2,952 research outputs found
Frequency selective reflection and transmission at a layer composed of a periodic dielectric
The feasibility of using a periodic dielectric layer, composed of alternating bars having dielectric constants epsilon sub 1 and epsilon sub 2, as a frequency selective subreflector in order to permit feed separation in large aperture reflecting antenna systems was examined. For oblique incidence, it is found that total transmission and total reflection can be obtained at different frequencies for proper choices of epsilon sub 1, epsilon 2, and the geometric parameters. The frequencies of total reflection and transmission can be estimated from wave phenomena occurring in a layer of uniform dielectric constant equal to the average for the periodic layers. About some of the frequencies of total transmission, the bandwidth for 90% transmission is found to be 40%. However, the bandwidth for 90% reflection is always found to be much narrower; the greatest value found being 2.5%
Evidence for a Finite Temperature Insulator
In superconductors the zero-resistance current-flow is protected from
dissipation at finite temperatures (T) by virtue of the short-circuit condition
maintained by the electrons that remain in the condensed state. The recently
suggested finite-T insulator and the "superinsulating" phase are different
because any residual mechanism of conduction will eventually become dominant as
the finite-T insulator sets-in. If the residual conduction is small it may be
possible to observe the transition to these intriguing states. We show that the
conductivity of the high magnetic-field insulator terminating superconductivity
in amorphous indium-oxide exhibits an abrupt drop, and seem to approach a zero
conductance at T<0.04 K. We discuss our results in the light of theories that
lead to a finite-T insulator
A Masked Pruning Approach for Dimensionality Reduction in Communication-Efficient Federated Learning Systems
Federated Learning (FL) represents a growing machine learning (ML) paradigm
designed for training models across numerous nodes that retain local datasets,
all without directly exchanging the underlying private data with the parameter
server (PS). Its increasing popularity is attributed to notable advantages in
terms of training deep neural network (DNN) models under privacy aspects and
efficient utilization of communication resources. Unfortunately, DNNs suffer
from high computational and communication costs, as well as memory consumption
in intricate tasks. These factors restrict the applicability of FL algorithms
in communication-constrained systems with limited hardware resources.
In this paper, we develop a novel algorithm that overcomes these limitations
by synergistically combining a pruning-based method with the FL process,
resulting in low-dimensional representations of the model with minimal
communication cost, dubbed Masked Pruning over FL (MPFL). The algorithm
operates by initially distributing weights to the nodes through the PS.
Subsequently, each node locally trains its model and computes pruning masks.
These low-dimensional masks are then transmitted back to the PS, which
generates a consensus pruning mask, broadcasted back to the nodes. This
iterative process enhances the robustness and stability of the masked pruning
model. The generated mask is used to train the FL model, achieving significant
bandwidth savings. We present an extensive experimental study demonstrating the
superior performance of MPFL compared to existing methods. Additionally, we
have developed an open-source software package for the benefit of researchers
and developers in related fields.Comment: 12 pages, 9 figure
The Geometric Maximum Traveling Salesman Problem
We consider the traveling salesman problem when the cities are points in R^d
for some fixed d and distances are computed according to geometric distances,
determined by some norm. We show that for any polyhedral norm, the problem of
finding a tour of maximum length can be solved in polynomial time. If
arithmetic operations are assumed to take unit time, our algorithms run in time
O(n^{f-2} log n), where f is the number of facets of the polyhedron determining
the polyhedral norm. Thus for example we have O(n^2 log n) algorithms for the
cases of points in the plane under the Rectilinear and Sup norms. This is in
contrast to the fact that finding a minimum length tour in each case is
NP-hard. Our approach can be extended to the more general case of quasi-norms
with not necessarily symmetric unit ball, where we get a complexity of
O(n^{2f-2} log n).
For the special case of two-dimensional metrics with f=4 (which includes the
Rectilinear and Sup norms), we present a simple algorithm with O(n) running
time. The algorithm does not use any indirect addressing, so its running time
remains valid even in comparison based models in which sorting requires Omega(n
\log n) time. The basic mechanism of the algorithm provides some intuition on
why polyhedral norms allow fast algorithms.
Complementing the results on simplicity for polyhedral norms, we prove that
for the case of Euclidean distances in R^d for d>2, the Maximum TSP is NP-hard.
This sheds new light on the well-studied difficulties of Euclidean distances.Comment: 24 pages, 6 figures; revised to appear in Journal of the ACM.
(clarified some minor points, fixed typos
Recommended from our members
Structure of amyloid-β (20-34) with Alzheimer's-associated isomerization at Asp23 reveals a distinct protofilament interface.
Amyloid-β (Aβ) harbors numerous posttranslational modifications (PTMs) that may affect Alzheimer's disease (AD) pathogenesis. Here we present the 1.1 Å resolution MicroED structure of an Aβ 20-34 fibril with and without the disease-associated PTM, L-isoaspartate, at position 23 (L-isoAsp23). Both wild-type and L-isoAsp23 protofilaments adopt β-helix-like folds with tightly packed cores, resembling the cores of full-length fibrillar Aβ structures, and both self-associate through two distinct interfaces. One of these is a unique Aβ interface strengthened by the isoaspartyl modification. Powder diffraction patterns suggest a similar structure may be adopted by protofilaments of an analogous segment containing the heritable Iowa mutation, Asp23Asn. Consistent with its early onset phenotype in patients, Asp23Asn accelerates aggregation of Aβ 20-34, as does the L-isoAsp23 modification. These structures suggest that the enhanced amyloidogenicity of the modified Aβ segments may also reduce the concentration required to achieve nucleation and therefore help spur the pathogenesis of AD
Solving a "Hard" Problem to Approximate an "Easy" One: Heuristics for Maximum Matchings and Maximum Traveling Salesman Problems
We consider geometric instances of the Maximum Weighted Matching Problem
(MWMP) and the Maximum Traveling Salesman Problem (MTSP) with up to 3,000,000
vertices. Making use of a geometric duality relationship between MWMP, MTSP,
and the Fermat-Weber-Problem (FWP), we develop a heuristic approach that yields
in near-linear time solutions as well as upper bounds. Using various
computational tools, we get solutions within considerably less than 1% of the
optimum.
An interesting feature of our approach is that, even though an FWP is hard to
compute in theory and Edmonds' algorithm for maximum weighted matching yields a
polynomial solution for the MWMP, the practical behavior is just the opposite,
and we can solve the FWP with high accuracy in order to find a good heuristic
solution for the MWMP.Comment: 20 pages, 14 figures, Latex, to appear in Journal of Experimental
Algorithms, 200
Excitation of guided waves in layered structures with negative refraction
We study the electromagnetic beam reflection from layered structures that
include the so-called double-negative materials, also called left-handed
metamaterials. We predict that such structures can demonstrate a giant lateral
Goos-Hanchen shift of the scattered beam accompanied by splitting of the
reflected and transmitted beams due to the resonant excitation of surface waves
at the interfaces between the conventional and double-negative materials as
well as due to excitation of leaky modes in the layered structures. The beam
shift can be either positive or negative, depending on the type of the guided
waves excited by the incoming beam. We also perform finite-difference
time-domain simulations and confirm the major effects predicted analytically.Comment: 13 pqages, 10 figures. Also available at
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-2-48
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