4,031 research outputs found
Detecting multivariate interactions in spatial point patterns with Gibbs models and variable selection
We propose a method for detecting significant interactions in very large
multivariate spatial point patterns. This methodology develops high dimensional
data understanding in the point process setting. The method is based on
modelling the patterns using a flexible Gibbs point process model to directly
characterise point-to-point interactions at different spatial scales. By using
the Gibbs framework significant interactions can also be captured at small
scales. Subsequently, the Gibbs point process is fitted using a
pseudo-likelihood approximation, and we select significant interactions
automatically using the group lasso penalty with this likelihood approximation.
Thus we estimate the multivariate interactions stably even in this setting. We
demonstrate the feasibility of the method with a simulation study and show its
power by applying it to a large and complex rainforest plant population data
set of 83 species
A Modified Scheme of Triplectic Quantization
A modified version of triplectic quantization, first introduce by Batalin and
Martnelius, is proposed which makes use of two independent master equations,
one for the action and one for the gauge functional such that the initial
classical action also obeys that master equation.Comment: 8 page
Loop Integrands for Scattering Amplitudes from the Riemann Sphere
The scattering equations on the Riemann sphere give rise to remarkable
formulae for tree-level gauge theory and gravity amplitudes. Adamo, Casali and
Skinner conjectured a one-loop formula for supergravity amplitudes based on
scattering equations on a torus. We use a residue theorem to transform this
into a formula on the Riemann sphere. What emerges is a framework for loop
integrands on the Riemann sphere that promises to have wide application, based
on off-shell scattering equations that depend on the loop momentum. We present
new formulae, checked explicitly at low points, for supergravity and
super-Yang-Mills amplitudes and for n-gon integrands at one loop. Finally, we
show that the off-shell scattering equations naturally extend to arbitrary loop
order, and we give a proposal for the all-loop integrands for supergravity and
planar super-Yang-Mills theory.Comment: 5 pages, 1 figure. v3: published versio
On the efficient Monte Carlo implementation of path integrals
We demonstrate that the Levy-Ciesielski implementation of Lie-Trotter
products enjoys several properties that make it extremely suitable for
path-integral Monte Carlo simulations: fast computation of paths, fast Monte
Carlo sampling, and the ability to use different numbers of time slices for the
different degrees of freedom, commensurate with the quantum effects. It is
demonstrated that a Monte Carlo simulation for which particles or small groups
of variables are updated in a sequential fashion has a statistical efficiency
that is always comparable to or better than that of an all-particle or
all-variable update sampler. The sequential sampler results in significant
computational savings if updating a variable costs only a fraction of the cost
for updating all variables simultaneously or if the variables are independent.
In the Levy-Ciesielski representation, the path variables are grouped in a
small number of layers, with the variables from the same layer being
statistically independent. The superior performance of the fast sampling
algorithm is shown to be a consequence of these observations. Both mathematical
arguments and numerical simulations are employed in order to quantify the
computational advantages of the sequential sampler, the Levy-Ciesielski
implementation of path integrals, and the fast sampling algorithm.Comment: 14 pages, 3 figures; submitted to Phys. Rev.
Fermion-Boson Interactions and Quantum Algebras
Quantum Algebras (q-algebras) are used to describe interactions between
fermions and bosons. Particularly, the concept of a su_q(2) dynamical symmetry
is invoked in order to reproduce the ground state properties of systems of
fermions and bosons interacting via schematic forces. The structure of the
proposed su_q(2) Hamiltonians, and the meaning of the corresponding deformation
parameters, are discussed.Comment: 20 pages, 10 figures. Physical Review C (in press
Biomass Properties and Gasification Behavior of 7-Year-Old Siberian Elm
Studies were conducted to establish baseline information for use in characterizing Siberian elm as energy or fiber feedstock. Biomass properties were determined. Calorific value (4,698 cal/g) and specific gravity (0.55) of Siberian elm are similar to those of soft elms, and its fiber length is relatively short (1.00 mm). Ash content was 1.65%. Gasification produces a medium energy gas (3 x 106 cal/m3) with yield varying from 0.17 to 0.96 m3/kg over a gasification temperature range of 600 to 700 C. Ovendry yields were 9.8 t/ha annually at 700 trees/ha
Quantifying differences between primary cortical areas in humans based on laminar profiles in in vivo MRI data
This paper presents an approach for mapping the human cortical architecture in vivo based on quantitative MRI indices of myelin. We automatically construct laminar profiles in several primary cortical areas and investigate different sampling strategies. The results demonstrate that our method is able to distinguish these areas at specific cortical depths
Parton distribution functions from nonlocal light-cone operators with definite twist
We introduce the chiral-even and chiral-odd quark distributions as forward
matrix elements of related bilocal quark operators with well-defined
(geometric) twist. Thereby, we achieve a Lorentz invariant classification of
these distributions which differ from the conventional ones by explicitly
taking into account the necessary trace terms. The relations between both kinds
of distribution functions are given and the mismatch between their different
definition of twist is discussed. Wandzura-Wilczek--like relations between the
conventional distributions (based on dynamical twist) are derived by means of
geometric twist distribution functions.Comment: 17 pages, REVTEX, Extended version, The Introduction has been
rewritten, Setion V "Wandzura-Wilczek--like relations" and App. B are added;
Sign errors are correcte
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