37,519 research outputs found
Comment on "Constraint Quantization of Open String in Background B field and Noncommutative D-brane"
In the paper "Constraint Quantization of Open String in Background field
and Noncommutative D-brane", it is claimed that the boundary conditions lead to
an infinite set of secondary constraints and Dirac brackets result in a
non-commutative Poisson structure for D-brain. Here we show that contrary to
the arguments in that paper, the set of secondary constraints on the boundary
is finite and the non-commutativity algebra can not be obtained by evaluating
the Dirac brackets.Comment: minor corrections, to appear in Phys.Lett.
Searching for a preferred direction with Union2.1 data
A cosmological preferred direction was reported from the type Ia supernovae
(SNe Ia) data in recent years. We use the Union2.1 data to give a simple
classification of such studies for the first time. Because the maximum
anisotropic direction is independent of isotropic dark energy models, we adopt
two cosmological models (CDM, CDM) for the hemisphere comparison
analysis and CDM model for dipole fit approach. In hemisphere
comparison method, the matter density and the equation of state of dark energy
are adopted as the diagnostic qualities in the CDM model and CDM
model, respectively. In dipole fit approach, we fit the fluctuation of distance
modulus. We find that there is a null signal for the hemisphere comparison
method, while a preferred direction () for the dipole fit method. This result indicates
that the dipole fit is more sensitive than the hemisphere comparison method.Comment: 8 pages, 2 figures, accepted for publication in MNRA
On canonical quantization of the gauged WZW model with permutation branes
In this paper we perform canonical quantization of the product of the gauged
WZW models on a strip with boundary conditions specified by permutation branes.
We show that the phase space of the -fold product of the gauged WZW model
on a strip with boundary conditions given by permutation branes is
symplectomorphic to the phase space of the double Chern-Simons theory on a
sphere with holes times the time-line with and gauge fields both
coupled to two Wilson lines. For the special case of the topological coset
we arrive at the conclusion that the phase space of the -fold product
of the topological coset on a strip with boundary conditions given by
permutation branes is symplectomorphic to the phase space of Chern-Simons
theory on a Riemann surface of the genus times the time-line with four
Wilson lines.Comment: 18 page
A novel implementation of method of optimality criterion in synthesizing spacecraft structures with natural frequency constraints
In the design of spacecraft structures, fine tuning the structure to achieve minimum weight with natural frequency constraints is a time consuming process. Here, a novel implementation of the method of optimality criterion (OC) is developed. In this new implementation of OC, the free vibration analysis results are used to compute the eigenvalue sensitivity data required for the formulation. Specifically, the modal elemental strain and kinetic energies are used. Additionally, normalized design parameters are introduced as a second level linking that allows design variables of different values to be linked together. With the use of this novel formulation, synthesis of structures with natural frequency constraint can be carried out manually using modal analysis results. Design examples are presented to illustrate this novel implementation of the optimality criterion method
The noncommutative harmonic oscillator in more than one dimensions
The noncommutative harmonic oscillator in arbitrary dimension is examined. It
is shown that the -genvalue problem can be decomposed into separate
harmonic oscillator equations for each dimension. The noncommutative plane is
investigated in greater detail. The constraints for rotationally symmetric
solutions and the corresponding two-dimensional harmonic oscillator are solved.
The angular momentum operator is derived and its -genvalue problem is
shown to be equivalent to the usual eigenvalue problem. The -genvalues
for the angular momentum are found to depend on the energy difference of the
oscillations in each dimension. Furthermore two examples of assymetric
noncommutative harmonic oscillator are analysed. The first is the
noncommutative two-dimensional Landau problem and the second is the
three-dimensional harmonic oscillator with symmetrically noncommuting
coordinates and momenta.Comment: 12 page
Optimal realizations of floating-point implemented digital controllers with finite word length considerations.
The closed-loop stability issue of finite word length (FWL) realizations is
investigated for digital controllers implemented in floating-point arithmetic.
Unlike the existing methods which only address the effect of the mantissa bits
in floating-point implementation to the sensitivity of closed-loop stability,
the sensitivity of closed-loop stability is analysed with respect to both the
mantissa and exponent bits of floating-point implementation. A computationally
tractable FWL closed-loop stability measure is then defined, and the method of
computing the value of this measure is given. The optimal controller realization
problem is posed as searching for a floating-point realization that maximizes
the proposed FWL closed-loop stability measure, and a numerical optimization
technique is adopted to solve for the resulting optimization problem. Simulation
results show that the proposed design procedure yields computationally efficient
controller realizations with enhanced FWL closed-loop stability performance
Improving Effective Surgical Delivery in Humanitarian Disasters: Lessons from Haiti
Kathryn Chu and colleagues describe the experiences of Médecins sans Frontières after the 2010 Haiti earthquake, and discuss how to improve delivery of surgery in humanitarian disasters
System identification and structural control on the JPL Phase B testbed
The primary objective of NASA's CSI program at JPL is to develop and demonstrate the CSI technology required to achieve high precision structural stability on large complex optical class spacecraft. The focus mission for this work is an orbiting interferometer telescope. Toward the realization of such a mission, a series of evolutionary testbed structures are being constructed. The JPL's CSI Phase B testbed is the second structure constructed in this series which is designed to study the pathlength control problem of the optical train of a stellar interferometer telescope mounted on a large flexible structure. A detailed description of this testbed can be found. This paper describes our efforts in the first phase of active structural control experiments of Phase B testbed using the active control approach where a single piezoelectric active member is used as an actuation device and the measurements include both colocated and noncolocated sensors. Our goal for this experiment is to demonstrate the feasibility of active structural control using both colocated and noncolocated measurements by means of successive control design and loop closing. More specifically, the colocated control loop was designed and closed first to provide good damping improvement over the frequency range of interest. The noncolocated controller was then designed with respect to a partially controlled structure to further improve the performance. Based on our approach, experimental closed-loop results have demonstrated significant performance improvement with excellent stability margins
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