12,953 research outputs found
On the existence of Hopf bifurcations in the sequential and distributive double phosphorylation cycle
Protein phosphorylation cycles are important mechanisms of the post
translational modification of a protein and as such an integral part of
intracellular signaling and control. We consider the sequential phosphorylation
and dephosphorylation of a protein at two binding sites. While it is known that
proteins where phosphorylation is processive and dephosphorylation is
distributive admit oscillations (for some value of the rate constants and total
concentrations) it is not known whether or not this is the case if both
phosphorylation and dephosphorylation are distributive. We study four
simplified mass action models of sequential and distributive phosphorylation
and show that for each of those there do not exist rate constants and total
concentrations where a Hopf bifurcation occurs. To arrive at this result we use
convex parameters to parameterize the steady state and Hurwitz matrices
Analytical parametrization of fusion barriers using proximity potentials
Using the three versions of proximity potentials, namely proximity 1977,
proximity 1988, and proximity 2000, we present a pocket formula for fusion
barrier heights and positions. This was achieved by analyzing as many as 400
reactions with mass between 15 and 296. Our parametrized formula can reproduced
the exact barrier heights and positions within an accuracy of . A
comparison with the experimental data is also in good agreement.Comment: 12 pages, 5 figure
Discrete Morse theory for computing cellular sheaf cohomology
Sheaves and sheaf cohomology are powerful tools in computational topology,
greatly generalizing persistent homology. We develop an algorithm for
simplifying the computation of cellular sheaf cohomology via (discrete)
Morse-theoretic techniques. As a consequence, we derive efficient techniques
for distributed computation of (ordinary) cohomology of a cell complex.Comment: 19 pages, 1 Figure. Added Section 5.
Order Reduction of the Radiative Heat Transfer Model for the Simulation of Plasma Arcs
An approach to derive low-complexity models describing thermal radiation for
the sake of simulating the behavior of electric arcs in switchgear systems is
presented. The idea is to approximate the (high dimensional) full-order
equations, modeling the propagation of the radiated intensity in space, with a
model of much lower dimension, whose parameters are identified by means of
nonlinear system identification techniques. The low-order model preserves the
main structural aspects of the full-order one, and its parameters can be
straightforwardly used in arc simulation tools based on computational fluid
dynamics. In particular, the model parameters can be used together with the
common approaches to resolve radiation in magnetohydrodynamic simulations,
including the discrete-ordinate method, the P-N methods and photohydrodynamics.
The proposed order reduction approach is able to systematically compute the
partitioning of the electromagnetic spectrum in frequency bands, and the
related absorption coefficients, that yield the best matching with respect to
the finely resolved absorption spectrum of the considered gaseous medium. It is
shown how the problem's structure can be exploited to improve the computational
efficiency when solving the resulting nonlinear optimization problem. In
addition to the order reduction approach and the related computational aspects,
an analysis by means of Laplace transform is presented, providing a
justification to the use of very low orders in the reduction procedure as
compared with the full-order model. Finally, comparisons between the full-order
model and the reduced-order ones are presented
Sensitivity of exclusive proton knockout spin observables to different Lorentz invariant representations of the NN interaction
Within the framework of the relativistic plane wave impulse approximation, we
study the observable consequences of employing a complete Lorentz invariant
representation of the NN scattering matrix in terms of 44 independent
amplitudes, as opposed to the previously-employed, but ambiguous, five-term
Lorentz invariant parametrization of the NN scattering matrix, for the
prediction of complete sets of exclusive () polarization
transfer observables. Two kinematic conditions are considered, namely proton
knockout from the state of Pb at an incident energy of 202
MeV for coplanar scattering angles (), as well as
an incident energy of 392 MeV for the angle pair (). The results indicate that certain spin observables are ideal
for discriminating between the two representations.Comment: 19 pages, 5 figures, Revtex, To be published in Phys. Rev.
Macaulay inverse systems revisited
Since its original publication in 1916 under the title "The Algebraic Theory
of Modular Systems", the book by F. S. Macaulay has attracted a lot of
scientists with a view towards pure mathematics (D. Eisenbud,...) or
applications to control theory (U. Oberst,...).However, a carefull examination
of the quotations clearly shows that people have hardly been looking at the
last chapter dealing with the so-called "inverse systems", unless in very
particular situations. The purpose of this paper is to provide for the first
time the full explanation of this chapter within the framework of the formal
theory of systems of partial differential equations (Spencer operator on
sections, involution,...) and its algebraic counterpart now called "algebraic
analysis" (commutative and homological algebra, differential modules,...). Many
explicit examples are fully treated and hints are given towards the way to work
out computer algebra packages.Comment: From a lecture at the International Conference : Application of
Computer Algebra (ACA 2008) july 2008, RISC, LINZ, AUSTRI
Bounds on universal new physics effects from fermion-antifermion production at LEP2
We consider lepton-antilepton annihilation into a fermion-antifermion pair at
variable c.m. energy. We propose for this process a simple parametrization of
the virtual effects of the most general model of new physics of
\underline{universal} type. This parametrization is based on a recent approach,
that uses the experimental results of LEP1, SLC as theoretical input. It
introduces \underline{three} functions whose energy dependence is argued to be
smooth and, in first approximation, negligible. A couple of representative
models of new physics are considered, as a support of the previous claim.
Explicit bounds are then derived for this type of new physics from the
available LEP2 data, and a discussion is given of the relevance in this respect
of the different experimental measurements. The method is then extended to
treat the case of two particularly simple models of {\it non universal} type,
for which it is possible to draw analogous conclusions.Comment: 15 pages, 3 tables and 4 figures. e-mail: [email protected]
Soft and Collinear Radiation and Factorization in Perturbation Theory and Beyond
Power corrections to differential cross sections near a kinematic threshold
are analysed by Dressed Gluon Exponentiation. Exploiting the factorization
property of soft and collinear radiation, the dominant radiative corrections in
the threshold region are resummed, yielding a renormalization-scale-invariant
expression for the Sudakov exponent. The interplay between Sudakov logs and
renormalons is clarified, and the necessity to resum the latter whenever power
corrections are non-negligible is emphasized. The presence of power-suppressed
ambiguities in the exponentiation kernel suggests that power corrections
exponentiate as well. This leads to a non-perturbative factorization formula
with non-trivial predictions on the structure of power corrections, which can
be contrasted with the OPE. Two examples are discussed. The first is
event-shape distributions in the two-jet region, where a wealth of precise data
provides a strong motivation for the improved perturbative technique and an
ideal situation to study hadronization. The second example is deep inelastic
structure functions. In contrast to event shapes, structure functions have an
OPE. However, since the OPE breaks down at large x, it does not provide a
practical framework for the parametrization of power corrections. Performing a
detailed analysis of twist 4 it is shown precisely how the twist-2 renormalon
ambiguity eventually cancels out. This analysis provides a physical picture
which substantiates the non-perturbative factorization conjecture.Comment: 11 pages, 6 postscript figures; talk presented at the XXXVII
Rencontres de Moriond `QCD and high energy hadronic interactions', Les Arcs,
France, and at the Workshop `Continuous Advances in QCD 2002/Arkadyfest',
Minnesot
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