318 research outputs found
A New monthly indicator of global real economic activity
In modelling macroeconomic time series, often a monthly indicator of global real economic
activity is used. We propose a new indicator, named World steel production, and compare it
to other existing indicators, precisely the Kilian’s index of global real economic activity and
the index of OECD World industrial production. We develop an econometric approach based
on desirable econometric properties in relation to the quarterly measure of World or global
gross domestic product to evaluate and to choose across different alternatives. The method is
designed to evaluate short-term, long-term and predictability properties of the indicators.
World steel production is proven to be the best monthly indicator of global economic activity
in terms of our econometric properties. Kilian’s index of global real economic activity also
accurately predicts World GDP growth rates. When extending the analysis to an out-ofsample
exercise, both Kilian’s index of global real economic activity and the World steel
production produce accurate forecasts for World GDP, confirming evidence provided by the
econometric properties. Specifically, a forecast combination of the three indices produces
statistically significant gains up to 40% at nowcast and more than 10% at longer horizons
relative to an autoregressive benchmark
World steel production: a new monthly indicator of global real economic activity
In this paper we propose a new indicator of monthly global real economic activity, named
world steel production. We use world steel production, OECD industrial production index
and Kilian’s rea index to forecast world real GDP, and key commodity prices. We find that
world steel production generates large statistically significant gains in forecasting world real
GDP and oil prices, relative to an autoregressive benchmark. A forecast combination of the
three indices produces statistically significant gains in forecasting world real GDP, oil,
natural gas, gold and fertilizer prices, relative to an autoregressive benchmark
Renormalization group of probabilistic cellular automata with one absorbing state
We apply a recently proposed dynamically driven renormalization group scheme
to probabilistic cellular automata having one absorbing state. We have found
just one unstable fixed point with one relevant direction. In the limit of
small transition probability one of the cellular automata reduces to the
contact process revealing that the cellular automata are in the same
universality class as that process, as expected. Better numerical results are
obtained as the approximations for the stationary distribution are improved.Comment: Errors in some formulas have been corrected. Additional material
available at http://mestre.if.usp.br/~javie
The egalitarian effect of search engines
Search engines have become key media for our scientific, economic, and social
activities by enabling people to access information on the Web in spite of its
size and complexity. On the down side, search engines bias the traffic of users
according to their page-ranking strategies, and some have argued that they
create a vicious cycle that amplifies the dominance of established and already
popular sites. We show that, contrary to these prior claims and our own
intuition, the use of search engines actually has an egalitarian effect. We
reconcile theoretical arguments with empirical evidence showing that the
combination of retrieval by search engines and search behavior by users
mitigates the attraction of popular pages, directing more traffic toward less
popular sites, even in comparison to what would be expected from users randomly
surfing the Web.Comment: 9 pages, 8 figures, 2 appendices. The final version of this e-print
has been published on the Proc. Natl. Acad. Sci. USA 103(34), 12684-12689
(2006), http://www.pnas.org/cgi/content/abstract/103/34/1268
Absence of epidemic threshold in scale-free networks with connectivity correlations
Random scale-free networks have the peculiar property of being prone to the
spreading of infections. Here we provide an exact result showing that a
scale-free connectivity distribution with diverging second moment is a
sufficient condition to have null epidemic threshold in unstructured networks
with either assortative or dissortative mixing. Connectivity correlations
result therefore ininfluential for the epidemic spreading picture in these
scale-free networks. The present result is related to the divergence of the
average nearest neighbors connectivity, enforced by the connectivity detailed
balance condition
Competition among memes in a world with limited attention
The wide adoption of social media has increased the competition among ideas for our finite attention. We employ a parsimonious agent-based model to study whether such a competition may affect the popularity of different memes, the diversity of information we are exposed to, and the fading of our collective interests for specific topics. Agents share messages on a social network but can only pay attention to a portion of the information they receive. In the emerging dynamics of information diffusion, a few memes go viral while most do not. The predictions of our model are consistent with empirical data from Twitter, a popular microblogging platform. Surprisingly, we can explain the massive heterogeneity in the popularity and persistence of memes as deriving from a combination of the competition for our limited attention and the structure of the social network, without the need to assume different intrinsic values among ideas
Disorder-induced phase transition in a one-dimensional model of rice pile
We propose a one-dimensional rice-pile model which connects the 1D BTW
sandpile model (Phys. Rev. A 38, 364 (1988)) and the Oslo rice-pile model
(Phys. Rev. lett. 77, 107 (1997)) in a continuous manner. We found that for a
sufficiently large system, there is a sharp transition between the trivial
critical behaviour of the 1D BTW model and the self-organized critical (SOC)
behaviour. When there is SOC, the model belongs to a known universality class
with the avalanche exponent .Comment: 10 pages, 7 eps figure
Application of a renormalization group algorithm to nonequilibrium cellular automata with one absorbing state
We improve a recently proposed dynamically driven renormalization group
algorithm for cellular automata systems with one absorbing state, introducing
spatial correlations in the expression for the transition probabilities. We
implement the renormalization group scheme considering three different
approximations which take into account correlations in the stationary
probability distribution. The improved scheme is applied to a probabilistic
cellular automaton already introduced in the literature.Comment: 7 pages, 4 figures, to be published in Phys. Rev.
Roughness of Sandpile Surfaces
We study the surface roughness of prototype models displaying self-organized
criticality (SOC) and their noncritical variants in one dimension. For SOC
systems, we find that two seemingly equivalent definitions of surface roughness
yields different asymptotic scaling exponents. Using approximate analytical
arguments and extensive numerical studies we conclude that this ambiguity is
due to the special scaling properties of the nonlinear steady state surface. We
also find that there is no such ambiguity for non-SOC models, although there
may be intermediate crossovers to different roughness values. Such crossovers
need to be distinguished from the true asymptotic behaviour, as in the case of
a noncritical disordered sandpile model studied in [10].Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
Fluctuations and correlations in sandpile models
We perform numerical simulations of the sandpile model for non-vanishing
driving fields and dissipation rates . Unlike simulations
performed in the slow driving limit, the unique time scale present in our
system allows us to measure unambiguously response and correlation functions.
We discuss the dynamic scaling of the model and show that
fluctuation-dissipation relations are not obeyed in this system.Comment: 5 pages, latex, 4 postscript figure
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