1,107 research outputs found
A \Gamma-structure on Lagrangian Grassmannians
For n odd the Lagrangian Grassmannian of \R^{2n} is a \Gamma-manifold.Comment: 6 pages; v2: new coauthor, nicer proo
Stability is not open
We give an example of a symplectic manifold with a stable hypersurface such
that nearby hypersurfaces are typically unstable.Comment: 9 page
Floer homology for magnetic fields with at most linear growth on the universal cover
The Floer homology of a cotangent bundle is isomorphic to loop space homology
of the underlying manifold, as proved by Abbondandolo-Schwarz, Salamon-Weber,
and Viterbo. In this paper we show that in the presence of a Dirac magnetic
monopole which admits a primitive with sublinear growth on the universal cover,
the Floer homology in atoroidal free homotopy classes is again isomorphic to
loop space homology. As a consequence we prove that for any atoroidal free
homotopy class and any sufficiently small T>0, any magnetic flow associated to
the Dirac magnetic monopole has a closed orbit of period T belonging to the
given free homotopy class. In the case where the Dirac magnetic monopole admits
a bounded primitive on the universal cover we also prove the Conley conjecture
for Hamiltonians that are quadratic at infinity, i.e., we show that such
Hamiltonians have infinitely many periodic orbits.Comment: 24 pages, V2 - minor corrections, final version to appear in JF
The contact geometry of the restricted 3-body problem
We show that the planar circular restricted three body problem is of
restricted contact type for all energies below the first critical value (action
of the first Lagrange point) and for energies slightly above it. This opens up
the possibility of using the technology of Contact Topology to understand this
particular dynamical system.Comment: 29 pages, 1 figur
Conformational Dependence of a Protein Kinase Phosphate Transfer Reaction
Atomic motions and energetics for a phosphate transfer reaction catalyzed by
the cAMP-dependent protein kinase (PKA) are calculated by plane-wave density
functional theory, starting from structures of proteins crystallized in both
the reactant conformation (RC) and the transition-state conformation (TC). In
the TC, we calculate that the reactants and products are nearly isoenergetic
with a 0.2 eV barrier; while phosphate transfer is unfavorable by over 1.2 eV
in the RC, with an even higher barrier. With the protein in the TC, the motions
involved in reaction are small, with only P and the catalytic proton
moving more than 0.5 \AA. Examination of the structures reveals that in the RC
the active site cleft is not completely closed and there is insufficient space
for the phosphorylated serine residue in the product state. Together, these
observations imply that the phosphate transfer reaction occurs rapidly and
reversibly in a particular conformation of the protein, and that the reaction
can be gated by changes of a few tenths of an \AA in the catalytic site.Comment: revtex4, 7 pages, 4 figures, to be submitted to Scienc
Model Independent Form Factors for Spin Independent Neutralino-Nucleon Scattering from Elastic Electron Scattering Data
Theoretical calculations of neutralino-nucleon interaction rates with various
nuclei are of great interest to direct dark matter searches such as CDMS,
EDELWEISS, ZEPLIN, and other experiments since they are used to establish upper
bounds on the WIMP-proton cross section. These interaction rates and cross
sections are generally computed with standard, one or two parameter
model-dependent nuclear form factors, which may not exactly mirror the actual
form factor for the particular nucleus in question. As is well known, elastic
electron scattering can allow for very precise determinations of nuclear form
factors and hence nuclear charge densities for spherical or near-spherical
nuclei. We use charge densities derived from elastic electron scattering data
to calculate model independent, analytic form factors for various target nuclei
important in dark matter searches, such as Si, Ge, S, Ca and others. We have
found that for nuclear recoils in the range of 1-100 keV significant
differences in cross sections and rates exist when the model independent form
factors are used: at 30 keV nuclear recoil the form factors squared differ by a
factor of 1.06 for Si, 1.11 for Ca, 1.27 for Ge, and 1.92
for Xe. We show the effect of different form factors on the upper limit
on the WIMP-proton cross section obtained with a hypothetical Ge
detector during a 100 kg-day effective exposure. Helm form factors with various
parameter choices differ at most by 10--20% from the best (Fourier Bessel) form
factor, and can approach it to better than 1% if the parameters are chosen to
mimic the actual nuclear density.Comment: 20 pages, 8 figure
Analytic models for mechanotransduction: gating a mechanosensitive channel
Analytic estimates for the forces and free energy generated by bilayer
deformation reveal a compelling and intuitive model for MscL channel gating
analogous to the nucleation of a second phase. We argue that the competition
between hydrophobic mismatch and tension results in a surprisingly rich story
which can provide both a quantitative comparison to measurements of opening
tension for MscL when reconstituted in bilayers of different thickness and
qualitative insights into the function of the MscL channel and other
transmembrane proteins
Hysteretic Optimization For Spin Glasses
The recently proposed Hysteretic Optimization (HO) procedure is applied to
the 1D Ising spin chain with long range interactions. To study its
effectiveness, the quality of ground state energies found as a function of the
distance dependence exponent, , is assessed. It is found that the
transition from an infinite-range to a long-range interaction at
is accompanied by a sharp decrease in the performance . The transition is
signaled by a change in the scaling behavior of the average avalanche size
observed during the hysteresis process. This indicates that HO requires the
system to be infinite-range, with a high degree of interconnectivity between
variables leading to large avalanches, in order to function properly. An
analysis of the way auto-correlations evolve during the optimization procedure
confirm that the search of phase space is less efficient, with the system
becoming effectively stuck in suboptimal configurations much earlier. These
observations explain the poor performance that HO obtained for the
Edwards-Anderson spin glass on finite-dimensional lattices, and suggest that
its usefulness might be limited in many combinatorial optimization problems.Comment: 6 pages, 9 figures. To appear in JSTAT. Author website:
http://www.bgoncalves.co
p-Adic Models of Ultrametric Diffusion Constrained by Hierarchical Energy Landscapes
We demonstrate that p-adic analysis is a natural basis for the construction
of a wide variety of the ultrametric diffusion models constrained by
hierarchical energy landscapes. A general analytical description in terms of
p-adic analysis is given for a class of models. Two exactly solvable examples,
i.e. the ultrametric diffusion constraned by the linear energy landscape and
the ultrametric diffusion with reaction sink, are considered. We show that such
models can be applied to both the relaxation in complex systems and the rate
processes coupled to rearrangenment of the complex surrounding.Comment: 14 pages, 6 eps figures, LaTeX 2.0
Thermodynamics of protein folding: a random matrix formulation
The process of protein folding from an unfolded state to a biologically
active, folded conformation is governed by many parameters e.g the sequence of
amino acids, intermolecular interactions, the solvent, temperature and chaperon
molecules. Our study, based on random matrix modeling of the interactions,
shows however that the evolution of the statistical measures e.g Gibbs free
energy, heat capacity, entropy is single parametric. The information can
explain the selection of specific folding pathways from an infinite number of
possible ways as well as other folding characteristics observed in computer
simulation studies.Comment: 21 Pages, no figure
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