2,137 research outputs found
On the Complexity of Solving Zero-Dimensional Polynomial Systems via Projection
Given a zero-dimensional polynomial system consisting of n integer
polynomials in n variables, we propose a certified and complete method to
compute all complex solutions of the system as well as a corresponding
separating linear form l with coefficients of small bit size. For computing l,
we need to project the solutions into one dimension along O(n) distinct
directions but no further algebraic manipulations. The solutions are then
directly reconstructed from the considered projections. The first step is
deterministic, whereas the second step uses randomization, thus being
Las-Vegas.
The theoretical analysis of our approach shows that the overall cost for the
two problems considered above is dominated by the cost of carrying out the
projections. We also give bounds on the bit complexity of our algorithms that
are exclusively stated in terms of the number of variables, the total degree
and the bitsize of the input polynomials
Minimal distance transformations between links and polymers: Principles and examples
The calculation of Euclidean distance between points is generalized to
one-dimensional objects such as strings or polymers. Necessary and sufficient
conditions for the minimal transformation between two polymer configurations
are derived. Transformations consist of piecewise rotations and translations
subject to Weierstrass-Erdmann corner conditions. Numerous examples are given
for the special cases of one and two links. The transition to a large number of
links is investigated, where the distance converges to the polymer length times
the mean root square distance (MRSD) between polymer configurations, assuming
curvature and non-crossing constraints can be neglected. Applications of this
metric to protein folding are investigated. Potential applications are also
discussed for structural alignment problems such as pharmacophore
identification, and inverse kinematic problems in motor learning and control.Comment: Submitted to J. Phys.:Condens. Matte
Fermi Detection of the Pulsar Wind Nebula HESS J1640-465
We present observations of HESS J1640-465 with the Fermi-LAT. The source is
detected with high confidence as an emitter of high-energy gamma-rays. The
spectrum lacks any evidence for the characteristic cutoff associated with
emission from pulsars, indicating that the emission arises primarily from the
pulsar wind nebula. Broadband modeling implies an evolved nebula with a low
magnetic field resulting in a high gamma-ray to X-ray flux ratio. The Fermi
emission exceeds predictions of the broadband model, and has a steeper
spectrum, possibly resulting from a distinct excess of low energy electrons
similar to what is inferred for both the Vela X and Crab pulsar wind nebulae.Comment: 6 pages, 5 figures, accepted for publication in Ap
Meta-Plaquette Expansion for the Triplet Excitation Spectrum in CaVO
We study antiferromagnetic, Heisenberg models with nearest and second
neighbor interactions on the one-fifth depleted square lattice which describes
the spin degrees of freedom in the spin-gap system CaVO. The
meta-plaquette expansion for the triplet excitation spectrum is extended to
fifth order, and the results are compared with experimental data on
CaVO. We attempt to locate the phase boundary between magnetically
ordered and gapped phases.Comment: 4 figure
Pulsar Wind Nebulae in the SKA era
Neutron stars lose the bulk of their rotational energy in the form of a
pulsar wind: an ultra-relativistic outflow of predominantly electrons and
positrons. This pulsar wind significantly impacts the environment and possible
binary companion of the neutron star, and studying the resultant pulsar wind
nebulae is critical for understanding the formation of neutron stars and
millisecond pulsars, the physics of the neutron star magnetosphere, the
acceleration of leptons up to PeV energies, and how these particles impact the
interstellar medium. With the SKA1 and the SKA2, it could be possible to study
literally hundreds of PWNe in detail, critical for understanding the many open
questions in the topics listed above.Comment: Comments: 10 pages, 3 figures, to be published in: "Advancing
Astrophysics with the Square Kilometre Array", Proceedings of Science,
PoS(AASKA14
On bi-Hamiltonian deformations of exact pencils of hydrodynamic type
In this paper we are interested in non trivial bi-Hamiltonian deformations of
the Poisson pencil \omega_{\lambda}=\omega_2+\lambda
\omega_1=u\delta'(x-y)+\f{1}{2}u_x\delta(x-y)+\lambda\delta'(x-y).
Deformations are generated by a sequence of vector fields ,
where each is homogenous of degree with respect to a grading
induced by rescaling. Constructing recursively the vector fields one
obtains two types of relations involving their unknown coefficients: one set of
linear relations and an other one which involves quadratic relations. We prove
that the set of linear relations has a geometric meaning: using
Miura-quasitriviality the set of linear relations expresses the tangency of the
vector fields to the symplectic leaves of and this tangency
condition is equivalent to the exactness of the pencil .
Moreover, extending the results of [17], we construct the non trivial
deformations of the Poisson pencil , up to the eighth order
in the deformation parameter, showing therefore that deformations are
unobstructed and that both Poisson structures are polynomial in the derivatives
of up to that order.Comment: 34 pages, revised version. Proof of Theorem 16 completely rewritten
due to an error in the first versio
Classical nonlinear response of a chaotic system: Langevin dynamics and spectral decomposition
We consider the classical response of a strongly chaotic Hamiltonian system.
