An Optimal Trade-off between Content Freshness and Refresh Cost

Caching is an effective mechanism for reducing bandwidth usage and alleviating server load. However, the use of caching entails a compromise between content freshness and refresh cost. An excessive refresh allows a high degree of content freshness at a greater cost of system resource. Conversely, a deficient refresh inhibits content freshness but saves the cost of resource usages. To address the freshness-cost problem, we formulate the refresh scheduling problem with a generic cost model and use this cost model to determine an optimal refresh frequency that gives the best tradeoff between refresh cost and content freshness. We prove the existence and uniqueness of an optimal refresh frequency under the assumptions that the arrival of content update is Poisson and the age-related cost monotonically increases with decreasing freshness. In addition, we provide an analytic comparison of system performance under fixed refresh scheduling and random refresh scheduling, showing that with the same average refresh frequency two refresh schedulings are mathematically equivalent in terms of the long-run average cost

Description of surgical and anaesthetic services provided at two district hospitals in KwaZulu-Natal in relation to the surgical norms and standards outlined in the Service Package for District Hospitals

Background: Provision of surgical services at district hospitals (DHS) is cost effective and important. The District Hospital Service Package for South Africa (package of services) specifies the services that a district hospital should provide. The aim of this study was to document the surgical services provided at two DHS in KwaZulu-Natal and to compare this with the recommendations in the package of services. Methods: In a retrospective quantitative study, data from 2008 were collected from the theatre register at two DHS. Data were analysed and results compared with the norms and standards in the package of services. Results were presented to staff at the hospitals, who then commented on the challenges of providing surgical services at DHS.Results: Only 60% and 30% respectively of procedures listed in the package of services were being carried out at the two hospitals. In total, 3 900 procedures were carried out over the year. Dundee Hospital offered a broader range of surgical procedures and anaesthetics than the Church of Scotland Hospital (COSH). COSH has a large obstetric burden, with 3 666 deliveries each year. A large number of procedures were being carried out by a single operator. Conclusions: Many surgical procedures are being carried out even though neither hospital provides the full complement of surgical services as specified in the package of services. The wide variation between the surgical services offered reflects the surgical and anaesthetic skills at the respective hospitals. Potential medico-legal hazards that require urgent attention were identified. A review of the package of services is essential to identify core procedures that must be provided at DHS.Keywords: surgical services, anaesthetic

Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide.

Excitation of ionic solids with extreme ultraviolet pulses creates localized core-level excitons, which in some cases couple strongly to the lattice. Here, core-level-exciton states of magnesium oxide are studied in the time domain at the Mg L_{2,3} edge with attosecond transient reflectivity spectroscopy. Attosecond pulses trigger the excitation of these short-lived quasiparticles, whose decay is perturbed by time-delayed near-infrared pulses. Combined with a few-state theoretical model, this reveals that the infrared pulse shifts the energy of bright (dipole-allowed) core-level-exciton states as well as induces features arising from dark core-level excitons. We report coherence lifetimes for the two lowest core-level excitons of 2.3±0.2 and 1.6±0.5  fs and show that these are primarily a consequence of strong exciton-phonon coupling, disclosing the drastic influence of structural effects in this ultrafast relaxation process

Illusory perceptions of space and time preserve cross-saccadic perceptual continuity

When voluntary saccadic eye movements are made to a silently ticking clock, observers sometimes think that the second hand takes longer than normal to move to its next position. For a short period, the clock appears to have stopped (chronostasis). Here we show that the illusion occurs because the brain extends the percept of the saccadic target backwards in time to just before the onset of the saccade. This occurs every time we move the eyes but it is only perceived when an external time reference alerts us to the phenomenon. The illusion does not seem to depend on the shift of spatial attention that accompanies the saccade. However, if the target is moved unpredictably during the saccade, breaking perception of the target's spatial continuity, then the illusion disappears. We suggest that temporal extension of the target's percept is one of the mechanisms that 'fill in' the perceptual 'gap' during saccadic suppression. The effect is critically linked to perceptual mechanisms that identify a target's spatial stability

Effects of Infrared Radiation on Skin Photo-Aging and Pigmentation

Infrared radiation is increasingly and uncritically used for cosmetic and wellness purposes, despite the poorly understood biologic effects of such treatments on humans. In the present study, we investigated the effects of infrared radiation on collagen and elastin production in dermal fibroblasts, as well as the clinical and histopathologic effects of infrared radiation on photo-aged facial skin lesions. In order to determine the effects of infrared radiation on collagen and elastin production, dermal fibroblasts were exposed to infrared radiation for varying lengths of time and collagen and elastin contents were subsequently determined. Additionally, 20 patients with mild to moderate facial wrinkles and hyperpigmented lesions received daily treatments of far infrared radiation (900 to 1000 µm) for six-months. During the treatment, patients and a medical observer conducted independent photographic and clinical evaluations every 4 weeks, and skin biopsies were obtained for histological analysis at baseline and one month post-treatment. We found that the content of collagen and elastin produced by the fibroblasts increased after infrared radiation, and that this increase was proportional to the duration of irradiation exposure. Following 6 months of treatment, all patients reported good (51-75%) improvements in skin texture and roughness. Additionally, patients noted fair (25-50%) improvement in color tone of the skin; however, improvements in hyperpigmented lesions were not observed. Objective medical evaluation of the patients indicated that roughness and laxity were fairly improved, but there was no significant improvement in hyperpigmented lesions. Histological examination failed to reveal any differences as well. These results suggest that infrared radiation may have beneficial effects on skin texture and wrinkles by increasing collagen and elastin contents from the stimulated fibroblasts. Therefore, skin treatment with infrared radiation may be an effective and safe non-ablative remodeling method, and may also be useful in the treatment of photo-aged skin