9,858 research outputs found
Duality, Monodromy and Integrability of Two Dimensional String Effective Action
The monodromy matrix, , is constructed for two dimensional
tree level string effective action. The pole structure of is
derived using its factorizability property. It is found that the monodromy
matrix transforms non-trivially under the non-compact T-duality group, which
leaves the effective action invariant and this can be used to construct the
monodromy matrix for more complicated backgrounds starting from simpler ones.
We construct, explicitly, for the exactly solvable
Nappi-Witten model, both when B=0 and , where these ideas can be
directly checked. We consider well known charged black hole solutions in the
heterotic string theory which can be generated by T-duality transformations
from a spherically symmetric `seed' Schwarzschild solution. We construct the
monodromy matrix for the Schwarzschild black hole background of the heterotic
string theory.Comment: 20 pages, to be published in Physical Review
Self-Duality and the KdV Hierarchy
We derive the entire KdV hierarchy as well as the recursion relations from
the self-duality condition on gauge fields in four dimensions.Comment: 7 page
Open Membranes, p-Branes and Noncommutativity of Boundary String Coordinates
We study the dynamics of an open membrane with a cylindrical topology, in the
background of a constant three form, whose boundary is attached to p-branes.
The boundary closed string is coupled to a two form potential to ensure gauge
invariance. We use the action, due to Bergshoeff, London and Townsend, to study
the noncommutativity properties of the boundary string coordinates. The
constrained Hamiltonian formalism due to Dirac is used to derive the
noncommutativity of coordinates. The chain of constraints is found to be finite
for a suitable gauge choice, unlike the case of the static gauge, where the
chain has an infinite sequence of terms. It is conjectured that the formulation
of closed string field theory may necessitate introduction of a star product
which is both noncommutative and nonassociative.Comment: 32page
Fuzzy logic control of telerobot manipulators
Telerobot systems for advanced applications will require manipulators with redundant 'degrees of freedom' (DOF) that are capable of adapting manipulator configurations to avoid obstacles while achieving the user specified goal. Conventional methods for control of manipulators (based on solution of the inverse kinematics) cannot be easily extended to these situations. Fuzzy logic control offers a possible solution to these needs. A current research program at SRI developed a fuzzy logic controller for a redundant, 4 DOF, planar manipulator. The manipulator end point trajectory can be specified by either a computer program (robot mode) or by manual input (teleoperator). The approach used expresses end-point error and the location of manipulator joints as fuzzy variables. Joint motions are determined by a fuzzy rule set without requiring solution of the inverse kinematics. Additional rules for sensor data, obstacle avoidance and preferred manipulator configuration, e.g., 'righty' or 'lefty', are easily accommodated. The procedure used to generate the fuzzy rules can be extended to higher DOF systems
Optimal pulse spacing for dynamical decoupling in the presence of a purely-dephasing spin-bath
Maintaining quantum coherence is a crucial requirement for quantum
computation; hence protecting quantum systems against their irreversible
corruption due to environmental noise is an important open problem. Dynamical
decoupling (DD) is an effective method for reducing decoherence with a low
control overhead. It also plays an important role in quantum metrology, where
for instance it is employed in multiparameter estimation. While a sequence of
equidistant control pulses (CPMG) has been ubiquitously used for decoupling,
Uhrig recently proposed that a non-equidistant pulse sequence (UDD) may enhance
DD performance, especially for systems where the spectral density of the
environment has a sharp frequency cutoff. On the other hand, equidistant
sequences outperform UDD for soft cutoffs. The relative advantage provided by
UDD for intermediate regimes is not clear. In this paper, we analyze the
relative DD performance in this regime experimentally, using solid-state
nuclear magnetic resonance. Our system-qubits are 13C nuclear spins and the
environment consists of a 1H nuclear spin-bath whose spectral density is close
to a normal (Gaussian) distribution. We find that in the presence of such a
bath, the CPMG sequence outperforms the UDD sequence. An analogy between
dynamical decoupling and interference effects in optics provides an intuitive
explanation as to why the CPMG sequence performs superior to any
non-equidistant DD sequence in the presence of this kind of environmental
noise.Comment: To be published in Phys. Rev. A. 15 pages, 16 figures. Presentation
of the work was improved. One Figure and some Refs. were adde
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
We study the dynamics of first-order cofinement-deconfinement phase
transition through nucleation of hadronic bubbles in an expanding quark gluon
plasma in the context of heavy ion collisions for interacting quark and hadron
gas and by incorporating the effects of curvature energy. We find that the
interactions reduce the delay in the phase transition whereas the curvature
energy has a mixed behavior. In contrast to the case of early Universe phase
transition, here lower values of surface tension increase the supercooling and
slow down the hadronization process. Higher values of bag pressure tend to
speed up the transition. Another interesting feature is the start of the
hadronization process as soon as the QGP is created.Comment: LaTeX, 17 pages including 14 postscript figure
An Empirical Analysis of Internet Use by U.S. Farmers
The Internet may reduce constraints on a farmerÂ’s ability to receive and manage information, regardless of where the farm is located or when the information is used. Using a count data estimation procedure, this study attempts to examine the key farm, operator, regional, and household characteristics that influence the number of Internet applications used by farm households. Findings indicate that educational level of the farm operator, farm size, farm diversification, off-farm income, off-farm investments, and regional location of the farm have a significant impact on the number of Internet applications used.computers, count data method, education, farm households, Internet applications, Farm Management,
INTERNET USAGE BY FARMERS: EVIDENCE FROM A NATIONAL SURVEY
The Internet may reduce constraints on a farmer's ability to receive and manage information, regardless of where the farm is located or when the information is used. Using a Count data estimation procedure, this study attempts to examine the key farm, operator, regional, and household characteristics that influence the number of Internet applications used by farm households. Results indicate that educational level of the farm operator, farm size, farm diversification, off-farm income, off-farm investments, and regional location of the farm have significant impact on the number of Internet applications.Research and Development/Tech Change/Emerging Technologies,
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