21,411 research outputs found
Cortical free association dynamics: distinct phases of a latching network
A Potts associative memory network has been proposed as a simplified model of
macroscopic cortical dynamics, in which each Potts unit stands for a patch of
cortex, which can be activated in one of S local attractor states. The internal
neuronal dynamics of the patch is not described by the model, rather it is
subsumed into an effective description in terms of graded Potts units, with
adaptation effects both specific to each attractor state and generic to the
patch. If each unit, or patch, receives effective (tensor) connections from C
other units, the network has been shown to be able to store a large number p of
global patterns, or network attractors, each with a fraction a of the units
active, where the critical load p_c scales roughly like p_c ~ (C S^2)/(a
ln(1/a)) (if the patterns are randomly correlated). Interestingly, after
retrieving an externally cued attractor, the network can continue jumping, or
latching, from attractor to attractor, driven by adaptation effects. The
occurrence and duration of latching dynamics is found through simulations to
depend critically on the strength of local attractor states, expressed in the
Potts model by a parameter w. Here we describe with simulations and then
analytically the boundaries between distinct phases of no latching, of
transient and sustained latching, deriving a phase diagram in the plane w-T,
where T parametrizes thermal noise effects. Implications for real cortical
dynamics are briefly reviewed in the conclusions
Global convergence of quorum-sensing networks
In many natural synchronization phenomena, communication between individual
elements occurs not directly, but rather through the environment. One of these
instances is bacterial quorum sensing, where bacteria release signaling
molecules in the environment which in turn are sensed and used for population
coordination. Extending this motivation to a general non- linear dynamical
system context, this paper analyzes synchronization phenomena in networks where
communication and coupling between nodes are mediated by shared dynamical quan-
tities, typically provided by the nodes' environment. Our model includes the
case when the dynamics of the shared variables themselves cannot be neglected
or indeed play a central part. Applications to examples from systems biology
illustrate the approach.Comment: Version 2: minor editions, added section on noise. Number of pages:
36
Symmetries, Stability, and Control in Nonlinear Systems and Networks
This paper discusses the interplay of symmetries and stability in the
analysis and control of nonlinear dynamical systems and networks. Specifically,
it combines standard results on symmetries and equivariance with recent
convergence analysis tools based on nonlinear contraction theory and virtual
dynamical systems. This synergy between structural properties (symmetries) and
convergence properties (contraction) is illustrated in the contexts of network
motifs arising e.g. in genetic networks, of invariance to environmental
symmetries, and of imposing different patterns of synchrony in a network.Comment: 16 pages, second versio
Matching the (DR4)-R-6 interaction at two-loops
The coefficient of the interaction in the low energy
expansion of the two-loop four-graviton amplitude in type II superstring theory
is known to be proportional to the integral of the Zhang-Kawazumi (ZK)
invariant over the moduli space of genus-two Riemann surfaces. We demonstrate
that the ZK invariant is an eigenfunction with eigenvalue 5 of the
Laplace-Beltrami operator in the interior of moduli space. Exploiting this
result, we evaluate the integral of the ZK invariant explicitly, finding
agreement with the value of the two-loop interaction predicted
on the basis of S-duality and supersymmetry. A review of the current
understanding of the interactions in type II superstring
theory compactified on a torus with and is
included.Comment: 40 pages, various typos and coefficients corrected in version
Comparison of dns of compressible and incompressible turbulent droplet-laden heated channel flow with phase transition
In this paper a turbulent channel flow with dispersed droplets is examined. The dispersed phase is allowed to have phase transition, which leads to heat and mass transfer between the phases, and correspondingly modulates turbulent flow properties. As a point of reference we examine the flow of water droplets in air, containing also the vapor of water. The key element of this study concerns the treatment of the carrier phase as either a compressible or an incompressible fluid. We compare simulation results obtained with a pseudo-spectral discretization for the incompressible flow to those obtained with a finite volume approach for the compressible flow. The compressible formulation is not tailored for low Mach flow and we need to resort to a Mach number that is artificially high for simulation feasibility. We discuss differences in fluid flow, heat- and mass transfer and dispersed droplet properties. The main conclusion is that both formulations give a good general correspondence. Flow properties such as velocity fields agree very closely, while heat transfer as characterized by the Nusselt number differs by around 25%. Droplet sizes are shown to be slightly larger, particularly in the center of the channel, in case the compressible formulation is chosen. A low-Mach compressible formulation is required for a fully quantitative comparison
Security policy refinement using data integration: a position paper.
In spite of the wide adoption of policy-based approaches for security management, and many existing treatments of policy verification and analysis, relatively little attention has been paid to policy refinement: the problem of deriving lower-level, runnable policies from higher-level policies, policy goals, and specifications. In this paper we present our initial ideas on this task, using and adapting concepts from data integration. We take a view of policies as governing the performance of an action on a target by a subject, possibly with certain conditions. Transformation rules are applied to these components of a policy in a structured way, in order to translate the policy into more refined terms; the transformation rules we use are similar to those of global-as-view database schema mappings, or to extensions thereof. We illustrate our ideas with an example. Copyright 2009 ACM
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