Stocks in the household portfolio: a look back at the 1990s

The growing prominence of stocks as a household asset in the 1990s encouraged the view that the United States had become a nation of zealous investors alert to every market development and eager to acquire new stocks. Yet an analysis of the factors behind the rise in the household equity share suggests that exceptionally high returns on stocks_rather than aggressive investment behavior_accounted for much of the increased importance of stocks.Financial markets ; Rate of return

Wireless Medical Sensor Networks: Design Requirements and Enabling Technologies

This article analyzes wireless communication protocols that could be used in healthcare environments (e.g., hospitals and small clinics) to transfer real-time medical information obtained from noninvasive sensors. For this purpose the features of the three currently most widely used protocols—namely, Bluetooth® (IEEE 802.15.1), ZigBee (IEEE 802.15.4), and Wi-Fi (IEEE 802.11)—are evaluated and compared. The important features under consideration include data bandwidth, frequency band, maximum transmission distance, encryption and authentication methods, power consumption, and current applications. In addition, an overview of network requirements with respect to medical sensor features, patient safety and patient data privacy, quality of service, and interoperability between other sensors is briefly presented. Sensor power consumption is also discussed because it is considered one of the main obstacles for wider adoption of wireless networks in medical applications. The outcome of this assessment will be a useful tool in the hands of biomedical engineering researchers. It will provide parameters to select the most effective combination of protocols to implement a specific wireless network of noninvasive medical sensors to monitor patients remotely in the hospital or at home

Nonlinear stability of spatially-periodic traveling-wave solutions of systems of reaction diffusion equations

Using spatial domain techniques developed by the authors and Myunghyun Oh in the context of parabolic conservation laws, we establish under a natural set of spectral stability conditions nonlinear asymptotic stability with decay at Gaussian rate of spatially periodic traveling-waves of systems of reaction diffusion equations. In the case that wave-speed is identically zero for all periodic solutions, we recover and slightly sharpen a well-known result of Schneider obtained by renormalization/Bloch transform techniques; by the same arguments, we are able to treat the open case of nonzero wave-speeds to which Schneider's renormalization techniques do not appear to appl

On the length of non-solutions to equations with constants in some linear groups

We show that for any finite-rank free group $\Gamma$, any word-equation in one variable of length $n$ with constants in $\Gamma$ fails to be satisfied by some element of $\Gamma$ of word-length $O(\log (n))$. By a result of the first author, this logarithmic bound cannot be improved upon for any finitely generated group $\Gamma$. Beyond free groups, our method (and the logarithmic bound) applies to a class of groups including $PSL_d(\mathbb{Z})$ for all $d \geq 2$, and the fundamental groups of all closed hyperbolic surfaces and $3$-manifolds.Comment: 11 pages, no figures. Comments welcome

Weak Lensing Peak Finding: Estimators, Filters, and Biases

Large catalogs of shear-selected peaks have recently become a reality. In order to properly interpret the abundance and properties of these peaks, it is necessary to take into account the effects of the clustering of source galaxies, among themselves and with the lens. In addition, the preferred selection of lensed galaxies in a flux- and size-limited sample leads to fluctuations in the apparent source density which correlate with the lensing field (lensing bias). In this paper, we investigate these issues for two different choices of shear estimators which are commonly in use today: globally-normalized and locally-normalized estimators. While in principle equivalent, in practice these estimators respond differently to systematic effects such as lensing bias and cluster member dilution. Furthermore, we find that which estimator is statistically superior depends on the specific shape of the filter employed for peak finding; suboptimal choices of the estimator+filter combination can result in a suppression of the number of high peaks by orders of magnitude. Lensing bias generally acts to increase the signal-to-noise \nu of shear peaks; for high peaks the boost can be as large as \Delta \nu ~ 1-2. Due to the steepness of the peak abundance function, these boosts can result in a significant increase in the abundance of shear peaks. A companion paper (Rozo et al., 2010) investigates these same issues within the context of stacked weak lensing mass estimates.Comment: 11 pages, 8 figures; comments welcom

Theoretical Transmission Spectra During Extrasolar Giant Planet Transits

The recent transit observation of HD 209458 b - an extrasolar planet orbiting a sun-like star - confirmed that it is a gas giant and determined that its orbital inclination is 85 degrees. This inclination makes possible investigations of the planet atmosphere. In this paper we discuss the planet transmission spectra during a transit. The basic tenet of the method is that the planet atmosphere absorption features will be superimposed on the stellar flux as the stellar flux passes through the planet atmosphere above the limb. The ratio of the planet's transparent atmosphere area to the star area is small, approximately 10^{-3} to 10^{-4}; for this method to work very strong planet spectral features are necessary. We use our models of close-in extrasolar giant planets to estimate promising absorption signatures: the alkali metal lines, in particular the Na I and K I resonance doublets, and the He I $2^3S$ - $2^3P$ triplet line at 1083.0 nm. If successful, observations will constrain the line-of-sight temperature, pressure, and density. The most important point is that observations will constrain the cloud depth, which in turn will distinguish between different atmosphere models. We also discuss the potential of this method for EGPs at different orbital distances and orbiting non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page

Weak lensing mass reconstructions of the ESO Distant Cluster Survey

We present weak lensing mass reconstructions for the 20 high-redshift clusters i n the ESO Distant Cluster Survey. The weak lensing analysis was performed on deep, 3-color optical images taken with VLT/FORS2, using a composite galaxy catalog with separate shape estimators measured in each passband. We find that the EDisCS sample is composed primarily of clusters that are less massive than t hose in current X-ray selected samples at similar redshifts, but that all of the fields are likely to contain massive clusters rather than superpositions of low mass groups. We find that 7 of the 20 fields have additional massive structures which are not associated with the clusters and which can affect the weak lensing mass determination. We compare the mass measurements of the remaining 13 clusters with luminosity measurements from cluster galaxies selected using photometric redshifts and find evidence of a dependence of the cluster mass-to-light ratio with redshift. Finally we determine the noise level in the shear measurements for the fields as a function of exposure time and seeing and demonstrate that future ground-based surveys which plan to perform deep optical imaging for use in weak lensing measurements must achieve point-spread functions smaller than a median of 0.6" FWHM.Comment: 35 pages, 24 figures, accepted to A&A, a version with better figure resolution can be found at http://www.mpa-garching.mpg.de/ediscs/papers.htm

Do Social Connections Reduce Moral Hazard? Evidence from the New York City Taxi Industry

This study investigates the role of social networks in aligning the incentives of economic agents in settings with incomplete contracts. We study the New York City taxi industry where taxis are often leased and lessee-drivers have worse driving outcomes than owner-drivers as a result of a moral hazard associated with incomplete leasing contracts. Using instrumental variables and fixed-effects analyses, we find that: (1) drivers leasing from members of their country-of-birth community exhibit significantly reduced effects of moral hazard; (2) network effects appear to operate primarily via social sanctions; and (3) network benefits can help to explain the organization of the industry in terms of which drivers and owners form business relationships.