13,118 research outputs found
Applications of symbolic computing methods to the dynamic analysis of large systems
Since the symbolic computing language is very well suited to the operations with algebraic equations, techniques use the transfer function concept as a tool for the analysis of large linear dynamic systems. Techniques were coded in the experimental symbolic computer language FORMAC. The first of these approaches, REDUCE 1, establishes the techniques and a computer program to symbolically reduce arbitrary block diagrams associated with large systems for desired transfer functions. Symbolic closed form solutions are determined in several forms including an expanded form in terms of the driving frequencies and system constants. Programs are also written to numerically evaluate the symbolic solutions. A second computer program, REDUCE 2, is also based on the use of symbolic computing methods and was written to accommodate large engineering systems
A graph theoretical analysis of the energy landscape of model polymers
In systems characterized by a rough potential energy landscape, local
energetic minima and saddles define a network of metastable states whose
topology strongly influences the dynamics. Changes in temperature, causing the
merging and splitting of metastable states, have non trivial effects on such
networks and must be taken into account. We do this by means of a recently
proposed renormalization procedure. This method is applied to analyze the
topology of the network of metastable states for different polypeptidic
sequences in a minimalistic polymer model. A smaller spectral dimension emerges
as a hallmark of stability of the global energy minimum and highlights a
non-obvious link between dynamic and thermodynamic properties.Comment: 15 pages, 15 figure
Where in the String Landscape is Quintessence
We argue that quintessence may reside in certain corners of the string
landscape. It arises as a linear combination of internal space components of
higher rank forms, which are axion-like at low energies, and may mix with
4-forms after compactification of the Chern-Simons terms to 4D due to internal
space fluxes. The mixing induces an effective mass term, with an action which
{\it preserves} the axion shift symmetry, breaking it spontaneously after the
background selection. With several axions, several 4-forms, and a low string
scale, as in one of the setups already invoked for dynamically explaining a
tiny residual vacuum energy in string theory, the 4D mass matrix generated by
random fluxes may have ultralight eigenmodes over the landscape, which are
quintessence. We illustrate how this works in simplest cases, and outline how
to get the lightest mass to be comparable to the Hubble scale now, . The shift symmetry protects the smallest mass from
perturbative corrections in field theory. Further, if the ultralight eigenmode
does not couple directly to any sector strongly coupled at a high scale, the
non-perturbative field theory corrections to its potential will also be
suppressed. Finally, if the compactification length is larger than the string
length by more than an order of magnitude, the gravitational corrections may
remain small too, even when the field value approaches .Comment: 8 pages RevTeX; added references, matches published versio
The Lense-Thirring effect in the Jovian system of the Galilean satellites and its measurability
In this paper we investigate the possibility of measuring the post-Newtonian
general relativistic gravitomagnetic Lense-Thirring effect in the Jovian system
of its Galilean satellites Io, Europa, Ganymede and Callisto in view of recent
developments in processing and modelling their optical observations spanning a
large time interval (125 years). The present day best observations have an
accuracy between several kilometers to few tens of kilometers, which is just
the order of magnitude of the Lense-Thirring shifts of the orbits of the
Galilean satellites over almost a century. From a comparison between analytical
development and numerical integration it turns out that, unfortunately, most of
the secular component of the gravitomagnetic signature is removed in the
process of fitting the initial conditions. Indeed, an estimation of the
magnitude of the Lense-Thirring effect in the ephemerides residuals is given;
the resulting residuals have a maximum magnitude of 20 meters only (over 125
years).Comment: Latex, 10 pages, 4 tables, 3 figures, 27 references. Invited paper
for a Special Issue of Int. J. Mod. Phys. D on the Lense-Thirring effect, D.
Grumiller edito
A Simple Geometric Representative for of a Point
For (or ) Donaldson theory on a 4-manifold , we construct a
simple geometric representative for of a point. Let be a generic
point in . Then the set is reducible , with
coefficient -1/4 and appropriate orientation, is our desired geometric
representative.Comment: Updated 2018 to published version. 8 pages, AmS-TeX, no figure
Charge Ordering and Spin Dynamics in NaV2O5
We report high-resolution neutron inelastic scattering experiments on the
spin excitations of NaV2O5. Below Tc, two branches associated with distinct
energy gaps are identified. From the dispersion and intensity of the spin
excitation modes, we deduce the precise zig-zag charge distribution on the
ladder rungs and the corresponding charge order (about 0.6). We argue that the
spin gaps observed in the low-T phase of this compound are primarily due to the
charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Direct observation of the high magnetic field effect on the Jahn-Teller state in TbVO4
We report the first direct observation of the influence of high magnetic
fields on the Jahn-Teller (JT) transition in TbVO4. Contrary to spectroscopic
and magnetic methods, X-ray diffraction directly measures the JT distortion;
the splitting between the (311)/(131) and (202)/(022) pairs of Bragg
reflections is proportional to the order parameter. Our experimental results
are compared to mean field calculations, taking into account all possible
orientations of the grains relative to the applied field, and qualitative
agreement is obtained.Comment: 11 pages, 4 figures, submitted to Phys. Rev. Let
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