Covering problems in edge- and node-weighted graphs

This paper discusses the graph covering problem in which a set of edges in an edge- and node-weighted graph is chosen to satisfy some covering constraints while minimizing the sum of the weights. In this problem, because of the large integrality gap of a natural linear programming (LP) relaxation, LP rounding algorithms based on the relaxation yield poor performance. Here we propose a stronger LP relaxation for the graph covering problem. The proposed relaxation is applied to designing primal-dual algorithms for two fundamental graph covering problems: the prize-collecting edge dominating set problem and the multicut problem in trees. Our algorithms are an exact polynomial-time algorithm for the former problem, and a 2-approximation algorithm for the latter problem, respectively. These results match the currently known best results for purely edge-weighted graphs.Comment: To appear in SWAT 201

Spatio-Temporal Scaling of Solar Surface Flows

The Sun provides an excellent natural laboratory for nonlinear phenomena. We use motions of magnetic bright points on the solar surface, at the smallest scales yet observed, to study the small scale dynamics of the photospheric plasma. The paths of the bright points are analyzed within a continuous time random walk framework. Their spatial and temporal scaling suggest that the observed motions are the walks of imperfectly correlated tracers on a turbulent fluid flow in the lanes between granular convection cells.Comment: Now Accepted by Physical Review Letter

The Coexistence of Multiple Distributions Systems for Financial Services: The Case of Property-Liability Insurance

Property-liability insurance is distributed by two different types of firms, those that distribute their product through independent agents, who represent more than one insurer,and direct writing insurers that distribute insurance through exclusive agents, who represent only one insurer. This paper analyzes the reasons for the long term coexistence of the independent agency and direct writing distribution systems. Two primary hypotheses explain the coexistence of independent and exclusive agents. The market imperfections hypothesis suggests that firms that use independent agents survive while providing essentially the same service as firms using exclusive agents because of market imperfections such as price regulation, slow diffusion of information in insurance markets, or search costs that permit inefficient firms to survive alongside efficient firms. Efficient firms are expected to earn super-normal risk-adjusted profits, while inefficient firms will earn risk-adjusted profits closer to normal levels. The product quality hypothesis suggests that higher costs of independent agents represent unobserved differences in product quality or service intensity, such as the providing of additional customer assistance with claims settlement,offering a greater variety of product choice sand reducing policyholder search costs. This hypothesis predicts normal risk-adjusted profits for both independent and exclusive agency firms. Because product quality in insurance is essentially unobserved, researchers have been unable to reach consensus on whether the market imperfections hypothesis or the product quality hypothesis is more consistent with the observed cost data. This lack of consensus leaves open the economic question of whether the market works well in solving the problem of minimizing product distribution costs and leaves unresolved the policy issue of whether marketing costs in property-liability insurance are excessive and perhaps should receive regulatory attention. The authors propose a new methodology for distinguishing between market imperfection sand product quality using frontier efficiency methods. They estimate both profit efficiency and cost efficiency for a sample of independent and exclusive agency insurers. Measuring profit efficiency helps to identify unobserved product quality differences because customers should be willing to pay extra for higher quality. This approach allows for the possibility that some firms may incur additional costs providing superior service and be compensated for these costs through higher revenues. Profit efficiency also implicitly incorporates the qualities floss control and risk management services,since insurers that more effectively control losses and manage risk should have higher average risk-adjusted profits but not necessarily lower costs than less effective insurers. The empirical results confirm that independent agency firms have higher costs on average than do direct writers. The principal finding of the study is that most of the average differential between the two groups of firms disappears in the profit function analysis. This is a robust result that holds both in the authors tables of averages and in the regression analysis and applies to both the standard and non-standard profit functions. Based on averages, the profit efficiency differential is at most one-third as large as the profit efficiency differential.Based on the regression analysis, the profit inefficiency differential is at most one-fourth as large as the cost inefficiency differential,and the profit inefficiency differential is not statistically significant in the more fully specified models that control for size,organizational form and business mix. The results provide strong support for the product quality hypothesis and do not support the market imperfections hypothesis. The higher costs of independent agents appear to be due almost entirely to the provision of higher quality services, which are compensated for by additional revenues. A significant public policy implication is that regulatory decisions should not be based on costs alone. The authors findings imply that marketing cost differentials among insurers are mostly attributable to service differentials rather than to inefficiency and therefore do not represent social costs. The profit inefficiency results show that there is room for improvement in both the independent and direct writing segments of the industry. However, facilitating competition is likely to be a more effective approach to increasing efficiency than restrictive price regulation.

Conductance quantization in mesoscopic graphene

Using a generalized Landauer approach we study the non-linear transport in mesoscopic graphene with zig-zag and armchair edges. We find that for clean systems, the low-bias low-temperature conductance, G, of an armchair edge system in quantized as G/t=4 n e^2/h, whereas for a zig-zag edge the quantization changes to G/t t=4(n+1/2)e^2/h, where t is the transmission probability and n is an integer. We also study the effects of a non-zero bias, temperature, and magnetic field on the conductance. The magnetic field dependence of the quantization plateaus in these systems is somewhat different from the one found in the two-dimensional electron gas due to a different Landau level quantization.Comment: 6 pages, 9 figures. Final version published in Physical Review

Analysis Of Measured Transport Properties Of Domain Walls In Magnetic Nanowires And Films

Existing data for soft magnetic materials of critical current for domain-wall motion, wall speed driven by a magnetic field, and wall electrical resistance, show that all three observable properties are related through a single parameter: the wall mobility $\mu$. The reciprocal of $\mu$ represents the strength of viscous friction between domain wall and conduction-electron gas. And $\mu$ is a function of the wall width, which depends in turn on the aspect ratio t/w, where t and w are the thickness and width of the sample. Over four orders of magnitude of $\mu$, the data for nanowires show $\mu\propto (t/w)^{-2.2}$. This dependence is in approximate agreement with the prediction of the 1984 Berger theory based on s-d exchange. On the other hand, it is inconsistent with the prediction of the 2004 Tatara and Kohno theory, and of the 2004 Zhang and Li theory.Comment: 7 pages, 1 figure; submitted to Phys. Rev.

Conglomeration Versus Strategic Focus: Evidence from the Insurance Industry

We provide evidence on the validity of the conglomeration hypothesis versus the strategic focus hypothesis for financial institutions using data on U.S. insurance companies. We distinguish between the hypotheses using profit scope economies, which measures the relative efficiency of joint versus specialized production, taking both costs and revenues into account. The results suggest that the conglomeration hypothesis dominates for some types of financial service providers and the strategic focus hypothesis dominates for other types. This may explain the empirical puzzle of why joint producers and specialists both appear to be competitively viable in the long run.

Fluctuation Superconductivity in Mesoscopic Aluminum Rings

Fluctuations are important near phase transitions, where they can be difficult to describe quantitatively. Superconductivity in mesoscopic rings is particularly intriguing because the critical temperature is an oscillatory function of magnetic field. There is an exact theory for thermal fluctuations in one-dimensional superconducting rings, which are therefore expected to be an excellent model system. We measure the susceptibility of many rings, one ring at a time, using a scanning SQUID that can isolate magnetic signals from seven orders of magnitude larger background applied flux. We find that the fluctuation theory describes the results and that a single parameter characterizes the ways in which the fluctuations are especially important at magnetic fields where the critical temperature is suppressed.Comment: Reprinted with permission from AAA