107 research outputs found
Tight Bound for Sum of Heterogeneous Random Variables: Application to Chance Constrained Programming
We study a tight Bennett-type concentration inequality for sums of
heterogeneous and independent variables, defined as a one-dimensional
minimization. We show that this refinement, which outperforms the standard
known bounds, remains computationally tractable: we develop a polynomial-time
algorithm to compute confidence bounds, proved to terminate with an
epsilon-solution. From the proposed inequality, we deduce tight
distributionally robust bounds to Chance-Constrained Programming problems. To
illustrate the efficiency of our approach, we consider two use cases. First, we
study the chance-constrained binary knapsack problem and highlight the
efficiency of our cutting-plane approach by obtaining stronger solution than
classical inequalities (such as Chebyshev-Cantelli or Hoeffding). Second, we
deal with the Support Vector Machine problem, where the convex conservative
approximation we obtain improves the robustness of the separation hyperplane,
while staying computationally tractable
A Rank-Based Reward between a Principal and a Field of Agents: Application to Energy Savings
We consider a problem where a Principal aims to design a reward function to a
field of heterogeneous agents. In our setting, the agents compete with each
other through their rank within the population in order to obtain the best
reward. We first explicit the equilibrium for the mean-field game played by the
agents, and then characterize the optimal reward in the homogeneous setting.
For the general case of a heterogeneous population, we develop a numerical
approach, which is then applied to the specific case study of the market of
Energy Saving Certificates
Quadratic Regularization of Unit-Demand Envy-Free Pricing Problems and Application to Electricity Markets
We consider a profit-maximizing model for pricing contracts as an extension
of the unit-demand envy-free pricing problem: customers aim to choose a
contract maximizing their utility based on a reservation bill and multiple
price coefficients (attributes). A classical approach supposes that the
customers have deterministic utilities; then, the response of each customer is
highly sensitive to price since it concentrates on the best offer. A second
approach is to consider logit model to add a probabilistic behavior in the
customers' choices. To circumvent the intrinsic instability of the former and
the resolution difficulties of the latter, we introduce a quadratically
regularized model of customer's response, which leads to a quadratic program
under complementarity constraints (QPCC). This allows to robustify the
deterministic model, while keeping a strong geometrical structure. In
particular, we show that the customer's response is governed by a polyhedral
complex, in which every polyhedral cell determines a set of contracts which is
effectively chosen. Moreover, the deterministic model is recovered as a limit
case of the regularized one. We exploit these geometrical properties to develop
an efficient pivoting heuristic, which we compare with implicit or non-linear
methods from bilevel programming. These results are illustrated by an
application to the optimal pricing of electricity contracts on the French
market.Comment: 37 pages, 9 figures; adding a section on the pricing of electricity
contract
Ergodic control of a heterogeneous population and application to electricity pricing
We consider a control problem for a heterogeneous population composed of
customers able to switch at any time between different contracts, depending not
only on the tariff conditions but also on the characteristics of each
individual. A provider aims to maximize an average gain per time unit,
supposing that the population is of infinite size. This leads to an ergodic
control problem for a "mean-field" MDP in which the state space is a product of
simplices, and the population evolves according to a controlled linear
dynamics. By exploiting contraction properties of the dynamics in Hilbert's
projective metric, we show that the ergodic eigenproblem admits a solution.
