Valiente Kroon's obstructions to smoothness at infinity

We conjecture an interpretation in terms of multipole moments of the obstructions to smoothness at infinity found for time-symmetric, conformally-flat initial data by Valiente Kroon (Comm. Math. Phys. 244 (2004), 133-156).Comment: To appear in GR

Multipath Parameter Estimation from OFDM Signals in Mobile Channels

We study multipath parameter estimation from orthogonal frequency division multiplex signals transmitted over doubly dispersive mobile radio channels. We are interested in cases where the transmission is long enough to suffer time selectivity, but short enough such that the time variation can be accurately modeled as depending only on per-tap linear phase variations due to Doppler effects. We therefore concentrate on the estimation of the complex gain, delay and Doppler offset of each tap of the multipath channel impulse response. We show that the frequency domain channel coefficients for an entire packet can be expressed as the superimposition of two-dimensional complex sinusoids. The maximum likelihood estimate requires solution of a multidimensional non-linear least squares problem, which is computationally infeasible in practice. We therefore propose a low complexity suboptimal solution based on iterative successive and parallel cancellation. First, initial delay/Doppler estimates are obtained via successive cancellation. These estimates are then refined using an iterative parallel cancellation procedure. We demonstrate via Monte Carlo simulations that the root mean squared error statistics of our estimator are very close to the Cramer-Rao lower bound of a single two-dimensional sinusoid in Gaussian noise.Comment: Submitted to IEEE Transactions on Wireless Communications (26 pages, 9 figures and 3 tables

On the Complexity of Nash Equilibria of Action-Graph Games

We consider the problem of computing Nash Equilibria of action-graph games (AGGs). AGGs, introduced by Bhat and Leyton-Brown, is a succinct representation of games that encapsulates both "local" dependencies as in graphical games, and partial indifference to other agents' identities as in anonymous games, which occur in many natural settings. This is achieved by specifying a graph on the set of actions, so that the payoff of an agent for selecting a strategy depends only on the number of agents playing each of the neighboring strategies in the action graph. We present a Polynomial Time Approximation Scheme for computing mixed Nash equilibria of AGGs with constant treewidth and a constant number of agent types (and an arbitrary number of strategies), together with hardness results for the cases when either the treewidth or the number of agent types is unconstrained. In particular, we show that even if the action graph is a tree, but the number of agent-types is unconstrained, it is NP-complete to decide the existence of a pure-strategy Nash equilibrium and PPAD-complete to compute a mixed Nash equilibrium (even an approximate one); similarly for symmetric AGGs (all agents belong to a single type), if we allow arbitrary treewidth. These hardness results suggest that, in some sense, our PTAS is as strong of a positive result as one can expect

Nonresident Winter Visitor: Comments About Montana

Presents written comments of winter visitors to Montana. Use frequency of comment categories as well as summer comments vs. winter comments are examined

A Time to Question: The Role of the Black Church in British Society

In this essay I raise some questions concerning the role of Black faith and religious institutions in Britain. It seems to me that certain assertions made -concerning the progressive nature of this role have remained unquestioned. Lest this be perceived as yet another attack on Black faith from an outsider, it will be presented in terms of an exercise in self-criticism. I will use a collection of papers on Black theology in Britain, which I co-edited, to illustrate my argument concerning the limitations of our faith-based radicalism. Given that the aim here is less precise conceptual clarity and more broad political mapping, debates concerning the exact composition of the Black Church in Britain need not detain us. It is suffice for current purposes to say that I will use the term to refer to those denominations and fellowships that could be regarded as Black-led, as well as Black Christians involved in white-led churches. Furthermore, discussions concerning the exact ethnic or national origin of the churches are less important than the trends in their concerns, structures and responses to social injustice that seem to cut across ethnic boundaries

The Formation and Deployment of SWAT in the Ivory Towers

The purpose of this study is to investigate the formation of SWAT teams on college campuses and some of the reasoning behind their deployments. The study provides an overview of the early formation of SWAT, policing in a college setting and finally the creation of such teams that began in municipal policing but have migrated into campus law enforcement. Data for this study was collected using a 20 question survey sent to college campuses that are members of IACLEA or International association of college law enforcement administrators. The return rate was 37% and the surveys were analyzed using SPSS version 20. The results provide some support for the idea that campus SWAT teams were not formed for the traditional role of SWAT, such as the barricaded subject, terrorist incident but for high risk warrant service, drug incidents