Quasi-Newton-Based Preconditioning and Damped Quasi-Newton Schemes for Nonlinear Conjugate Gradient Methods

In this paper, we deal with matrix-free preconditioners for Nonlinear Conjugate Gradient (NCG) methods. In particular, we review proposals based on quasi-Newton updates, and either satisfying the secant equation or a secant-like equation at some of the previous iterates. Conditions are given proving that, in some sense, the proposed preconditioners also approximate the inverse of the Hessian matrix. In particular, the structure of the preconditioners depends both on low-rank updates along with some specific parameters. The low-rank updates are obtained as by-product of NCG iterations. Moreover, we consider the possibility to embed damped techniques within a class of preconditioners based on quasi-Newton updates. Damped methods have proved to be effective to enhance the performance of quasi-Newton updates, in those cases where the Wolfe linesearch conditions are hardly fulfilled. The purpose is to extend the idea behind damped methods also to improve NCG schemes, following a novel line of research in the literature. The results, which summarize an extended numerical experience using large-scale CUTEst problems, is reported, showing that these approaches can considerably improve the performance of NCG methods

On Optimal Buffer Allocation for Guaranteeing Quality of Service in Multimedia Internet Broadcasting for Mobile Networks

In this paper the problem of guaranteeing Quality of Service via optimal buffer allocation is considered: namely, we compute the optimal buffer with respect to a real-world dataset accounting for the expected data traffic volume for a predefined set of business users belonging to a mobile network scenario. Two distinct approaches are followed to tackle this issue, in a static and dynamic fashion, respectively: the former relies on nonlinear programming, while the latter relies on model predictive control. The proposed formulation in terms of optimal static buffer allocation enables the minimization of the wasted amount of data traffic in order to prevent users from paying the assignment of extra resources in addition to the available traffic bundle. Instead, optimal dynamic buffer allocation pushes resource optimization forward by enabling personalization: indeed, tracking the prediction of data traffic consumption for each user allows to satisfy personalized Quality of Service guarantees. Numerical simulations show the effectiveness of the proposed approaches in terms of buffer saving and decrease in Quality of Service mismatch

The late complications of totally implantable central venous access ports: The results from an Italian multicenter prospective observation study

a b s t r a c t Purpose: The principal aim of this study is to analyze the incidence of late complications in oncologic patients with totally implanted central venous access ports. Methods: A prospective multicenter observational study was conducted in 26 Italian oncologic outpatient clinics. 1076 cancer patients with Totally Implanted Central Venous Access Ports (TIAP) were observed. 515 devices were observed in patients under treatment and 561 in patients who went to the outpatient clinic only for flushing. Results: Late complications observed in patients under treatment were: 3 pocket infections (0.09/1000 days of port observation), 1 cutaneous infection (0.03/1000 days of port observation), 8 occlusions (0.24/ 1000 days of port observation) and 12 others. In patients using the device only for flushing we observed 4 cases of device related bacteremia (0.04/ 1000 days of port observation), 1 pocket infection (0.01/1000 days of port observation), 1 cutaneous infection (0.01/1000 days of port observation), 3 occlusions (0.03/1000 days of port observation) and 7 other complications. Conclusions: The low incidence of complications suggests that TIAP is safe and reliable for long term intermittent venous access. Our results support the use of TIAP in the oncology patients