9,524 research outputs found
The Use of Historical Data in Rule-Based Modelling for Scenarios to Improve Resilience within the Building Stock
Digital documentation has become integral to the preservation, analysis and communication of historical sites. New platforms are now being developed that involve complex 3D models and allow the analysis of spatial data. These include procedural modelling, a technique that enables the rapid development of ‘dynamic’ 3D environments, and generation of simulations for entire cities, resulting in low cost, high resolution 3D city models. Though procedural modelling has been used in the context of archaeology to ‘recreate’ cities at specific historic time points, the use of historical data in the development of rule-based procedural models for current cities has been little explored. Here, we test the extent to which construction age data, historical building regulations and architectural knowledge can be used in the generation of procedural rules, and the level of detail and potential impact that these models may have. Rather than creating an accurate representation of the city, we instead seek to simulate the way in which urban areas are likely to behave under certain conditions, in order to test what-if? planning scenarios. This allows us to explore more flexible ways of digitally ‘creating’ cities, past and present, and to gain insights into underlying ‘rules’ that govern their physical form
Ten Years of Solar Change as Monitored by SBUV and SBUV/2
Observations of the Sun by the Solar Backscatter Ultraviolet (SBUV) instrument aboard Nimbus 7 and the SBUV/2 instrument aboard NOAA-9 reveal variations in the solar irradiance from 1978, to 1988. The maximum to minimum solar change estimated from the Heath and Schlesinger Mg index and wavelength scaling factors is about 4 percent from 210 to 260 nm and 8 percent for 180 to 210 nm; direct measurements of the solar change give values of 1 to 3 percent and 5 to 7 percent, respectively, for the same wavelength range. Solar irradiances were high from the start of observations, late in 1978, until 1983, declined until early 1985, remained approximately constant until mid-1987, and then began to rise. Peak-to-peak 27-day rotational modulation amplitudes were as large as 6 percent at solar maximum and 1 to 2 percent at solar minimum. During occasional intervals of the 1979 to 1983 maximum and again during 1988, the dominant rotational modulation period was 13.5 days. Measurements near 200 to 205 nm show the same rotational modulation behavior but cannot be used to track long-term changes in the Sun because of uncertainties in the characterization of long-term instrument sensitivity changes
Gene-history correlation and population structure
Correlation of gene histories in the human genome determines the patterns of
genetic variation (haplotype structure) and is crucial to understanding genetic
factors in common diseases. We derive closed analytical expressions for the
correlation of gene histories in established demographic models for genetic
evolution and show how to extend the analysis to more realistic (but more
complicated) models of demographic structure. We identify two contributions to
the correlation of gene histories in divergent populations: linkage
disequilibrium, and differences in the demographic history of individuals in
the sample. These two factors contribute to correlations at different length
scales: the former at small, and the latter at large scales. We show that
recent mixing events in divergent populations limit the range of correlations
and compare our findings to empirical results on the correlation of gene
histories in the human genome.Comment: Revised and extended version: 26 pages, 5 figures, 1 tabl
The Time Machine: A Simulation Approach for Stochastic Trees
In the following paper we consider a simulation technique for stochastic
trees. One of the most important areas in computational genetics is the
calculation and subsequent maximization of the likelihood function associated
to such models. This typically consists of using importance sampling (IS) and
sequential Monte Carlo (SMC) techniques. The approach proceeds by simulating
the tree, backward in time from observed data, to a most recent common ancestor
(MRCA). However, in many cases, the computational time and variance of
estimators are often too high to make standard approaches useful. In this paper
we propose to stop the simulation, subsequently yielding biased estimates of
the likelihood surface. The bias is investigated from a theoretical point of
view. Results from simulation studies are also given to investigate the balance
between loss of accuracy, saving in computing time and variance reduction.Comment: 22 Pages, 5 Figure
The Anderson prescription for surfaces and impurities
We test the Anderson prescription [1], a BCS formalism for describing
superconductivity in inhomogeneous systems, and compare results with those
obtained from the Bogoliubov-de Gennes formalism, using the attractive Hubbard
model with surfaces and nonmagnetic impurities. The Anderson approach captures
the essential features of the spatial variation of the gap parameter and
electron density around a surface or an impurity over a wide range of
parameters. It breaks down, however, in the strong-coupling regime for a weak
impurity potential.
[1] P. W. Anderson, J. Phys. Chem. Solids 11, 26 (1959).Comment: 4 pages, 4 figure
Extremality of Gaussian quantum states
We investigate Gaussian quantum states in view of their exceptional role
within the space of all continuous variables states. A general method for
deriving extremality results is provided and applied to entanglement measures,
secret key distillation and the classical capacity of Bosonic quantum channels.
We prove that for every given covariance matrix the distillable secret key rate
and the entanglement, if measured appropriately, are minimized by Gaussian
states. This result leads to a clearer picture of the validity of frequently
made Gaussian approximations. Moreover, it implies that Gaussian encodings are
optimal for the transmission of classical information through Bosonic channels,
if the capacity is additive.Comment: 4 page
First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI
We present the first results using the Reuven Ramaty High-Energy Solar
Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated
with active regions, sunspots or flares (the quiet Sun). Using a newly
developed chopping technique (fan-beam modulation) during seven periods of
offpointing between June 2005 to October 2006, we obtained upper limits over
3-200 keV for the quietest times when the GOES12 1-8A flux fell below
Wm. These values are smaller than previous limits in the 17-120 keV
range and extend them to both lower and higher energies. The limit in 3-6 keV
is consistent with a coronal temperature MK. For quiet Sun periods
when the GOES12 1-8A background flux was between Wm and
Wm, the RHESSI 3-6 keV flux correlates to this as a power-law,
with an index of . The power-law correlation for microflares has
a steeper index of . We also discuss the possibility of
observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun
limits to estimate the axion-to-photon coupling constant for two different
axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter
Nonlinearity and stochasticity in the density--velocity relation
We present results of the investigations of the statistical properties of a
joint density and velocity divergence probability distribution function (PDF)
in the mildly non-linear regime. For that purpose we use both perturbation
theory results, extended here for a top-hat filter, and numerical simulations.
In particular we derive the quantitative (complete as possible up to third
order terms) and qualitative predictions for constrained averages and
constrained dispersions -- which describe the nonlinearities and the
stochasticity properties beyond the linear regime -- and compare them against
numerical simulations. We find overall a good agreement for constrained
averages; however, the agreement for constrained dispersions is only
qualitative. Scaling relations for the Omega-dependence of these quantities are
satisfactory reproduced.
Guided by our analytical and numerical results, we finally construct a robust
phenomenological description of the joint PDF in a closed analytic form. The
good agreement of our formula with results of N-body simulations for a number
of cosmological parameters provides a sound validation of the presented
approach.
Our results provide a basis for a potentially powerful tool with which it is
possible to analyze galaxy survey data in order to test the gravitational
instability paradigm beyond the linear regime and put useful constraints on
cosmological parameters. In particular we show how the nonlinearity in the
density--velocity relation can be used to break the so-called Omega-bias
degeneracy in cosmic density-velocity comparisons.Comment: 12 pages, 11 figures; revised version with minor changes in the
presentation, accepted for publication in MNRA
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