1,326 research outputs found
Optimal Crowdsourcing Contests
We study the design and approximation of optimal crowdsourcing contests.
Crowdsourcing contests can be modeled as all-pay auctions because entrants must
exert effort up-front to enter. Unlike all-pay auctions where a usual design
objective would be to maximize revenue, in crowdsourcing contests, the
principal only benefits from the submission with the highest quality. We give a
theory for optimal crowdsourcing contests that mirrors the theory of optimal
auction design: the optimal crowdsourcing contest is a virtual valuation
optimizer (the virtual valuation function depends on the distribution of
contestant skills and the number of contestants). We also compare crowdsourcing
contests with more conventional means of procurement. In this comparison,
crowdsourcing contests are relatively disadvantaged because the effort of
losing contestants is wasted. Nonetheless, we show that crowdsourcing contests
are 2-approximations to conventional methods for a large family of "regular"
distributions, and 4-approximations, otherwise.Comment: The paper has 17 pages and 1 figure. It is to appear in the
proceedings of ACM-SIAM Symposium on Discrete Algorithms 201
Auditing: Active Learning with Outcome-Dependent Query Costs
We propose a learning setting in which unlabeled data is free, and the cost
of a label depends on its value, which is not known in advance. We study binary
classification in an extreme case, where the algorithm only pays for negative
labels. Our motivation are applications such as fraud detection, in which
investigating an honest transaction should be avoided if possible. We term the
setting auditing, and consider the auditing complexity of an algorithm: the
number of negative labels the algorithm requires in order to learn a hypothesis
with low relative error. We design auditing algorithms for simple hypothesis
classes (thresholds and rectangles), and show that with these algorithms, the
auditing complexity can be significantly lower than the active label
complexity. We also discuss a general competitive approach for auditing and
possible modifications to the framework.Comment: Corrections in section
Time-reversal in dynamically-tuned zero-gap periodic systems
We show that short pulses propagating in zero-gap periodic systems can be
reversed with 100% efficiency by using weak non-adiabatic tuning of the wave
velocity at time-scales that can be much slower than the period. Unlike
previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses
with simple structures. Our scheme may thus open the way to time-reversal in a
variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter
Spatio-Temporal Low Count Processes with Application to Violent Crime Events
There is significant interest in being able to predict where crimes will
happen, for example to aid in the efficient tasking of police and other
protective measures. We aim to model both the temporal and spatial dependencies
often exhibited by violent crimes in order to make such predictions. The
temporal variation of crimes typically follows patterns familiar in time series
analysis, but the spatial patterns are irregular and do not vary smoothly
across the area. Instead we find that spatially disjoint regions exhibit
correlated crime patterns. It is this indeterminate inter-region correlation
structure along with the low-count, discrete nature of counts of serious crimes
that motivates our proposed forecasting tool. In particular, we propose to
model the crime counts in each region using an integer-valued first order
autoregressive process. We take a Bayesian nonparametric approach to flexibly
discover a clustering of these region-specific time series. We then describe
how to account for covariates within this framework. Both approaches adjust for
seasonality. We demonstrate our approach through an analysis of weekly reported
violent crimes in Washington, D.C. between 2001-2008. Our forecasts outperform
standard methods while additionally providing useful tools such as prediction
intervals
Properties of low-lying states in a diffusive quantum dot and Fock-space localization
Motivated by an experiment by Sivan et al. (Europhys. Lett. 25, 605 (1994))
and by subsequent theoretical work on localization in Fock space, we study
numerically a hierarchical model for a finite many-body system of Fermions
moving in a disordered potential and coupled by a two-body interaction. We
focus attention on the low-lying states close to the Fermi energy. Both the
spreading width and the participation number depend smoothly on excitation
energy. This behavior is in keeping with naive expectations and does not
display Anderson localization. We show that the model reproduces essential
features of the experiment by Sivan et al.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let
Energy Level Alignment at Molecule-Metal Interfaces from an Optimally-Tuned Range-Separated Hybrid Functional
The alignment of the frontier orbital energies of an adsorbed molecule with
the substrate Fermi level at metal-organic interfaces is a fundamental
observable of significant practical importance in nanoscience and beyond.
Typical density functional theory calculations, especially those using local
and semi-local functionals, often underestimate level alignment leading to
inaccurate electronic structure and charge transport properties. In this work,
we develop a new fully self-consistent predictive scheme to accurately compute
level alignment at certain classes of complex heterogeneous molecule-metal
interfaces based on optimally-tuned range-separated hybrid functionals.
Starting from a highly accurate description of the gas-phase electronic
structure, our method by construction captures important nonlocal surface
polarization effects via tuning of the long-range screened exchange in a
range-separated hybrid in a non-empirical and system-specific manner. We
implement this functional in a plane-wave code and apply it to several
physisorbed and chemisorbed molecule-metal interface systems. Our results are
in quantitative agreement with experiments, both the level alignment and work
function changes. Our approach constitutes a new practical scheme for accurate
and efficient calculations of the electronic structure of molecule-metal
interfaces.Comment: 15 pages, 8 figure
Quasiparticle Lifetime in a Finite System: A Non--Perturbative Approach
The problem of electron--electron lifetime in a quantum dot is studied beyond
perturbation theory by mapping it onto the problem of localization in the Fock
space. We identify two regimes, localized and delocalized, corresponding to
quasiparticle spectral peaks of zero and finite width, respectively. In the
localized regime, quasiparticle states are very close to single particle
excitations. In the delocalized state, each eigenstate is a superposition of
states with very different quasiparticle content. A transition between the two
regimes occurs at the energy , where is
the one particle level spacing, and is the dimensionless conductance. Near
this energy there is a broad critical region in which the states are
multifractal, and are not described by the Golden Rule.Comment: 13 pages, LaTeX, one figur
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