410 research outputs found
Optimally Adapted Meshes for Finite Elements of Arbitrary Order and W1p Norms
Given a function f defined on a bidimensional bounded domain and a positive
integer N, we study the properties of the triangulation that minimizes the
distance between f and its interpolation on the associated finite element
space, over all triangulations of at most N elements. The error is studied in
the W1p norm and we consider Lagrange finite elements of arbitrary polynomial
order m-1. We establish sharp asymptotic error estimates as N tends to infinity
when the optimal anisotropic triangulation is used. A similar problem has been
studied earlier, but with the error measured in the Lp norm. The extension of
this analysis to the W1p norm is crucial in order to match more closely the
needs of numerical PDE analysis, and it is not straightforward. In particular,
the meshes which satisfy the optimal error estimate are characterized by a
metric describing the local aspect ratio of each triangle and by a geometric
constraint on their maximal angle, a second feature that does not appear for
the Lp error norm. Our analysis also provides with practical strategies for
designing meshes such that the interpolation error satisfies the optimal
estimate up to a fixed multiplicative constant. We discuss the extension of our
results to finite elements on simplicial partitions of a domain of arbitrary
dimension, and we provide with some numerical illustration in two dimensions.Comment: 37 pages, 6 figure
Smooth-AP: Smoothing the Path Towards Large-Scale Image Retrieval
Optimising a ranking-based metric, such as Average Precision (AP), is
notoriously challenging due to the fact that it is non-differentiable, and
hence cannot be optimised directly using gradient-descent methods. To this end,
we introduce an objective that optimises instead a smoothed approximation of
AP, coined Smooth-AP. Smooth-AP is a plug-and-play objective function that
allows for end-to-end training of deep networks with a simple and elegant
implementation. We also present an analysis for why directly optimising the
ranking based metric of AP offers benefits over other deep metric learning
losses. We apply Smooth-AP to standard retrieval benchmarks: Stanford Online
products and VehicleID, and also evaluate on larger-scale datasets: INaturalist
for fine-grained category retrieval, and VGGFace2 and IJB-C for face retrieval.
In all cases, we improve the performance over the state-of-the-art, especially
for larger-scale datasets, thus demonstrating the effectiveness and scalability
of Smooth-AP to real-world scenarios.Comment: Accepted at ECCV 202
A Graphene Surface Force Balance
We report a method for transferring graphene, grown
by chemical vapor deposition, which produces ultraflat graphene
surfaces (root-mean-square roughness of 0.19 nm) free from
polymer residues over macroscopic areas (>1 cm2). The critical
step in preparing such surfaces involves the use of an intermediate
mica template, which itself is atomically smooth. We demonstrate
the compatibility of these model surfaces with the surface force
balance, opening up the possibility of measuring normal and lateral
forces, including friction and adhesion, between two graphene sheets
either in contact or across a liquid medium. The conductivity of the
graphene surfaces allows forces to be measured while controlling the
surface potential. This new apparatus, the graphene surface force
balance, is expected to be of importance to the future understanding
of graphene in applications from lubrication to electrochemical energy storage systems
Knaster's problem for -symmetric subsets of the sphere
We prove a Knaster-type result for orbits of the group in
, calculating the Euler class obstruction. Among the consequences
are: a result about inscribing skew crosspolytopes in hypersurfaces in , and a result about equipartition of a measures in
by -symmetric convex fans
Description of the two-nucleon system on the basis of the Bargmann representation of the S matrix
For the effective-range function , a pole approximation that
involves a small number of parameters is derived on the basis of the Bargmann
representation of the matrix. The parameters of this representation, which
have a clear physical meaning, are related to the parameters of the Bargmann
matrix by simple equations. By using a polynomial least-squares fit to the
function at low energies, the triplet low-energy parameters of
neutron-proton scattering are obtained for the latest experimental data of
Arndt et al. on phase shifts. The results are fm, fm, and fm. With allowance for the values found for the
low-energy scattering parameters and for the pole parameter, the pole
approximation of the function provides an excellent description
of the triplet phase shift for neutron-proton scattering over a wide energy
range (MeV), substantially improving the
description at low energies as well. For the experimental phase shifts of Arndt
et al., the triplet shape parameters of the effective-range expansion
are obtained by using the pole approximation. The description of the phase
shift by means of the effective-range expansion featuring values found for the
low-energy scattering parameters proves to be fairly accurate over a broad
energy region extending to energy values approximately equal to the energy at
