The BARISTA: A model for bid arrivals in online auctions

The arrival process of bidders and bids in online auctions is important for studying and modeling supply and demand in the online marketplace. A popular assumption in the online auction literature is that a Poisson bidder arrival process is a reasonable approximation. This approximation underlies theoretical derivations, statistical models and simulations used in field studies. However, when it comes to the bid arrivals, empirical research has shown that the process is far from Poisson, with early bidding and last-moment bids taking place. An additional feature that has been reported by various authors is an apparent self-similarity in the bid arrival process. Despite the wide evidence for the changing bidding intensities and the self-similarity, there has been no rigorous attempt at developing a model that adequately approximates bid arrivals and accounts for these features. The goal of this paper is to introduce a family of distributions that well-approximate the bid time distribution in hard-close auctions. We call this the BARISTA process (Bid ARrivals In STAges) because of its ability to generate different intensities at different stages. We describe the properties of this model, show how to simulate bid arrivals from it, and how to use it for estimation and inference. We illustrate its power and usefulness by fitting simulated and real data from eBay.com. Finally, we show how a Poisson bidder arrival process relates to a BARISTA bid arrival process.Comment: Published in at http://dx.doi.org/10.1214/07-AOAS117 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

Highly Accurate Determination of Heterogeneously Stacked Van-der-Waals Materials by Optical Microspectroscopy

The composition of Van-der-Waals heterostructures is conclusively determined using a hybrid evaluation scheme of data acquired by optical microspectroscopy. This scheme deploys a parameter set comprising both change in reflectance and wavelength shift of distinct extreme values in reflectance spectra. Furthermore, the method is supported by an accurate analytical model describing reflectance of multilayer systems acquired by optical microspectroscopy. This approach allows uniquely for discrimination of 2D materials like graphene and hBN and, thus, quantitative analysis of Van-der-Waals heterostructures containing structurally very similar materials. The physical model features a transfer matrix method which allows for flexible, modular description of complex optical systems and may easily be extended to individual setups. It accounts for numerical apertures of applied objective lenses and a glass fiber which guides the light into the spectrometer by two individual weighting functions. The scheme is proven by highly accurate quantification of the number of layers of graphene and hBN in Van-der-Waals heterostructures. In this exemplary case, the fingerprint of graphene involves distinct deviations of reflectance accompanied by additional wavelength shifts of extreme values. In contrast to graphene the fingerprint of hBN reveals a negligible deviation in absolute reflectance causing this material being only detectable by spectral shifts of extreme values.Comment: 12 pages, 4 figure

Effects of Selection on Morphological Characteristics in \u3ci\u3eSetaria sphacelata\u3c/i\u3e (Schumach.) Moss

Cattle production in Florida is limited by the lack of available warm-season forages adapted to the cool winters of the region. Setaria (S. sphacelata) has demonstrated green growth during cool conditions in subtropical climates around the world, and has the potential to fill this niche in cattle production in Florida. Four populations were selected in Gainesville and Ona, Florida: two cycles of selection for increased head number, one cycle for increased leaf width and one cycle for grazing tolerance. The objectives of this research were to compare these populations morphologically to determine changes due to selection. Selection for leaf width increased this trait while maintaining other morphological characteristics constant. Selection for head number decreased plant height and increased head number in each cycle, and decreased leaf width and inflorescence length in the second cycle. Selection for grazing regrowth resulted in reduced plant height and inflorescence length. Concern regarding susceptibility of Setaria to chinch bug and possible winter killing in Florida remain to be resolved before this species may be commercialized in Florida

Existence of Open Loop Equilibria for Disturbed Stackelberg Games

In this work, we derive necessary and sufficient conditions for the existence of an hierarchic equilibrium of a disturbed two player linear quadratic game with open loop information structure. A convexity condition guarantees the existence of a unique Stackelberg equilibria; this solution is first obtained in terms of a pair of symmetric Riccati equations and also in terms of a coupled of system of Riccati equations. In this latter case, the obtained equilibrium controls are of feedback type

Seleção de genótipos de Panicum maximum Jacq. para tolerância ao frio.

Com o objetivo de avaliar a tolerância de genótipos de P. maximum sob estresse térmico por baixas temperaturas, foi conduzido um experimento com dois genótipos e três cultivares (Mombaça, Massai e Tanzânia), no laboratório de fisiologia vegetal na Embrapa Gado de Corte, em três câmaras climáticas, durante 30 dias. O delineamento experimental foi de blocos casualizados com três repetições em esquema fatorial 5x3, com cinco genótipos e três temperaturas noturnas (12°C, 4°C e 0°C)

Approaches to calculate the dielectric function of ZnO around the band gap

Being one of the most sensitive methods for optical thin film metrology ellipsometry is widely used for the characterization of zinc oxide (ZnO), a key material for optoelectronics, photovoltaics, and printable electronics and in a range of critical applications. The dielectric function of ZnO has a special feature around the band gap dominated by a relatively sharp absorption feature and an excitonic peak. In this work we summarize and compare direct (point-by-point) and parametric approaches for the description of the dielectric function. We also investigate how the choice of the wavelength range influences the result, the fit quality and the sensitivity. Results on ZnO layers prepared by sputtering are presented

Quantum Interference of Coulomb Interaction and Disorder: Phase Shift of Friedel Oscillations and an Instability of the Fermi Sea

We investigate the influence of interference between Coulomb interaction and impurity scattering on the static electronic response $\chi (0,q)$ in disordered metals to leading order in the effective Coulomb interaction. When the transport relaxation time $\tau _{tr}$ is much shorter than the quasiparticle life time, we find a \mbox{sgn}(2p_F-q)/\sqrt{|2p_F-q|} divergence of the polarization function at the Fermi surface ($q=2p_F$). It causes a phase shift of the Friedel oscillations as well as an enhancement of their amplitude. Our results are consistent with experiments and may be relevant for understanding the stability of the amorphous state of certain alloys against crystallization.Comment: 11 pages, 4 PostScript figures appended as a self-extracting tar archive; includes output instruction