8,584 research outputs found
Computational coarse graining of a randomly forced 1-D Burgers equation
We explore a computational approach to coarse graining the evolution of the
large-scale features of a randomly forced Burgers equation in one spatial
dimension. The long term evolution of the solution energy spectrum appears
self-similar in time. We demonstrate coarse projective integration and coarse
dynamic renormalization as tools that accelerate the extraction of macroscopic
information (integration in time, self-similar shapes, and nontrivial dynamic
exponents) from short bursts of appropriately initialized direct simulation.
These procedures solve numerically an effective evolution equation for the
energy spectrum without ever deriving this equation in closed form.Comment: 21 pages, 7 figure
Cumulene Molecular Wire Conductance from First Principles
We present first principles calculations of current-voltage characteristics
(IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire
junctions with realistic contacts. The transport properties are calculated
using full self-consistent ab initio NEGF-DFT methods under external bias. The
conductance of the cumulene wires shows oscillatory behavior depending on the
number of carbon atoms (double bonds). Among all conjugated oligomers, we find
that cumulene wires with odd number of carbon atoms yield the highest
conductance with metallic-like ballistic transport behavior. The reason is the
high density of states in broad LUMO levels spanning the Fermi level of the
electrodes. The transmission spectrum and the conductance depend only weakly on
applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V.
Cumulene wires are therefore potential candidates for metallic connections in
nanoelectronic applications.Comment: Accepted in Phys. Rev. B; 5 pages and 6 figure
Top quark pair production via collision in the littlest Higgs model with T-parity at the ILC
In the littlest Higgs model with T-parity, we studied the contributions of
the new particles to the top-quark pair production via collision
at the International Linear Collider. We calculated the top-quark pair
production cross section and found this process can generate significantly
relative correction. The result may be a sensitive probe of the littlest Higgs
model with T-parity
Avoiding unnecessary demerging and remerging of multiâcommodity integer flows
Resource flows may merge and demerge at a network node. Sometimes several demerged flows may be immediately merged again, but in different combinations compared to before they were demerged. However, the demerging is unnecessary in the first place if the total resources at each of the network nodes involved remains unchanged. We describe this situation as âunnecessary demerging and remerging (UDR)â of flows, which would incur unnecessary operations and costs in practice. Multiâcommodity integer flows in particular will be considered in this paper. This deficiency could be theoretically overcome by means of fixedâcharge variables, but the practicality of this approach is restricted by the difficulty in solving the corresponding integer linear program (ILP). Moreover, in a problem where the objective function has many cost elements, it would be helpful if such operational costs are optimized implicitly. This paper presents a heuristic branching method within an ILP solver for removing UDR without the use of fixedâcharge variables. We use the concept of âflow potentialsâ (different from âflow residuesâ for maxâflows) guided by which underutilized arcs are heuristically banned, thus reducing occurrences of UDR. Flow connection bigraphs and flow connection groups (FCGs) are introduced. We prove that if certain conditions are met, fully utilizing an arc will guarantee an improvement within an FCG. Moreover, a location subâmodel is given when the former cannot guarantee an improvement. More importantly, the heuristic approach can significantly enhance the full fixedâcharge model by warmâstarting. Computational experiments based on realâworld instances have shown the usefulness of the proposed methods
Secure & Encrypted Accessing and Sharing of Data in Distributed Virtual Cloud: A Review
Cloud Computing has been accepted as the next generation architecture of IT Enterprise. The Cloud computing idea offers dynamically scalable resources provisioned as a service over and the Internet Economic benefits are the main driver for the Cloud, since it promises the reduction of capital expenditure and operational expenditure Placing critical data in the hands of a cloud provider should come with the guarantee of security and availability for data and in use. various alternatives available for storage services, while data confidentiality is the solutions for the database as a service pattern are still undeveloped This architecture is supporting purely distributed clients to connect directly to an encrypted cloud database, and to execute simultaneous and independent operations including those modifying the database structure. The Access control policy is set out in which only authorised users are able to decrypt the stored information. This scheme prevents from replay attacks and supports formation, modification, and reading data stored in the cloud. This unique attribute, however, creates many new security challenges which have not been well understood. Security is to protect data from danger and vulnerability. There are various dangers and vulnerabilities to be handle. Various security issues and some of their solution are explained and are concentrating mainly on public cloud security issues and their solutions. Data should always be encrypted in a time when stored and transmitted
On Approximating Restricted Cycle Covers
A cycle cover of a graph is a set of cycles such that every vertex is part of
exactly one cycle. An L-cycle cover is a cycle cover in which the length of
every cycle is in the set L. The weight of a cycle cover of an edge-weighted
graph is the sum of the weights of its edges.
We come close to settling the complexity and approximability of computing
L-cycle covers. On the one hand, we show that for almost all L, computing
L-cycle covers of maximum weight in directed and undirected graphs is APX-hard
and NP-hard. Most of our hardness results hold even if the edge weights are
restricted to zero and one.
On the other hand, we show that the problem of computing L-cycle covers of
maximum weight can be approximated within a factor of 2 for undirected graphs
and within a factor of 8/3 in the case of directed graphs. This holds for
arbitrary sets L.Comment: To appear in SIAM Journal on Computing. Minor change
Negative-weight percolation
We describe a percolation problem on lattices (graphs, networks), with edge
weights drawn from disorder distributions that allow for weights (or distances)
of either sign, i.e. including negative weights. We are interested whether
there are spanning paths or loops of total negative weight. This kind of
percolation problem is fundamentally different from conventional percolation
problems, e.g. it does not exhibit transitivity, hence no simple definition of
clusters, and several spanning paths/loops might coexist in the percolation
regime at the same time. Furthermore, to study this percolation problem
numerically, one has to perform a non-trivial transformation of the original
graph and apply sophisticated matching algorithms.
Using this approach, we study the corresponding percolation transitions on
large square, hexagonal and cubic lattices for two types of disorder
distributions and determine the critical exponents. The results show that
negative-weight percolation is in a different universality class compared to
conventional bond/site percolation. On the other hand, negative-weight
percolation seems to be related to the ferromagnet/spin-glass transition of
random-bond Ising systems, at least in two dimensions.Comment: v1: 4 pages, 4 figures; v2: 10 pages, 7 figures, added results, text
and reference
- âŠ