which this phase shift changes sign, this being indicative of a high accuracy
and a considerable value of the effective-range expansion in describing
experimental data on nucleon-nucleon scattering. The properties of the deuteron
that were calculated by using various approximations of the effective-range
function comply well with their experimental values.Comment: 39 pages, 3 figure
Linear Relaxation Processes Governed by Fractional Symmetric Kinetic Equations
We get fractional symmetric Fokker - Planck and Einstein - Smoluchowski
kinetic equations, which describe evolution of the systems influenced by
stochastic forces distributed with stable probability laws. These equations
generalize known kinetic equations of the Brownian motion theory and contain
symmetric fractional derivatives over velocity and space, respectively. With
the help of these equations we study analytically the processes of linear
relaxation in a force - free case and for linear oscillator. For a weakly
damped oscillator we also get kinetic equation for the distribution in slow
variables. Linear relaxation processes are also studied numerically by solving
corresponding Langevin equations with the source which is a discrete - time
approximation to a white Levy noise. Numerical and analytical results agree
quantitatively.Comment: 30 pages, LaTeX, 13 figures PostScrip
Entropic Uncertainty Relations in Quantum Physics
Uncertainty relations have become the trademark of quantum theory since they
were formulated by Bohr and Heisenberg. This review covers various
generalizations and extensions of the uncertainty relations in quantum theory
that involve the R\'enyi and the Shannon entropies. The advantages of these
entropic uncertainty relations are pointed out and their more direct connection
to the observed phenomena is emphasized. Several remaining open problems are
mentionedComment: 35 pages, review pape
The role of glacier mice in the invertebrate colonisation of glacial surfaces: the moss balls of the Falljökull, Iceland
Glacier surfaces have a surprisingly complex ecology. Cryoconite holes contain diverse invertebrate communities while other invertebrates, such as Collembola often graze on algae and windblown dead organic on the glacier surface. Glacier mice (ovoid unattached moss balls) occur on some glaciers worldwide. Studies of these glacier mice have concentrated on their occurrence and mode of formation. There are no reports of the invertebrate communities. But, such glacier mice may provide a suitable favourable habitat and refuge for a variety of invertebrate groups to colonise the glacier surface. Here we describe the invertebrate fauna of the glacier mice (moss balls) of the Falljökull, Iceland. The glacier mice were composed of Racomitrium sp. and varied in size from 8.0 to 10.0 cm in length. All glacier mice studied contained invertebrates. Two species of Collembola were present. Pseudisotoma sensibilis (Tullberg, 1876) was numerically dominant with between 12 and 73 individuals per glacier mouse while Desoria olivacea (Tullberg, 1871) occurred but in far lower numbers. Tardigrada and Nematoda had mean densities of approximately 200 and 1,000 respectively. No Acari, Arachnida or Enchytraeidae were observed which may be related to the difficulty these groups have in colonizing the glacier mice. We suggest that glacier mice provide an unusual environmentally ameliorated microhabitat for an invertebrate community dwelling on a glacial surface. The glacier mice thereby enable an invertebrate fauna to colonise an otherwise largely inhospitable location with implications for carbon flow in the system
Exotic smooth structures and symplectic forms on closed manifolds
We give a short proof of the (known) result that there are no Kaehler
structures on exotic tori. This yields a negative solution to a problem posed
by Benson and Gordon. W discuss the symplectic version of the problem and
analyze results which yield an evidence for the conjecture that there are no
symplectic structures on exotic tori.Comment: AMSLaTeX, 16 pages, a new version. A survey of the symplectic version
of the problem is adde
Comparison of crystallization characteristics and mechanical properties of polypropylene processed by ultrasound and conventional micro injection molding
YesUltrasound injection molding has emerged as an alternative production route for the manufacturing of micro-scale polymeric components, where it offers significant benefits over the conventional micro-injection molding process. In this work, the effects of ultrasound melting on the mechanical and morphological properties of micro-polypropylene parts were characterized. The ultrasound injection molding process was experimentally compared to the conventional micro-injection molding process using a novel mold, which allows mounting on both machines and visualization of the melt flow for both molding processes. Direct measurements of the flow front speed and temperature distributions were performed using both conventional and thermal high-speed imaging techniques. The manufacturing of micro-tensile specimens allowed the comparison of the mechanical properties of the parts obtained with the different processes. The results indicated that the ultrasound injection molding process could be an efficient alternative to the conventional process
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