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Risk reduction and real estate portfolio size
Despite a number of papers that discuss the advantages of increased size on risk levels in real estate portfolios there is remarkably little empirical evidence based on actual portfolios. The objective of this paper is to remedy this deficiency by examining the portfolio risk of a large sample of actual property data over the period 1981 to 1996. The results show that all that can be said is that portfolios of properties of a large size, on the average, tend to have lower risks than small sized portfolios. More importantly portfolios of a few properties can have very high or very low risk
Using ultra-short pulses to determine particle size and density distributions
We analyze the time dependent response of strongly scattering media (SSM) to
ultra-short pulses of light. A random walk technique is used to model the
optical scattering of ultra-short pulses of light propagating through media
with random shapes and various packing densities. The pulse spreading was found
to be strongly dependent on the average particle size, particle size
distribution, and the packing fraction. We also show that the intensity as a
function of time-delay can be used to analyze the particle size distribution
and packing fraction of an optically thick sample independently of the presence
of absorption features. Finally, we propose an all new way to measure the shape
of ultra-short pulses that have propagated through a SSM.Comment: 15 pages, 29 figures, accepted for publication in Optics Express will
update with full reference when it is availabl
On the "Poisson Trick" and its Extensions for Fitting Multinomial Regression Models
This article is concerned with the fitting of multinomial regression models
using the so-called "Poisson Trick". The work is motivated by Chen & Kuo (2001)
and Malchow-M{\o}ller & Svarer (2003) which have been criticized for being
computationally inefficient and sometimes producing nonsense results. We first
discuss the case of independent data and offer a parsimonious fitting strategy
when all covariates are categorical. We then propose a new approach for
modelling correlated responses based on an extension of the Gamma-Poisson
model, where the likelihood can be expressed in closed-form. The parameters are
estimated via an Expectation/Conditional Maximization (ECM) algorithm, which
can be implemented using functions for fitting generalized linear models
readily available in standard statistical software packages. Compared to
existing methods, our approach avoids the need to approximate the intractable
integrals and thus the inference is exact with respect to the approximating
Gamma-Poisson model. The proposed method is illustrated via a reanalysis of the
yogurt data discussed by Chen & Kuo (2001)
The structures, binding energies and vibrational frequencies of Ca3 and Ca4: An application of the CCSD(T) method
The Ca3 and Ca4 metallic clusters have been investigated using state-of-the-art ab initio quantum mechanical methods. Large atomic natural orbital basis sets have been used in conjunction with the singles and doubles coupled-cluster (CCSD) method, a coupled-cluster method that includes a perturbational estimate of connected triple excitations, denoted CCSD(T), and the multireference configuration interaction (MRCI) method. The equilibrium geometries, binding energies and harmonic vibrational frequencies have been determined with each of the methods so that the accuracy of the coupled-cluster methods may be assessed. Since the CCSD(T) method reproduces the MRCI results very well, cubic and quartic force fields of Ca3 and Ca4 have been determined using this approach and used to evaluate the fundamental vibrational frequencies. The infrared intensities of both the e' mode of Ca3 and the t2 mode of Ca4 are found to be small. The results obtained in this study are compared and contrasted with those from our earlier studies on small Be and Mg clusters
Comparison of the quadratic configuration interaction and coupled cluster approaches to electron correlation including the effect of triple excitations
The recently proposed quadratic configuration interaction (QCI) method is compared with the more rigorous coupled cluster (CC) approach for a variety of chemical systems. Some of these systems are well represented by a single-determinant reference function and others are not. The finite order singles and doubles correlation energy, the perturbational triples correlation energy, and a recently devised diagnostic for estimating the importance of multireference effects are considered. The spectroscopic constants of CuH, the equilibrium structure of cis-(NO)2 and the binding energies of Be3, Be4, Mg3, and Mg4 were calculated using both approaches. The diagnostic for estimating multireference character clearly demonstrates that the QCI method becomes less satisfactory than the CC approach as non-dynamical correlation becomes more important, in agreement with a perturbational analysis of the two methods and the numerical estimates of the triple excitation energies they yield. The results for CuH show that the differences between the two methods become more apparent as the chemical systems under investigation becomes more multireference in nature and the QCI results consequently become less reliable. Nonetheless, when the system of interest is dominated by a single reference determinant both QCI and CC give very similar results
Kink-Antikink Unbinding Transition in the Two Dimensional Fully Frustrated XY Model
We carry out the first numerical simulations to directly confirm the
existence of a kink-antikink unbinding transition, T_w, along Ising-like domain
walls in the two dimensional fully frustrated XY model. We comment on the
possible implications of kink-antikink unbinding for the bulk phase transitions
of the model.Comment: 8 pages, 10 figures - expanded version of original submission as
accepted by Phys. Rev. B. Some correction to the analysis lead to an increase
in the value of the kink unbinding transition temperatur
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