145,471 research outputs found
Green functions for the TASEP with sublattice parallel update
We consider the totally asymmetric simple exclusion process (TASEP) in
discrete time with the sublattice parallel dynamics describing particles moving
to the right on the one-dimensional infinite chain with equal hoping
probabilities. Using sequentially two mappings, we show that the model is
equivalent to the TASEP with the backward-ordered sequential update in the case
when particles start and finish their motion not simultaneously. The Green
functions are obtained exactly in a determinant form for different initial and
final conditions.Comment: 11 pages, 4 figure
Supersymmetric Kaluza-Klein reductions of M-waves and MKK-monopoles
We investigate the Kaluza-Klein reductions to ten dimensions of the purely
gravitational half-BPS M-theory backgrounds: the M-wave and the Kaluza-Klein
monopole. We determine the moduli space of smooth (supersymmetric) Kaluza-Klein
reductions by classifying the freely-acting spacelike Killing vectors which
preserve some Killing spinor. As a consequence we find a wealth of new
supersymmetric IIA configurations involving composite and/or bound-state
configurations of waves, D0 and D6-branes, Kaluza-Klein monopoles in type IIA
and flux/nullbranes, and some other new configurations. Some new features
raised by the geometry of the Taub-NUT space are discussed, namely the
existence of reductions with no continuous moduli. We also propose an
interpretation of the flux 5-brane in terms of the local description (close to
the branes) of a bound state of D6-branes and ten-dimensional Kaluza-Klein
monopoles.Comment: 36 pages (v2: Reference added, "draft" mode disabled; v3: two
singular reductions discarded, appendix on spin structures added, references
updated
Type IIA Superstrings, Chiral Symmetry, and N=1 4D Gauge Theory Dualities
We study N=1 four dimensional gauge theories as the world volume theory of
D4-branes between NS 5-branes. We find constructions for a number of known
field theory dualities involving groups, coupled by
matter fields F in the representation, in terms of branes of
type IIA string theory. The dual gauge group follows from simply reversing the
ordering of the NS 5-branes and the D6-branes while conserving magnetic charge
on the world volume of the branes. We interpret many field theory phenomena
such as deformation of the superpotential W = \Tr (F\tilde F)^{k+1} in terms
of the position of branes. By looking to D-branes for guidance, we find new N=1
dualities involving arbitrary numbers of gauge groups. We propose a mechanism
for enhanced chiral symmetry in the brane construction which, we conjecture, is
associated with tensionless threebranes in six dimensions.Comment: 32 pages, 9 figures, uses harvmac, tables, and eps
A Self-Organizing Neural Model of Motor Equivalent Reaching and Tool Use by a Multijoint Arm
This paper describes a self-organizing neural model for eye-hand coordination. Called the DIRECT model, it embodies a solution of the classical motor equivalence problem. Motor equivalence computations allow humans and other animals to flexibly employ an arm with more degrees of freedom than the space in which it moves to carry out spatially defined tasks under conditions that may require novel joint configurations. During a motor babbling phase, the model endogenously generates movement commands that activate the correlated visual, spatial, and motor information that are used to learn its internal coordinate transformations. After learning occurs, the model is capable of controlling reaching movements of the arm to prescribed spatial targets using many different combinations of joints. When allowed visual feedback, the model can automatically perform, without additional learning, reaches with tools of variable lengths, with clamped joints, with distortions of visual input by a prism, and with unexpected perturbations. These compensatory computations occur within a single accurate reaching movement. No corrective movements are needed. Blind reaches using internal feedback have also been simulated. The model achieves its competence by transforming visual information about target position and end effector position in 3-D space into a body-centered spatial representation of the direction in 3-D space that the end effector must move to contact the target. The spatial direction vector is adaptively transformed into a motor direction vector, which represents the joint rotations that move the end effector in the desired spatial direction from the present arm configuration. Properties of the model are compared with psychophysical data on human reaching movements, neurophysiological data on the tuning curves of neurons in the monkey motor cortex, and alternative models of movement control.National Science Foundation (IRI 90-24877); Office of Naval Research (N00014-92-J-1309); Air Force Office of Scientific Research (F49620-92-J-0499); National Science Foundation (IRI 90-24877
M(atrix) Theory on an Orbifold and Twisted Membrane
M(atrix) theory on an orbifold and classical two-branes therein are studied
with particular emphasis to heterotic M(atrix) theory on relevant to
strongly coupled heterotic and dual Type IA string theories. By analyzing
orbifold condition on Chan-Paton factors, we show that three choice of gauge
group are possible for heterotic M(atrix) theory: SO(2N), SO(2N+1) or USp(2N).
By examining area-preserving diffeomorphism that underlies the M(atrix) theory,
we find that each choices of gauge group restricts possible topologies of
two-branes. The result suggests that only the choice of SO(2N) or SO(2N+1)
groups allows open two-branes, hence, relevant to heterotic M(atrix) theory. We
show that requirement of both local vacuum energy cancellation and of
worldsheet anomaly cancellation of resulting heterotic string identifies
supersymmetric twisted sector spectra with sixteen fundamental representation
spinors from each of the two fixed points. Twisted open and closed two-brane
configurations are obtained in the large N limit.Comment: latex, 23 pages, version to appear in Nuclear Physics
A Self-Organizing Neural Network Model for Redundant Sensory-Motor Control, Motor Equivalence, and Tool Use
A neural network is introduced which provides a solution of the classical motor equivalence problem, whereby many different joint configurations of a redundant manipulator can all be used to realize a desired trajectory in 3-D space. To do this, the network self-organizes a mapping from motion directions in 3-D space to velocity commands in joint space. Computer simulations demonstrate that, without any additional learning, the network can generate accurate movement commands that compensate for variable tool lengths, clamping of joints, distortions of visual input by a prism, and unexpected limb perturbations. Blind reaches have also been simulated.National Science Foundation (IRI-87-16960, IRI-90-24877
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