2,229 research outputs found
Improved Second-Order Bounds for Prediction with Expert Advice
This work studies external regret in sequential prediction games with both
positive and negative payoffs. External regret measures the difference between
the payoff obtained by the forecasting strategy and the payoff of the best
action. In this setting, we derive new and sharper regret bounds for the
well-known exponentially weighted average forecaster and for a new forecaster
with a different multiplicative update rule. Our analysis has two main
advantages: first, no preliminary knowledge about the payoff sequence is
needed, not even its range; second, our bounds are expressed in terms of sums
of squared payoffs, replacing larger first-order quantities appearing in
previous bounds. In addition, our most refined bounds have the natural and
desirable property of being stable under rescalings and general translations of
the payoff sequence
Measurements of the CERN PS longitudinal resistive coupling impedance
The longitudinal coupling impedance of the CERN PS has been studied in the past years in order to better understand collective effects which could produce beam intensity limitations for the LHC Injectors Upgrade project. By measuring the incoherent quadrupole synchrotron frequency vs beam intensity, the inductive impedance was evaluated and compared with the impedance model obtained by taking into account the contribution of the most important machine devices. In this paper, we present the results of the measurements performed during a dedicated campaign, of the real part of the longitudinal coupling impedance by means of the synchronous phase shift vs beam intensity. The phase shift has been measured by using two different techniques: in one case, we injected in the machine two bunches, one used as a reference with constant intensity, and the second one changing its intensity; in the second case, more conventional, we measured the bunch position with respect to the RF signal of the 40 MHz cavities. The obtained dependence
of the synchrotron phase with intensity is then related to the loss factor and the resistive coupling impedance, which is compared to the real part of the PS impedance model
Non-Gaussianity of quantum states: an experimental test on single-photon added coherent states
Non Gaussian states and processes are useful resources in quantum information
with continuous variables. An experimentally accessible criterion has been
proposed to measure the degree of non Gaussianity of quantum states, based on
the conditional entropy of the state with a Gaussian reference. Here we adopt
such criterion to characterise an important class of non classical states,
single-photon added coherent states. Our studies demonstrate the reliability
and sensitivity of this measure, and use it to quantify how detrimental is the
role of experimental imperfections in our realisation
Local and Global Analytic Solutions for a Class of Characteristic Problems of the Einstein Vacuum Equations in the "Double Null Foliation Gauge"
The main goal of this work consists in showing that the analytic solutions
for a class of characteristic problems for the Einstein vacuum equations have
an existence region larger than the one provided by the Cauchy-Kowalevski
theorem due to the intrinsic hyperbolicity of the Einstein equations. To prove
this result we first describe a geometric way of writing the vacuum Einstein
equations for the characteristic problems we are considering, in a gauge
characterized by the introduction of a double null cone foliation of the
spacetime. Then we prove that the existence region for the analytic solutions
can be extended to a larger region which depends only on the validity of the
apriori estimates for the Weyl equations, associated to the "Bel-Robinson
norms". In particular if the initial data are sufficiently small we show that
the analytic solution is global. Before showing how to extend the existence
region we describe the same result in the case of the Burger equation, which,
even if much simpler, nevertheless requires analogous logical steps required
for the general proof. Due to length of this work, in this paper we mainly
concentrate on the definition of the gauge we use and on writing in a
"geometric" way the Einstein equations, then we show how the Cauchy-Kowalevski
theorem is adapted to the characteristic problem for the Einstein equations and
we describe how the existence region can be extended in the case of the Burger
equation. Finally we describe the structure of the extension proof in the case
of the Einstein equations. The technical parts of this last result is the
content of a second paper.Comment: 68 page
Federated Learning with Communication Delay in Edge Networks
Federated learning has received significant attention as a potential solution
for distributing machine learning (ML) model training through edge networks.
