1,353 research outputs found
THE BATTLE OVER STOCKHOLDERS VOICE: A CRITIQUE OF AGAR V. JUDY AND THE STANDARD OF REVIEW PROBLEM IN MANIPULATION OF STOCKHOLDERSâ FIRST AMENDMENT RIGHTS
Subset feedback vertex set is fixed parameter tractable
The classical Feedback Vertex Set problem asks, for a given undirected graph
G and an integer k, to find a set of at most k vertices that hits all the
cycles in the graph G. Feedback Vertex Set has attracted a large amount of
research in the parameterized setting, and subsequent kernelization and
fixed-parameter algorithms have been a rich source of ideas in the field.
In this paper we consider a more general and difficult version of the
problem, named Subset Feedback Vertex Set (SUBSET-FVS in short) where an
instance comes additionally with a set S ? V of vertices, and we ask for a set
of at most k vertices that hits all simple cycles passing through S. Because of
its applications in circuit testing and genetic linkage analysis SUBSET-FVS was
studied from the approximation algorithms perspective by Even et al.
[SICOMP'00, SIDMA'00].
The question whether the SUBSET-FVS problem is fixed-parameter tractable was
posed independently by Kawarabayashi and Saurabh in 2009. We answer this
question affirmatively. We begin by showing that this problem is
fixed-parameter tractable when parametrized by |S|. Next we present an
algorithm which reduces the given instance to 2^k n^O(1) instances with the
size of S bounded by O(k^3), using kernelization techniques such as the
2-Expansion Lemma, Menger's theorem and Gallai's theorem. These two facts allow
us to give a 2^O(k log k) n^O(1) time algorithm solving the Subset Feedback
Vertex Set problem, proving that it is indeed fixed-parameter tractable.Comment: full version of a paper presented at ICALP'1
Evaluating the Performance of TCP Stacks for High-Speed Networks.
In this paper we present experimental results evaluating
the performance of the Scalable-TCP, HS-TCP, BICTCP,
FAST-TCP and H-TCP proposals for changes to the TCP
congestion control algorithm to improve performance in highspeed network paths
Evaluating the Performance of TCP Stacks for High-Speed Networks.
In this paper we present experimental results evaluating
the performance of the Scalable-TCP, HS-TCP, BICTCP,
FAST-TCP and H-TCP proposals for changes to the TCP
congestion control algorithm to improve performance in highspeed network paths
A close examination of the structure and dynamics of HC(NH2)2PbI3 by MD simulations and group theory
International audienceThe formamidinium lead iodide hybrid perovskite is studied using first principles molecular dynamics simulations and further analyzed using group theory. The simulations are performed on large supercells containing 768 atoms under isothermal and fully anisotropic isobaric conditions. Two trajectories, one at 300 K and another at 450 K, were extended for over 50 ps in order to perform a detailed assessment of the rotational dynamic of the organic cations. The characteristic rotations of the cation are analyzed by defining two rotation axes. It is found that the formamidinium molecules rotate preferentially around the direction parallel to the line connecting the two nitrogen atoms. The rotational dynamics shows some characteristics already observed in methylammonium lead iodide, like a heterogeneous dynamic at room temperature that disappears at 450 K. The orientational probability of the molecules is explored in terms of an expansion in cubic harmonics up to the 12 th order. It reveals a strong directionality at room temperature that relaxes when increasing the temperature. These findings are further rationalized using Landau and group theories suggesting a mixed displacive/order-disorder structural instability at lower temperatures
Vibrational properties of 2H-PbI2 semiconductors studied via Density Functional Theory calculations
International audienceDensity Functional Theory is used to study the vibrational properties of 2H-PbI2 semiconductor. The Born charge tensors are determined. Calculated phonon frequencies at the Brillouin zone center are compared to Raman scattering and IR absorption measurements. The computed Raman spectra show a good agreement with available experimental data. The simulated phonon dispersion curves are compared with triple-axis neutron scattering measurements
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
Computational design of high performance hybrid perovskite on silicon tandem solar cells
In this study, the optoelectronic properties of a monolithically integrated
series-connected tandem solar cell are simulated. Following the large success
of hybrid organic-inorganic perovskites, which have recently demonstrated large
efficiencies with low production costs, we examine the possibility of using the
same perovskites as absorbers in a tandem solar cell. The cell consists in a
methylammonium mixed bromide-iodide lead perovskite, CH3NH3PbI3(1-x)Br3x (0 < x
< 1), top sub-cell and a single-crystalline silicon bottom sub-cell. A Si-based
tunnel junction connects the two sub-cells. Numerical simulations are based on
a one-dimensional numerical drift-diffusion model. It is shown that a top cell
absorbing material with 20% of bromide and a thickness in the 300-400 nm range
affords current matching with the silicon bottom cell. Good interconnection
between single cells is ensured by standard n and p doping of the silicon at
5.10^19cm-3 in the tunnel junction. A maximum efficiency of 27% is predicted
for the tandem cell, exceeding the efficiencies of stand-alone silicon (17.3%)
and perovskite cells (17.9%) taken for our simulations, and more importantly,
that of the record crystalline Si cells.Comment: 17 pages, 7 figure
Necessity of Superposition of Macroscopically Distinct States for Quantum Computational Speedup
For quantum computation, we investigate the conjecture that the superposition
of macroscopically distinct states is necessary for a large quantum speedup.
Although this conjecture was supported for a circuit-based quantum computer
performing Shor's factoring algorithm [A. Ukena and A. Shimizu, Phys. Rev. A69
(2004) 022301], it needs to be generalized for it to be applicable to a large
class of algorithms and/or other models such as measurement-based quantum
computers. To treat such general cases, we first generalize the indices for the
superposition of macroscopically distinct states. We then generalize the
conjecture, using the generalized indices, in such a way that it is
unambiguously applicable to general models if a quantum algorithm achieves
exponential speedup. On the basis of this generalized conjecture, we further
extend the conjecture to Grover's quantum search algorithm, whose speedup is
large but quadratic. It is shown that this extended conjecture is also correct.
Since Grover's algorithm is a representative algorithm for unstructured
problems, the present result further supports the conjecture.Comment: 18 pages, 5 figures. Fixed typos throughout the manuscript. This
version has been publishe
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