This allows us to obtain optimal strategies, and to quantify the gap between
steady-state strategies and optimal ones. We illustrate this approach on
examples from electricity pricing, and show in particular that the optimal
policies may be cyclic-alternating between discount and profit taking stages
Spectroscopic measurement of the excitation spectrum on effectively curved spacetimes in a polaritonic fluid of light
Quantum fields in regions of extreme spacetime curvature give rise to a
wealth of effects, like Hawking radiation at the horizon of black holes. While
quantum field theory can only be studied theoretically in black holes, it can
be tested in controlled laboratory experiments. Typically, a fluid accelerating
from sub- to supersonic speed will create an effectively curved spacetime for
the acoustic field, with an apparent horizon where the speed of the fluid
equals the speed of sound. Here we create effective curved spacetimes with a
quantum fluid of light, with smooth and steep acoustic horizons and various
supersonic fluid speeds. We use a recently developed spectroscopy method to
measure the spectrum of acoustic excitations on these spacetimes, thus
observing negative energy modes in the supersonic regions. This demonstrates
the potential of quantum fluids of light for the study of field theories on
curved spacetimes.Comment: 5 pages, 3 figure
Spectrum of collective excitations of a quantum fluid of polaritons
We use a recently developed high-resolution coherent probe spectroscopy
method to investigate the dispersion of collective excitations of a polaritonic
quantum fluid. We measure the dispersion relation with high energy and
wavenumber resolution, which allows us to determine the speed of sound in the
fluid and to evidence the contribution of an excitonic reservoir. We report on
the generation of collective excitations at negative energies, on the ghost
branch of the dispersion curve. Precursors of dynamical instabilities are also
identified. Our methods open the way to the precise study of quantum
hydrodynamics of quantum fluids of light
Observation of the diffusive Nambu-Goldstone mode of a non-equilibrium phase transition
Second-order phase transitions are governed by spontaneous symmetry breaking,
which yield collective excitations with a gapless spectrum called
Nambu-Goldstone (NG) modes. While NG modes in conservative systems are
propagating excitations, non-equilibrium phase transitions have been predicted
to feature a diffusive NG mode. We present the first experimental evidence of a
diffusive NG mode in a non-equilibrium Bose-Einstein condensate of microcavity
polaritons. The NG mode is observed as a spectral narrowing in the
spectroscopic response of the condensate. Additionally, explicitly breaking the
symmetry causes the opening of a gap in the spectrum and the disappearance of
the NG mode. Our observations confirm the diffusive dynamics of the NG mode of
non-equilibrium phase transitions and establish a promising framework to
investigate fundamental questions in statistical mechanics.Comment: 8 pages + refs + appendix, 6 figure
Sensitivity and specifi city of HAT Sero-K-SeT, a rapid diagnostic test for serodiagnosis of sleeping sickness caused by Trypanosoma brucei gambiense: a case-control study
Background Human African trypanosomiasis (HAT) is a life-threatening infection aff ecting rural populations in sub-
Saharan Africa. Large-scale population screening by antibody detection with the Card Agglutination Test for
Trypanosomiasis (CATT)/Trypanosoma brucei (T b) gambiense helped reduce the number of reported cases of
gambiense HAT to fewer than 10 000 in 2011. Because low case numbers lead to decreased cost-eff ectiveness of such
active screening, we aimed to assess diagnostic accuracy of a rapid serodiagnostic test (HAT Sero-K-SeT) applicable in
primary health-care centres.
Methods In our case-control study, we assessed participants older than 11 years who presented for HAT Sero-K-SeT
and CATT/T b gambiense at primary care centres or to mobile teams (and existing patients with confi rmed disease
status at these centres) in Bandundu Province, DR Congo. We defi ned cases as patients with trypanosomes that had
been identifi ed in lymph node aspirate, blood, or cerebrospinal fl uid. During screening, we recruited controls without
previous history of HAT or detectable trypanosomes in blood or lymph who resided in the same area as the cases. We
assessed diagnostic accuracy of three antibody detection tests for gambiense HAT: HAT Sero-K-SeT and
CATT/T b gambiense (done with venous blood at the primary care centres) and immune trypanolysis (done with
plasma at the Institute of Tropical Medicine, Antwerp, Belgium).
Findings Between June 6, 2012, and Feb 25, 2013, we included 134 cases and 356 controls. HAT Sero-K-SeT had a
sensitivity of 0·985 (132 true positives, 95% CI 0·947–0·996) and a specifi city of 0·986 (351 true negatives, 0·968–0·994),
which did not diff er signifi cantly from CATT/T b gambiense (sensitivity 95% CI 0·955, 95% CI 0·906–0·979 [128 true
positives] and specifi city 0·972, 0·949–0·985 [346 true negatives]) or immune trypanolysis (sensitivity 0·985,
0·947–0·996 [132 true positives] and specifi city 0·980, 0·960–0·990 [349 true negatives]).
Interpretation The diagnostic accuracy of HAT Sero-K-SeT is adequate for T b gambiense antibody detection in local
health centres and could be used for active screening whenever a cold chain and electricity supply are unavailable and
CATT/T b gambiense cannot be done
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