This work addresses an important consideration of federated learning at the
network edge: communication delays between the edge nodes and the aggregator. A
technique called FedDelAvg (federated delayed averaging) is developed, which
generalizes the standard federated averaging algorithm to incorporate a
weighting between the current local model and the delayed global model received
at each device during the synchronization step. Through theoretical analysis,
an upper bound is derived on the global model loss achieved by FedDelAvg, which
reveals a strong dependency of learning performance on the values of the
weighting and learning rate. Experimental results on a popular ML task indicate
significant improvements in terms of convergence speed when optimizing the
weighting scheme to account for delays.Comment: Accepted for publication at IEEE Global Communications Conference
(Globecom 2020
Ice-bridging frustration by self-ejection of single droplets results in superior anti-frosting surfaces
Surfaces capable of delaying the frosting passively and facilitating its
removal are highly desirable in fields where ice introduces inefficiencies and
risks. Coalescence jumping, enabled by highly hydrophobic surfaces, is already
exploited to slow down the frosting but it is insufficient to completely
eliminate the propagation by ice-bridging. We show how the self-ejection of
single condensation droplets can frustrate the ice bridges of all the
condensation droplets leading to a frost velocity lower than 0.5 um/s thus
dropping below the current limits of passive surfaces by a factor of at least
2. Arrays of truncated microcones, covered by uniformly hydrophobic
nanostructures, enable individual condensation droplets to growth and
self-propel towards the top of the microstructures and to self-eject once a
precise volume is reached. The independency of self-ejection on the neighbour
droplets allows a precise control on the droplets' size and distance
distributions and the ice-bridging frustration. The most performant
microstructures tend to cones with a sharp tip on which the percentage of
self-ejection is maximum. Looking towards applications, tapered microstructures
allow maximising the percentage of self-ejecting drops while maintaining a
certain mechanical strength. Further, it is shown that inserted pinning sites
are not essential, which greatly facilitates manufacturing
Late Epiphrenic-Retroperitoneal Fistula of an Esophageal Diverticulum Ten Years after Surgical Excision: Report of the First Case
Background: Esophageal fistula is a rare complication of an epiphrenic diverticulum and represents a difficult diagnostic challenge. In the majority of cases in the English literature, the fistula is secondary to the spontaneous perforation of the epiphrenic diverticulum and in only one case an esophagobronchial fistula occurs after resection of an epiphrenic diverticulum. No case of esophageal fistula communicating with the retroperitoneum tissues was found. Case Report: We present a case of 84 years old male admitted in 2017 in our Hospital for the appearance of epigastralgia and cough with traces of blood to sputum, asthenia and night sweats. Ten years before he underwent to a hiatal hernia plastic sec. Nissen and subsequently was reoperated for exeresis of diverticulum of the distal third of the esophagus by thoracotomy. In August 2008, because of esophageal substenosis an operation of posterior gastropexy sec. Hill with the demolition of the previous hiatoplasty was performed. In 2017, a computerized tomography shows a voluminous abscess in the right paravertebral region with development along the muscular plane of the ipsilateral psoas and a fistular path of about 4 cm between the posterior wall of the esophagus and the retroperitoneal collection of the abdomen. Conclusions: Esophageal fistula represents a complex problem of epiphrenic diverticulum and rarely a hidden complication of surgery. Surgery is the treatment of choice in suitable patients
The role of the internal heat gains for artificial lighting on the energy performance of buildings
This paper aims to propose a procedure for calculating the energy performance indexes of buildings considering the seasonality of internal gains due to artificial lighting with a monthly quasi-steady-state energy balance. The proposed methodology evaluates the heat gains due to the integrated natural-artificial lighting system with the Lighting Energy Numerical Indicator (LENI). For the evaluation of buildings’ global energy performance and for some energy services, this contribution cannot be considered constant annually as depend strongly by climate conditions. The effect of daylighting, type of light sourcesluminaires, building orientation and shading devices could influence lighting contribution of the internal heat gains. Then, the proposed methodology evaluates the internal heat gains with monthly energy balances. This methodology was applied to the case study of the "Brancaccio" retirement home in Matera (IT) for which the values of the energy performance indexes were compared with the standard normative approach using constant internal heat gains. The results of this work underline the importance of performing a detailed analysis that considers the availability of natural light in the different months of the year, the efficiency of the different lighting systems and their power installed per unit of area as a function of the lighting comfort requirements in the different types of environments
- …