8,647 research outputs found
Measurement of point velocities in turbulent liquid flow
Turbulent water flow velocity distribution using hot-wire anemometer and photographic technique
The Minimum Wiener Connector
The Wiener index of a graph is the sum of all pairwise shortest-path
distances between its vertices. In this paper we study the novel problem of
finding a minimum Wiener connector: given a connected graph and a set
of query vertices, find a subgraph of that connects all
query vertices and has minimum Wiener index.
We show that The Minimum Wiener Connector admits a polynomial-time (albeit
impractical) exact algorithm for the special case where the number of query
vertices is bounded. We show that in general the problem is NP-hard, and has no
PTAS unless . Our main contribution is a
constant-factor approximation algorithm running in time
.
A thorough experimentation on a large variety of real-world graphs confirms
that our method returns smaller and denser solutions than other methods, and
does so by adding to the query set a small number of important vertices
(i.e., vertices with high centrality).Comment: Published in Proceedings of the 2015 ACM SIGMOD International
Conference on Management of Dat
Expression and regulation of Cek-8, a cell to cell signalling receptor in developing chick limb buds
Wakefield damping for the CLIC crab cavity
A crab cavity is required in the CLIC to allow effective head-on collision of
bunches at the IP. A high operating frequency is preferred as the deflection
voltage required for a given rotation angle and the RF phase tolerance for a
crab cavity are inversely proportional to the operating frequency. The short
bunch spacing of the CLIC scheme and the high sensitivity of the crab cavity to
dipole kicks demand very high damping of the inter-bunch wakes, the major
contributor to the luminosity loss of colliding bunches. This paper
investigates the nature of the wakefields in the CLIC crab cavity and the
possibility of using various damping schemes to suppress them effectively
Population inversion in optically pumped asymmetric quantum well terahertz lasers
Intersubband carrier lifetimes and population ratios are calculated for three- and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga(1 – x)Al(x)As quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation. © 1997 American Institute of Physics
Demand side participation for frequency containment in the web of cells architecture
A large number of demand side management schemes have been proposed in literature for provision of frequency control ancillary services to the network. However, it is assumed that all the flexible devices within the network are managed and controlled under one demand side management (DSM) scheme. In this paper, two independent demand side management schemes control the portfolio of flexible devices within a web of cells architecture. A methodology and scenarios for analysis of the performance of more than one DSM scheme within the same network have been realized using a real-time power hardware-in-the-loop co-simulation platform, and the paper presents this as a basis for investigations of such arrangements
Invariant Homology on Standard Model Manifolds
Torus-fibered Calabi-Yau threefolds Z, with base dP_9 and fundamental group
pi_1(Z)=Z_2 X Z_2, are reviewed. It is shown that Z=X/(Z_2 X Z_2), where X=B
X_{P_1} B' are elliptically fibered Calabi-Yau threefolds that admit a freely
acting Z_2 X Z_2 automorphism group. B and B' are rational elliptic surfaces,
each with a Z_2 X Z_2 group of automorphisms. It is shown that the Z_2 X Z_2
invariant classes of curves of each surface have four generators which produce,
via the fiber product, seven Z_2 X Z_2 invariant generators in H_4(X,Z). All
invariant homology classes are computed explicitly. These descend to produce a
rank seven homology group H_4(Z,Z) on Z. The existence of these homology
classes on Z is essential to the construction of anomaly free, three family
standard-like models with suppressed nucleon decay in both weakly and strongly
coupled heterotic superstring theory.Comment: 57 pages, 13 figure
Torus-Fibered Calabi-Yau Threefolds with Non-Trivial Fundamental Group
We construct smooth Calabi-Yau threefolds Z, torus-fibered over a dP_9 base,
with fundamental group Z_2 X Z_2. To do this, the structure of rational
elliptic surfaces is studied and it is shown that a restricted subset of such
surfaces admit at least a Z_2 X Z_2 group of automorphisms. One then constructs
Calabi-Yau threefolds X as the fiber product of two such dP_9 surfaces,
demonstrating that the involutions on the surfaces lift to a freely acting Z_2
X Z_2 group of automorphisms on X. The threefolds Z are then obtained as the
quotient Z=X/(Z_2 X Z_2). These Calabi-Yau spaces Z admit stable, holomorphic
SU(4) vector bundles which, in conjunction with Z_2 X Z_2 Wilson lines, lead to
standard-like models of particle physics with naturally suppressed nucleon
decay.Comment: 60 pages, 13 figures, Typos correcte
A bio-inspired image coder with temporal scalability
We present a novel bio-inspired and dynamic coding scheme for static images.
Our coder aims at reproducing the main steps of the visual stimulus processing
in the mammalian retina taking into account its time behavior. The main novelty
of this work is to show how to exploit the time behavior of the retina cells to
ensure, in a simple way, scalability and bit allocation. To do so, our main
source of inspiration will be the biologically plausible retina model called
Virtual Retina. Following a similar structure, our model has two stages. The
first stage is an image transform which is performed by the outer layers in the
retina. Here it is modelled by filtering the image with a bank of difference of
Gaussians with time-delays. The second stage is a time-dependent
analog-to-digital conversion which is performed by the inner layers in the
retina. Thanks to its conception, our coder enables scalability and bit
allocation across time. Also, our decoded images do not show annoying artefacts
such as ringing and block effects. As a whole, this article shows how to
capture the main properties of a biological system, here the retina, in order
to design a new efficient coder.Comment: 12 pages; Advanced Concepts for Intelligent Vision Systems (ACIVS
2011
Knots, Braids and BPS States in M-Theory
In previous work we considered M-theory five branes wrapped on elliptic
Calabi-Yau threefold near the smooth part of the discriminant curve. In this
paper, we extend that work to compute the light states on the worldvolume of
five-branes wrapped on fibers near certain singular loci of the discriminant.
We regulate the singular behavior near these loci by deforming the discriminant
curve and expressing the singularity in terms of knots and their associated
braids. There braids allow us to compute the appropriate string junction
lattice for the singularity and,hence to determine the spectrum of light BPS
states. We find that these techniques are valid near singular points with N=2
supersymmetry.Comment: 38 page
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