failures. This work focuses on developing techniques to analyze a manipulator's workspace and identify regions in which tasks, characterized by sequences of point-to-point moves, can be completed even with such failures. Measures of fault tolerance are formulated to allow for the evaluation of the workspace.This work was supported by Sandia National Labs under contract no. AL-3011 and by NSF under contract no. MIP-9708309
Analysis of the post-fault behavior of robotic manipulators, An
Includes bibliographical references.Operations in hazardous or remote environments are invariably performed by robots. The hostile nature of the environments, however, increase the likelihood of failures for robots used in such applications. The difficulty and delay in the detection and consequent correction of these faults makes the post-fault performance of the robots particularly important. This work investigates the behavior of robots experiencing undetected locked-joint failures in a general class of tasks characterized by point-to-point motion. The robot is considered to have "converged" to a task position and orientation if all its joints come to rest when the end-effector is at that position. It is seen that the post-fault behavior may be classified into three categories: 1) The robot converges to the task position; 2) the robot converges to a position other than the task position; or 3) the robot does not converge, but keeps moving forever. The specific conditions for convergence are identified, and the different behaviors illustrated with examples of simple planar manipulators.This work was supported by Sandia National Laboratories under contract number AL-3011
Fully-dynamic Approximation of Betweenness Centrality
Betweenness is a well-known centrality measure that ranks the nodes of a
network according to their participation in shortest paths. Since an exact
computation is prohibitive in large networks, several approximation algorithms
have been proposed. Besides that, recent years have seen the publication of
dynamic algorithms for efficient recomputation of betweenness in evolving
networks. In previous work we proposed the first semi-dynamic algorithms that
recompute an approximation of betweenness in connected graphs after batches of
edge insertions.
In this paper we propose the first fully-dynamic approximation algorithms
(for weighted and unweighted undirected graphs that need not to be connected)
with a provable guarantee on the maximum approximation error. The transfer to
fully-dynamic and disconnected graphs implies additional algorithmic problems
that could be of independent interest. In particular, we propose a new upper
bound on the vertex diameter for weighted undirected graphs. For both weighted
and unweighted graphs, we also propose the first fully-dynamic algorithms that
keep track of such upper bound. In addition, we extend our former algorithm for
semi-dynamic BFS to batches of both edge insertions and deletions.
Using approximation, our algorithms are the first to make in-memory
computation of betweenness in fully-dynamic networks with millions of edges
feasible. Our experiments show that they can achieve substantial speedups
compared to recomputation, up to several orders of magnitude
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