Ballerina - Pirouettes in Search of Gamma Bursts

The cosmological origin of gamma ray bursts has now been established with reasonable certainty. Many more bursts will need to be studied to establish the typical distance scale, and to map out the large diversity in properties which have been indicated by the first handful of events. We are proposing Ballerina, a small satellite to provide accurate positions and new data on the gamma-ray bursts. We anticipate a detection rate an order of magnitude larger than obtained from Beppo-SAX.Comment: A&AS in press, proceedings of the Workshop "Gamma Ray Bursts in the Afterglow Era" in Rome, November 199

Scattering of dislocated wavefronts by vertical vorticity and the Aharonov-Bohm effect II: Dispersive waves

Previous results on the scattering of surface waves by vertical vorticity on shallow water are generalized to the case of dispersive water waves. Dispersion effects are treated perturbatively around the shallow water limit, to first order in the ratio of depth to wavelength. The dislocation of the incident wavefront, analogous to the Aharonov-Bohm effect, is still observed. At short wavelengths the scattering is qualitatively similar to the nondispersive case. At moderate wavelengths, however, there are two markedly different scattering regimes according to wether the capillary length is smaller or larger than $\sqrt{3}$ times depth. The dislocation is characterized by a parameter that depends both on phase and group velocity. The validity range of the calculation is the same as in the shallow water case: wavelengths small compared to vortex radius, and low Mach number. The implications of these limitations are carefully considered.Comment: 30 pages, 11 figure

Elastic forces that do no work and the dynamics of fast cracks

Elastic singularities such as crack tips, when in motion through a medium that is itself vibrating, are subject to forces orthogonal to the direction of motion and thus impossible to determine by energy considerations alone. This fact is used to propose a universal scenario, in which three dimensionality is essential, for the dynamic instability of fast cracks in thin brittle materials.Comment: 8 pages Latex, 1 Postscript figur

A Mission Statement for Mutual Funds in Shareholder Litigation

This Article analyzes the conduct of mutual funds in shareholder litigation. We begin by reviewing the basic forms of shareholder litigation and the benefits such claims might offer mutual fund investors. We then investigate, through an in-depth docket review, whether and how the ten largest mutual funds participate in share-holder litigation. We find that although shareholder suits offer potential benefits, the largest mutual funds have essentially forfeited their use of litigation. This finding is particularly striking given that index funds and other long-term oriented mutual funds generally cannot sell their shares when they are dissatisfied with company performance, leaving them with only two levers in corporate governance—voting and suing. Mutual funds vote, but they do not sue. We analyze potential explanations for the failure of mutual funds to litigate on behalf of their investors. Collective action problems and conflicts of interest raise significant obstacles to mutual fund participation in shareholder litigation. Yet, we argue, there are situations in which shareholder litigation could create value for mutual fund investors. We therefore turn to the normative question: How should mutual funds litigate on behalf of their investors? Answering this question allows us to articulate a mission statement for mutual funds in shareholder litigation. Our mission statement is grounded on the perspective of the broadly diversified “market investor.” The repeat-play incentives and broad diversification of many mutual funds, index funds in particular, suggests that they could create value by focusing principally on deterrence objectives. Mutual funds should bring share-holder suits against portfolio companies when doing so would meaningfully enhance deterrence. They should also scrutinize the litigation brought by other share-holders, objecting to outcomes that fail to promote meaningful deterrence. At the same time, mutual funds should focus on compensatory goals in litigation against nonportfolio defendants because extra portfolio claims do not raise circularity concerns. In addition, mutual funds should consider whether litigation can be used to implement corporate governance reforms. Finally, in all cases, mutual funds should closely monitor litigation agency costs. We close by suggesting ways in which the incentives of mutual funds might be restructured to bring about these changes

Boson Sampling from Gaussian States

We pose a generalized Boson Sampling problem. Strong evidence exists that such a problem becomes intractable on a classical computer as a function of the number of Bosons. We describe a quantum optical processor that can solve this problem efficiently based on Gaussian input states, a linear optical network and non-adaptive photon counting measurements. All the elements required to build such a processor currently exist. The demonstration of such a device would provide the first empirical evidence that quantum computers can indeed outperform classical computers and could lead to applications

A Protocol for Generating Random Elements with their Probabilities

We give an AM protocol that allows the verifier to sample elements x from a probability distribution P, which is held by the prover. If the prover is honest, the verifier outputs (x, P(x)) with probability close to P(x). In case the prover is dishonest, one may hope for the following guarantee: if the verifier outputs (x, p), then the probability that the verifier outputs x is close to p. Simple examples show that this cannot be achieved. Instead, we show that the following weaker condition holds (in a well defined sense) on average: If (x, p) is output, then p is an upper bound on the probability that x is output. Our protocol yields a new transformation to turn interactive proofs where the verifier uses private random coins into proofs with public coins. The verifier has better running time compared to the well-known Goldwasser-Sipser transformation (STOC, 1986). For constant-round protocols, we only lose an arbitrarily small constant in soundness and completeness, while our public-coin verifier calls the private-coin verifier only once

Blimp-1-dependent and -independent natural antibody production by B-1 and B-1-derived plasma cells.

Natural antibodies contribute to tissue homeostasis and protect against infections. They are secreted constitutively without external antigenic stimulation. The differentiation state and regulatory pathways that enable continuous natural antibody production by B-1 cells, the main cellular source in mice, remain incompletely understood. Here we demonstrate that natural IgM-secreting B-1 cells in the spleen and bone marrow are heterogeneous, consisting of (a) terminally differentiated B-1-derived plasma cells expressing the transcriptional regulator of differentiation, Blimp-1, (b) Blimp-1+, and (c) Blimp-1neg phenotypic B-1 cells. Blimp-1neg IgM-secreting B-1 cells are not simply intermediates of cellular differentiation. Instead, they secrete similar amounts of IgM in wild-type and Blimp-1-deficient (PRDM-1ΔEx1A) mice. Blimp-1neg B-1 cells are also a major source of IgG3. Consequently, deletion of Blimp-1 changes neither serum IgG3 levels nor the amount of IgG3 secreted per cell. Thus, the pool of natural antibody-secreting B-1 cells is heterogeneous and contains a distinct subset of cells that do not use Blimp-1 for initiation or maximal antibody secretion

SUSApp: a mobile app for measuring and comparing questionnaire-based usability assessments

Usability questionnaires are one of the most used methods to measure usability in terms of the user’s subjective satisfaction. However, most of the usability questionnaires do not provide a complete environment to store measurements and compare different usability values of application categories and versions over the long term, which makes it difficult to study the usability of a software product or even the usability of different versions of such products over time, hindering the facility to obtain comparisons and thresholds in usability measurements for different product lines. In this paper we present SUSApp, a tool conceived for the analysis of usability through the SUS (System Usability Scale) questionnaire, which is one of the most popular ones. This tool was conceived for mobile platforms, and it is intended to easily analyze usability by storing and recovering past evaluations, and allowing to statistically compare usability measurements among different software products and applications categories. In addition, a user testing is presented. This has provided acceptable usability results concerning SUSApp in an experiment with real usersThis work has been partially supported by the funding projects «eMadrid-CM», granted by the Madrid Research Council (project code S2013/ICE-2715), and «Flexor» granted by the Spanish Government (project code TIN2014-52129-R