N=4 BPS black holes and octonionic twistors

Stationary, spherically symmetric solutions of N=2 supergravity in 3+1 dimensions have been shown to correspond to holomorphic curves on the twistor space of the quaternionic-K\"ahler space which arises in the dimensional reduction along the time direction. In this note, we generalize this result to the case of 1/4-BPS black holes in N=4 supergravity, and show that they too can be lifted to holomorphic curves on a "twistor space" Z, obtained by fibering the Grassmannian F=SO(8)/U(4) over the moduli space in three-dimensions SO(8,n_v+2)/SO(8)xSO(n_v+2). This provides a kind of octonionic generalization of the standard constructions in quaternionic geometry, and may be useful for generalizing the known BPS black hole solutions, and finding new non-BPS extremal solutions.Comment: 30 pages, one figure, uses JHEP3.cl

Determinants of Advertising Effectiveness: The Development of an International Advertising Elasticity Database and a Meta-Analysis

Increasing demand for marketing accountability requires an efficient allocation of marketing expenditures. Managers who know the elasticity of their marketing instruments can allocate their budgets optimally. Meta-analyses offer a basis for deriving benchmark elasticities for advertising. Although they provide a variety of valuable insights, a major shortcoming of prior meta-analyses is that they report only generalized results as the disaggregated raw data are not made available. This problem is highly relevant because coding of empirical studies, at least to a certain extent, involves subjective judgment. For this reason, meta-studies would be more valuable if researchers and practitioners had access to disaggregated data allowing them to conduct further analyses of individual, e.g., product-level-specific, interests. We are the first to address this gap by providing (1) an advertising elasticity database (AED) and (2) empirical generalizations about advertising elasticities and their determinants. Our findings indicate that the average current-period advertising elasticity is 0.09, which is substantially smaller than the value 0f 0.12 that was recently reported by Sethuraman, Tellis, and Briesch (2011). Furthermore, our meta-analysis reveals a wide range of significant determinants of advertising elasticity. For example, we find that advertising elasticities are higher (i) for hedonic and experience goods than for other goods; (ii) for new than for established goods; (iii) when advertising is measured in gross rating points (GRP) instead of absolute terms; and (iv) when the lagged dependent or lagged advertising variable is omitted.advertising effectiveness, advertising elasticity, advertising elasticity database, meta-analysis, empirical marketing generalizations

Providing health checks as incentives to retain blood donors — evidence from two field experiments

The collection of blood given by donors has proven to be a substantial societal and a managerial challenge. Consequently, blood donation services seek for incentive mechanisms to retain donors. However, economic or material rewards might entail negative side effects such as motivational crowding out or even attracting “bad blood”. In an effort to increase the retention of established blood donors, we conducted two randomized field trials (N1 = 53,257, N2 = 31,522) in cooperation with the German Red Cross Blood Donation Service and tested the effectiveness of an incentive strategy that is directly related to the blood donation itself: offering a comprehensive blood health check. Contrary to previous related research, we found substantial positive effects of a comprehensive blood health check incentive on donation behavior. In addition, unlike previous studies, we examine effects of repeated exposure to this incentive and do not find any wearout effects. Considering the positive effect of this incentive on donor retention and the relative low cost for providing this service to donors, our findings suggest that offering comprehensive blood health check incentives is a viable and cost-efficient marketing strategy to increase the retention among previous donors even if offered over the longer run.Accepted manuscrip

Should they stay or should they go? Reactivation and Termination of Low-Tier Customers: Effects on Satisfaction, Word-of-Mouth, and Purchases

Many companies face the problem of having a substantial number of low-tier customers ? clients at the bottom of the customer pyramid. For this segment, it is necessary to either reactivate or terminate the customer relationships to increase profitability. Managers seek to learn more about marketing actions targeted towards low-tier customers and their response towards these actions. Therefore, we conducted a large field experiment in which we implemented a ?last call? marketing action for a large sample of low-tier customers of a catalogue retailer (N = 12,000). The action aims at sales reactivation, but in case a customer should not react, the relationship will be terminated. We measure customer response in terms of satisfaction, (positive and negative) word-of-mouth, and purchase behavior. We find no harmful effects from relationship termination, such as dissatisfaction or negative word-of-mouth. The results indicate that the ?last call? marketing action reactivates a small fraction of the low-tier customers. These customers remain active in the months following the action period. We discuss managerial implications of our findings and future research on low-tier customer segments.

The Cost Impact of Spam Filters: Measuring the Effect of Information System Technologies in Organizations

More than 70% of global e-mail traffic consists of unsolicited and commercial direct marketing, also known as spam. Dealing with spam incurs high costs for organizations, prompting efforts to try to reduce spam-related costs by installing spam filters. Using modern econometric methods to reduce the selection bias of installing a spam filter, we deploy a unique data setting implemented at a German university to measure the costs associated with spam and the costs savings of spam filters. The applied methodological framework can easily be transferred to estimate the effect of other IS technologies (e.g., SAP) implemented in organizations. Our findings indicate that central IT costs are of little relevance since the majority of spam costs stem from employees who spend working time identifying and deleting spam. The working time losses caused by spam are approximately 1,200 minutes per employee per year; these costs could be reduced by roughly 35% through the installation of a spam filter mechanism. The individual efficiency of a spam filter installation depends on the amount of spam that is received and on the level of knowledge about spam.propensity score matching, treatment effects, spam filter, spam

A Second Order Penalized Direct Forcing for Hybrid Cartesian/Immersed Boundary Flow Simulations

International audienceFlows around complex stationary/moving solids take an important place in life-science context or in many engineering applications. Usually, these problems are solved by body-fitted approaches on unstructured meshes with boundary conditions directly imposed on the domain boundary. Another way is using immersed boundary (IB) techniques: the physical domain is immersed in a fixed fictitious one of simpler geometry on Cartesian grids. It allows to use efficient, fast and accurate numerical methods avoiding the tedious task of re-meshing in case of time varying geometry. In contrast, one needs specific methods to take into account the IB conditions (IBC). Here, we propose a second order penalized direct forcing method for unsteady incompressible flows with Dirichlet's IBC. It consists in adding a penalized forcing term to the initial problem, applied only on Cartesian nodes near the IB, in order to bring back the variable to the imposed one. Regarding Navier-Stokes solvers using a projection scheme, the forcing term is distributed both in the velocity prediction and in the correction equations. It leads to a natural way to prescribe the pressure boundary conditions around obstacles. Numerical experiments, performed for laminar flows around static/moving solids, assess the validity and illustrate the ability of our method, showing in particular a quadratic convergence rate

Declutching control of a wave energy converter

International audienceWhen hydraulic Power Take Off (PTO) is used to convert the mechanical energy of a wave energy converter into a more useful form of energy, the PTO force needs to be controlled. Continuous controlled variation of the PTO force can be approximated by a set of discrete values. This can be implemented using either variable displacement pumps or several hydraulic cylinders or several high pressure accumulators with different pressure levels. This pseudo continuous control could lead to a complex PTO with a lot of components. A simpler way for controlling this hydraulic PTO is declutching control, which consists in switching on and off alternatively the wave energy converter's PTO. This can be achieved practically using a simple bypass valve. In this paper, the control law of the valve is determined by using the optimal command theory. It is shown that, theoretically when considering a wave activated body type of WEC, declutching control can lead to energy absorption performance at least equivalent to that of pseudo-continuous control. The method is then applied to the case of the SEAREV wave energy converter, and it is shown than declutching control can even lead to a higher energy absorption, both in regular and irregular waves