Optimal inventory decisions when offering layaway

This paper presents an inventory management policy for a retailer offering a layaway programme. Layaway is a service provided by retailers that allows budget constrained consumers who have sufficiently high valuations to pay for a product in several instalments rather than at once and obtain the product that has been reserved for them at the end of the payment period. If a consumer defaults on payments, then the reserved item is released back into store inventory. In this paper, we first determine the retailer's optimal order decisions when layaway is offered. We find that the order quantity under a layaway programme decreases with the likelihood of consumers not finishing their layaway plans and that it is not always profitable for a retailer to offer a layaway programme. We then identify the market conditions under which the retailer would benefit from a layaway programme. Lastly, we consider an extension to capture the influence of the timing of consumer defaults

Low-temperature synthesis under low oxygen pressure and thermodynamic properties of YbFe2O4–δ

Received: 27.11.2017. Accepted: 26.03.2018. Published: 10.05.2018.The mixed-valence compound YbFe2O4–δ was synthesized using a modified method that allows employing gaseous atmosphere with a controlled ratio of inert gas and oxygen. The stability range for YbFe2O4–δ was determined at 1090 °C under reducing conditions. Thermodynamic characteristics for the formation of YbFe2O4–δ from the simple oxides and from elements in the temperature range 700–910 °C have been calculated. The obtained results allow specifying the low-temperature part of P-T-X diagram for the Yb-Fe-O system.The study was done in accordance with the state quota for IMET UB RAS, theme No. 0396-2015-0075, using equipment of CCU “Ural-M”

A hybrid version of the tilted axis cranking model and its application to ^{128}Ba

A hybrid version the deformed nuclear potential is suggested, which combines a spherical Woods Saxon potential with a deformed Nilsson potential. It removes the problems of the conventional Nilsson potential in the mass 130 region. Based on the hybrid potential, tilted axis cranking calculations are carried out for the magnetic dipole band in ^{128}Ba.Comment: 10 pages 6 figure

Optical characterization of Mg-doped GaN films grown by metalorganic chemical vapor phase deposition

Scanning electron microscopy, micro-Raman, and photoluminescence (PL) measurements are reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor phase deposition. The surface morphology, structural, and optical properties of GaN samples with Mg concentrations ranging from 1019 to 1021 cm−3 have been studied. In the scanning micrographs large triangular pyramids are observed, probably due to stacking fault formation and three-dimensional growth. The density and size of these structures increase with the amount of magnesium incorporated in the samples. In the photoluminescence spectra, intense lines were found at 3.36 and 3.31 eV on the triangular regions, where the presence of cubic inclusions was confirmed by micro-Raman measurements. The excitation dependence and temperature behavior of these lines enable us to identify their excitonic nature. From our study we conclude that the interface region between these defects and the surrounding wurtzite GaN could be responsible for PL [email protected] ; [email protected] ; [email protected]

Coats disease ‒ case report

Coats disease is a rare, in about 90% of cases unilateral idiopathic eye pathology. It is characterized by telangiectatic and aneurysmally altered retinal blood vessels and progressive intra- and subretinal exudation, which can lead to exudative retinal detachment. Most often the symptoms manifest in the advanced stages. Purpose: To report a case with Coats disease. 10 year old boy, admitted to the Eye Clinic of the University Alexandrovska Hospital – Sofia, with leucocoria and strabismus of the right eye. Method: Full ophthalmic and orthoptic examination, OCT. Results: 3B stage of Coats disease of the right eye was established. The chance for improvement after surgery was evaluated as very little, so our patient remained under observation with protective glasses and prevention of complications. Conclusion: It is of great importance to early diagnose the disease, so to have more treatment options

Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478

Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.Comment: 20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysic

Residual strain effects on the two-dimensional electron gas concentration of AlGaN/GaN heterostructures

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly [email protected] ; [email protected]

Variations of Infiltration and Electronic Contact in Mesoscopic Perovskite Solar Cells Revealed by High‐Resolution Multi‐Mapping Techniques

A combination of high‐resolution mapping techniques is developed to probe the homogeneity and defects of mesoscopic perovskite solar cells. Three types of cells using a one‐step infiltration process with methylammonium lead iodide (MAPbI3) or 5‐ammoniumvaleric acid‐MAPbI3 solutions, or two‐step process with MAPbI3 solution are investigated. The correlation between photoluminescence, photocurrent, electroluminescence, and Raman maps gives a detailed understanding of the different infiltration mechanisms, electronic contact at interfaces, and effect on local photocurrent for the cells. The one‐step MAPbI3 cell has very limited infiltration of the perovskite solution which results in poor device performance. High loading of the mesopores of the TiO2 and ZrO2 scaffold is observed when using 5‐ammoniumvaleric acid, but some micrometer‐sized non‐infiltrated areas remain due to dense carbon flakes hindering perovskite infiltration. The two‐step cell has a complex morphology with features having either beneficial or detrimental effects on the local photocurrent. The results not only provide key insights to achieving better infiltration and homogeneity of the perovskite film in mesoporous devices but can also aid further work on planar devices to develop efficient extraction layers. Moreover, this multi‐mapping approach allows the correlation of the local photophysical properties of full perovskite devices, which would be challenging to obtain by other techniques