4,444 research outputs found
Exchange Rate Models Are Not as Bad as You Think
Standard models of exchange rates, based on macroeconomic variables such as prices, interest rates, output, etc., are thought by many researchers to have failed empirically. We present evidence to the contrary. First, we emphasize the point that "beating a random walk" in forecasting is too strong a criterion for accepting an exchange rate model. Typically models should have low forecasting power of this type. We then propose a number of alternative ways to evaluate models. We examine in-sample fit, but emphasize the importance of the monetary policy rule, and its effects on expectations, in determining exchange rates. Next we present evidence that exchange rates incorporate news about future macroeconomic fundamentals, as the models imply. We demonstrate that the models might well be able to account for observed exchange-rate volatility. We discuss studies that examine the response of exchange rates to announcements of economic data. Then we present estimates of exchange-rate models in which expected present values of fundamentals are calculated from survey forecasts. Finally, we show that out-of-sample forecasting power of models can be increased by focusing on panel estimation and long-horizon forecasts.
Charging of ice-vapor interfaces
International audienceThe time resolved chemical composition of aerosol particles, formed by the oxidation of alpha-pinene has been investigated by liquid chromatography/mass spectrometry (LC-MS) using negative and positive ionisation methods (ESI(-) and APCI(+)). The experiments were performed at the EUPHORE facility in Valencia (Spain) under various experimental conditions, including dark ozone reactions, photosmog experiments with low NOx mixing ratios and reaction with OH radicals in the absence of NOx (H2O2-photolysis). Particles were sampled on PTFE filters at different stages of the reaction and extracted with methanol. The predominant products from alpha-pinene in the particulate phase are cis-pinic acid, cis-pinonic acid and hydroxy-pinonic acid isomers. Another major compound with molecular weight 172 was detected, possibly a hydroxy-carboxylic acid. These major compounds account for 50% to 80% of the identified aerosol products, depending on the time of sampling and type of experiment. In addition, more than 20 different products have been detected and structures have been tentatively assigned based on their molecular weight and responses to the different ionisation modes. The different experiments performed showed that the aerosol formation is mainly caused by the ozonolysis reaction. The highest aerosol yields were observed in the dark ozone experiments, for which also the highest ratios of mass of identified products to the formed aerosol mass were found (30% to 50%, assuming a density of 1 g cm-3)
Thermodynamics and the Global Optimization of Lennard-Jones clusters
Theoretical design of global optimization algorithms can profitably utilize
recent statistical mechanical treatments of potential energy surfaces (PES's).
Here we analyze the basin-hopping algorithm to explain its success in locating
the global minima of Lennard-Jones (LJ) clusters, even those such as \LJ{38}
for which the PES has a multiple-funnel topography, where trapping in local
minima with different morphologies is expected. We find that a key factor in
overcoming trapping is the transformation applied to the PES which broadens the
thermodynamic transitions. The global minimum then has a significant
probability of occupation at temperatures where the free energy barriers
between funnels are surmountable.Comment: 13 pages, 13 figures, revte
The double-funnel energy landscape of the 38-atom Lennard-Jones cluster
The 38-atom Lennard-Jones cluster has a paradigmatic double-funnel energy
landscape. One funnel ends in the global minimum, a face-centred-cubic (fcc)
truncated octahedron. At the bottom of the other funnel is the second lowest
energy minimum which is an incomplete Mackay icosahedron. We characterize the
energy landscape in two ways. Firstly, from a large sample of minima and
transition states we construct a disconnectivity tree showing which minima are
connected below certain energy thresholds. Secondly we compute the free energy
as a function of a bond-order parameter. The free energy profile has two
minima, one which corresponds to the fcc funnel and the other which at low
temperature corresponds to the icosahedral funnel and at higher temperatures to
the liquid-like state. These two approaches show that the greater width of the
icosahedral funnel, and the greater structural similarity between the
icosahedral structures and those associated with the liquid-like state, are the
cause of the smaller free energy barrier for entering the icosahedral funnel
from the liquid-like state and therefore of the cluster's preferential entry
into this funnel on relaxation down the energy landscape. Furthermore, the
large free energy barrier between the fcc and icosahedral funnels, which is
energetic in origin, causes the cluster to be trapped in one of the funnels at
low temperature. These results explain in detail the link between the
double-funnel energy landscape and the difficulty of global optimization for
this cluster.Comment: 12 pages, 11 figures, revte
Dilaton Contact Terms in the Bosonic and Heterotic Strings
Dilaton contact terms in the bosonic and heterotic strings are examined
following the recent work of Distler and Nelson on the bosonic and semirigid
strings. In the bosonic case dilaton two-point functions on the sphere are
calculated as a stepping stone to constructing a `good' coordinate family for
dilaton calculations on higher genus surfaces. It is found that dilaton-dilaton
contact terms are improperly normalized, suggesting that the interpretation of
the dilaton as the first variation of string coupling breaks down when other
dilatons are present. It seems likely that this can be attributed to the
tachyon divergence found in \TCCT. For the heterotic case, it is found that
there is no tachyon divergence and that the dilaton contact terms are properly
normalized. Thus, a dilaton equation analogous to the one in topological
gravity is derived and the interpretation of the dilaton as the string coupling
constant goes through.Comment: 44 pages, Figures now included. This replacement version includes the
7 figures as PostScript files appended to the end and the macros to insert
them into the text. Also some typos in intermediate formulae were correcte
A qualitative analysis of the interfaces between urban underground metro infrastructure and its environment in London
Peer reviewedPostprin
On the role of frameworks and smart mobility in addressing the rural mobility problem
Acknowledgements: We would like to acknowledge the European Commission in funding the SMARTA project (Contract n. No MOVE/B4/SER/2017-473/S 12.780134), which has enabled this research to take place. The insight papers and good practice cases referred to in this paper have been completed by members of the SMARTA Consortium: MemEx (IT), University of Aberdeen (UK), Transport & Mobility Leuven (BE), Vectos (UK) and European Integrated Projects (RO).Peer reviewedPostprin
Build an app and they will come? Lessons learnt from trialling the GetThereBus app in rural communities
Acknowledgements The research described here was supported by the award made by the RCUK Digital Economy programme to the dot.rural Digital Economy Hub; award reference: EP/G066051/1.Peer reviewedPostprin
World-Sheet Supersymmetry Without Contact Terms
Green and Seiberg showed that, in simple treatments of fermionic string
theory, it is necessary to introduce contact interactions when vertex operators
collide. Otherwise, certain superconformal Ward identities would be violated.
In this note, we show how these contact terms arise naturally when proper
account is taken of the superconformal geometry involved when punctures
collide. More precisely, we show that there is no contact term at all! Rather,
corrections arise to the ``na\"\i ve" formula when the boundary of moduli space
is described correctly.Comment: 14pp., 2 figures (included
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