2,104 research outputs found
Improvements on "Fast space-variant elliptical filtering using box splines"
It is well-known that box filters can be efficiently computed using
pre-integrations and local finite-differences
[Crow1984,Heckbert1986,Viola2001]. By generalizing this idea and by combining
it with a non-standard variant of the Central Limit Theorem, a constant-time or
O(1) algorithm was proposed in [Chaudhury2010] that allowed one to perform
space-variant filtering using Gaussian-like kernels. The algorithm was based on
the observation that both isotropic and anisotropic Gaussians could be
approximated using certain bivariate splines called box splines. The attractive
feature of the algorithm was that it allowed one to continuously control the
shape and size (covariance) of the filter, and that it had a fixed
computational cost per pixel, irrespective of the size of the filter. The
algorithm, however, offered a limited control on the covariance and accuracy of
the Gaussian approximation. In this work, we propose some improvements by
appropriately modifying the algorithm in [Chaudhury2010].Comment: 7 figure
Deterministic Global Attitude Estimation
A deterministic attitude estimation problem for a rigid body in an attitude
dependent potential field with bounded measurement errors is studied. An
attitude estimation scheme that does not use generalized coordinate
representations of the attitude is presented here. Assuming that the initial
attitude, angular velocity and measurement noise lie within given ellipsoidal
bounds, an uncertainty ellipsoid that bounds the attitude and the angular
velocity of the rigid body is obtained. The center of the uncertainty ellipsoid
provides point estimates, and its size gives the accuracy of the estimates. The
point estimates and the uncertainty ellipsoids are propagated using a Lie group
variational integrator and its linearization, respectively. The estimation
scheme is optimal in the sense that the attitude estimation error and the size
of the uncertainty ellipsoid is minimized at each measurement instant, and it
is global since the attitude is represented by a rotation matrix.Comment: IEEE Conference on Decision and Control, 2006. 6 pages, 6 figure
Effect of disorder on the electronic properties of graphene: a theoretical approach
In order to manipulate the properties of graphene, its very important to
understand the electronic structure in presence of disorder. We investigate,
within a tight-binding description, the effects of disorder in the on-site
(diagonal disorder) term in the Hamiltonian as well as in the hopping integral
(off-diagonal disorder) on the electronic dispersion and density of states by
augmented space recursion method. Extrinsic off-diagonal disorder is shown to
have dramatic effects on the two-dimensional Dirac-cone, including asymmetries
in the band structures as well as the presence of discontinuous bands in
certain limits. Disorder-induced broadening, related to the scattering length
(or life-time) of electrons, is modified significantly with the increasing
strength of disorder. We propose that our theory is suitable to study the
effects of disorder in other 2D materials, e.g., a boron nitride monolayer.Comment: 11 pages, 8 figure
The metallicity dependence of envelope inflation in massive stars
Recently it has been found that models of massive stars reach the Eddington
limit in their interior, which leads to dilute extended envelopes. We perform a
comparative study of the envelope properties of massive stars at different
metallicities, with the aim to establish the impact of the stellar metallicity
on the effect of envelope inflation. We analyse published grids of
core-hydrogen burning massive star models computed with metallicities
appropriate for massive stars in the Milky Way, the LMC and the SMC, the very
metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition.
Stellar models of all the investigated metallicities reach and exceed the
Eddington limit in their interior, aided by the opacity peaks of iron, helium
and hydrogen, and consequently develop inflated envelopes. Envelope inflation
leads to a redward bending of the zero-age main sequence and a broadening of
the main sequence band in the upper part of the Hertzsprung-Russell diagram. We
derive the limiting L/M-values as function of the stellar surface temperature
above which inflation occurs, and find them to be larger for lower metallicity.
While Galactic models show inflation above ~29 Msun, the corresponding mass
limit for Population III stars is ~150 Msun. While the masses of the inflated
envelopes are generally small, we find that they can reach 1-100 Msun in models
with effective temperatures below ~8000 K, with higher masses reached by models
of lower metallicity. Envelope inflation is expected to occur in sufficiently
massive stars at all metallicities, and is expected to lead to rapidly growing
pulsations, high macroturbulent velocities, and might well be related to the
unexplained variability observed in Luminous Blue Variables like S Doradus and
Eta Carina.Comment: 16 pages (with Appendix), accepted in A&
Interplay of 4f-3d Magnetism and Ferroelectricity in DyFeO3
DyFeO3 exhibits a weak ferromagnetism (TNFe ~ 645 K) that disappears below a
spin-reorientation (Morin) transition at TSRFe ~ 50 K. It is also known that
applied magnetic field induces ferroelectricity at the magnetic ordering
temperature of Dy-ions (TNDy ~ 4.5 K). Here, we show that the ferroelectricity
exists in the weak ferromagnetic state (TSRFe < T < TN,C) without applying
magnetic field, indicating the crucial role of weak ferromagnetism in inducing
ferroelectricity. 57Fe M\"ossbauer studies show that hyperfine field (Bhf)
deviates from mean field-like behaviour that is observed in the weak
ferromagnetic state and decreases below the onset of spin-reorientation
transition (80 K), implying that the Bhf above TSR had additional contribution
from Dy-ions due to induced magnetization by the weak ferromagnetic moment of
Fe-sublattice and below TSR, this contribution decreases due to collinear
ordering of Fe-sublattice. These results clearly demonstrate the presence of
magnetic interactions between Dy(4f) and Fe(3d) and their correlation with
ferroelectricity in the weak ferromagnetic state of DyFeO3.Comment: 5 pages, 6 figures, published in EP
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