The spectrum of such a system consists of discrete complex Ruelle-Pollicott
(RP) resonances which manifest themselves in the behavior of the correlation
and response functions. We interpret the RP resonances as the eigenstates and
eigenvalues of the Fokker-Planck operator obtained by adding an infinitesimal
noise term to the first-order Liouville operator. We demonstrate how the
deterministic expression for the linear response is reproduced in the limit of
vanishing noise. For the second-order response we establish an equivalence of
the spectral decomposition with infinitesimal noise and the long-time
asymptotic expansion for the deterministic case.Comment: 16 pages, 1 figur
Dimer Expansion Study of the Bilayer Square Lattice Frustrated Quantum Heisenberg Antiferromagnet
The ground state of the square lattice bilayer quantum antiferromagnet with
nearest () and next-nearest () neighbour intralayer interaction is
studied by means of the dimer expansion method up to the 6-th order in the
interlayer exchange coupling . The phase boundary between the spin-gap
phase and the magnetically ordered phase is determined from the poles of the
biased Pad\'e approximants for the susceptibility and the inverse energy gap
assuming the universality class of the 3-dimensional classical Heisenberg
model. For weak frustration, the critical interlayer coupling decreases
linearly with . The spin-gap phase persists down to
(single layer limit) for 0.45 \simleq \alpha \simleq 0.65. The crossover of
the short range order within the disordered phase is also discussed.Comment: 4 pages, 6 figures, One reference adde
Modulation of p53 activity by IκBα: Evidence suggesting a common phylogeny between NF-κB and p53 transcription factors
BACKGROUND: In this work we present evidence that the p53 tumor suppressor protein and NF-κB transcription factors could be related through common descent from a family of ancestral transcription factors regulating cellular proliferation and apoptosis. P53 is a homotetrameric transcription factor known to interact with the ankyrin protein 53BP2 (a fragment of the ASPP2 protein). NF-κB is also regulated by ankyrin proteins, the prototype of which is the IκB family. The DNA binding sequences of the two transcription factors are similar, sharing 8 out of 10 nucleotides. Interactions between the two proteins, both direct and indirect, have been noted previously and the two proteins play central roles in the control of proliferation and apoptosis. RESULTS: Using previously published structure data, we noted a significant degree of structural alignment between p53 and NF-κB p65. We also determined that IκBα and p53 bind in vitro through a specific interaction in part involving the DNA binding region of p53, or a region proximal to it, and the amino terminus of IκBα independently or cooperatively with the ankyrin 3 domain of IκBα In cotransfection experiments, κBα could significantly inhibit the transcriptional activity of p53. Inhibition of p53-mediated transcription was increased by deletion of the ankyrin 2, 4, or 5 domains of IκBα Co-precipitation experiments using the stably transfected ankyrin 5 deletion mutant of κBα and endogenous wild-type p53 further support the hypothesis that p53 and IκBα can physically interact in vivo. CONCLUSION: The aggregate results obtained using bacterially produced IκBα and p53 as well as reticulocyte lysate produced proteins suggest a correlation between in vitro co-precipitation in at least one of the systems and in vivo p53 inhibitory activity. These observations argue for a mechanism involving direct binding of IκBα to p53 in the inhibition of p53 transcriptional activity, analogous to the inhibition of NF-κB by κBα and p53 by 53BP2/ASPP2. These data furthermore suggest a role for ankyrin proteins in the regulation of p53 activity. Taken together, the NFκB and p53 proteins share similarities in structure, DNA binding sites and binding and regulation by ankyrin proteins in support of our hypothesis that the two proteins share common descent from an ancestral transcriptional factor
On the Extended Emission Around the Anomalous X-ray Pulsar 1E 1547.0-5408
We present an analysis of the extended emission around the anomalous X-ray
pulsar 1E 1547.0-5408 using four XMM-Newton observations taken with the source
in varying states of outburst as well as in quiescence. We find that the
extended emission flux is highly variable and strongly correlated with the flux
of the magnetar. Based on this result, as well as on spectral and energetic
considerations, we conclude that the extended emission is dominated by a
dust-scattering halo and not a pulsar wind nebula (PWN), as has been previously
argued. We obtain an upper limit on the 2-10 keV flux of a possible PWN of
4.7e-14 erg/s/cm^2, three times less than the previously claimed value,
implying an efficiency for conversion of spin-down energy into nebular
luminosity of <9e-4 (assuming a distance of 4 kpc). We do, however, find strong
evidence for X-ray emission from the supernova remnant shell surrounding the
pulsar, as previously reported.Comment: 16 pages, 3 tables, 4 figures, published in the Astrophysical Journa
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