Ferromagnetic ordering in graphs with arbitrary degree distribution

We present a detailed study of the phase diagram of the Ising model in random graphs with arbitrary degree distribution. By using the replica method we compute exactly the value of the critical temperature and the associated critical exponents as a function of the minimum and maximum degree, and the degree distribution characterizing the graph. As expected, there is a ferromagnetic transition provided < \infty. However, if the fourth moment of the degree distribution is not finite then non-trivial scaling exponents are obtained. These results are analyzed for the particular case of power-law distributed random graphs.Comment: 9 pages, 1 figur

Generating Innovations in Economic Variables

Stock prices should respond only to unpredictable components of economic news (‘innovations’) in efficient markets. While innovations used in empirical investigations of the economic underpinnings of stock market risk should at least satisfy this basic requirement this may not guarantee satisfactory research results. Three methods of generating innovations are evaluated for a variety of economic variables. First differencing produces unsatisfactory serially correlated innovations in general. Both ARIMA and Kalman Filter innovations are unpredictable, but in a further evaluation the component scores from Principal Components Analysis are regressed against economic innovations using PcGets. The results are far less noisy when Kalman Filter innovations are used.Macroeconomic variables, Innovations, stock returns, principal components analysis

Changes in the risk structure of stock returns. Consumer Confidence and the Dotcom Bubble.

Changes in the risk structure of stock returns may sometimes be very revealing. We examine economic variables that help explain principal components in UK stock returns, 01/1985 to 12/2001. The loading pattern on explanatory variables for the first component in a ‘bubble’ period is distinctive and consistent with a bubble/crash market. The second component shows a loading pattern on a Consumer Confidence variable in a pre-bubble period only. We observe apparently systematic changes in the structure of risk, and conjecture that Consumer Confidence captures a change in market sentiment that could be a signal for the evolution of stock prices.Macroeconomic variables, consumer confidence, stock returns, principal components analysis

The Incubator Hypothesis: Evidence from Five Cities

The purpose of this paper is to review the past evidence and to offer some new data to assess whether the incubator hypothesis can be empirically supported. In particular the two general aspects of the hypothesis will be tested. First, we will examine the proposition that highly centralized locations are attracting a disproportionate number of new firms and/or the employment associated with new firms. Second, we will test the hypothesis that new firms which are formed in high density areas move outward from such sites in their early years of existence in order to expand their productive activities. We refer to these as the "simple and "dynamic" hypotheses in the rest of the paper. Our analysis is based on the experience of all manufacturers in several U.S. cities. We recognize that it is quite possible that the hypothesis could hold for certain industries even if it is unsupported for all firms together. Our intent, however, is to test the validity of the hypothesis as a general theory of intraurban location behavior. The paper consists of three sections. The first two present evidence on the "simple" and "dynamic" hypotheses. The final section summarizes our findings and offers some conclusions.

Variability of the HeI5876 A line in early type chemically peculiar stars

Chemically peculiar stars present spectral and photometric variability with a single period. In the oblique rotator model, the non homogeneous distribution of elements on the stellar surface is at the origin of the observed variations. As to helium weak stars, it has been suggested that photometric and helium line equivalent width variations are out of phase. To understand the behaviour of helium in CP stars, we have obtained time resolved spectra of the HeI5876 A line for a sample of 16 chemically peculiar stars in the spectral range B3 -- A1 and belonging to different sub-groups. The HeI5876 A line is too weak to be measured in the spectra of the stars HD 24155, HD 41269, and HD 220825. No variation of the equivalent width of the selected He line has been revealed in the stars HD 22920, HD 24587, HD 36589, HD 49606, and HD 209515. The equivalent width variation of the HeI5876 A line is in phase with the photometric variability for the stars HD 43819, HD 171247 and HD 176582. On the contrary it is out of phase for the stars HD 28843, HD 182255 and HD 223640. No clear relation has been found for the stars HD 26571 and HD 177003.Comment: 8 pages, 9 figures. 1998, A&AS in pres

Quark mixing renormalization effects in the determination of |V_{tq}|

We study the numerical effects of several renormalization schemes of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix on the top-quark decay widths. We then employ these results to infer the relative shifts in the CKM parameters |V_{tq}|^2 due to the quark mixing renormalization corrections, assuming that they are determined directly from the top-quark partial decay widths, without imposing unitarity constraints. We also discuss the implications of these effects on the ratio R = Gamma(t -> Wb) / Gamma_t and the determination of |V_{tb}|^2.Comment: 10 pages, 3 table

Answer Set Planning Under Action Costs

Recently, planning based on answer set programming has been proposed as an approach towards realizing declarative planning systems. In this paper, we present the language Kc, which extends the declarative planning language K by action costs. Kc provides the notion of admissible and optimal plans, which are plans whose overall action costs are within a given limit resp. minimum over all plans (i.e., cheapest plans). As we demonstrate, this novel language allows for expressing some nontrivial planning tasks in a declarative way. Furthermore, it can be utilized for representing planning problems under other optimality criteria, such as computing ``shortest'' plans (with the least number of steps), and refinement combinations of cheapest and fastest plans. We study complexity aspects of the language Kc and provide a transformation to logic programs, such that planning problems are solved via answer set programming. Furthermore, we report experimental results on selected problems. Our experience is encouraging that answer set planning may be a valuable approach to expressive planning systems in which intricate planning problems can be naturally specified and solved