288,219 research outputs found
Volatility and Agent Adaptability in a Self-Organizing Market
We present results for the so-called `bar-attendance' model of market
behavior: adaptive agents, each possessing prediction rules chosen
randomly from a pool, attempt to attend a bar whose cut-off is . The global
attendance time-series has a mean near, but not equal to, . The variance, or
`volatility', can show a minimum with increasing adaptability of the individual
agents.Comment: 8 pages, 3 figs. [email protected], [email protected]
K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar Decays in the General MSSM
We reanalyze the rare decays K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar
in a general MSSM with conserved R-parity. Working in the mass eigenstate basis
and performing adaptive scanning of a large space of supersymmetric parameters,
16 parameters in the constrained scan and 63 in the extended scan, we find that
large departures from the Standard Model expectations are possible while
satisfying all existing constraints. Both branching ratios can be as large as a
few times 10^{-10} with Br(K_L -> pi^0 nu nu-bar) often larger than Br(K^+ ->
pi^+ nu nu-bar) and close to its model independent upper bound. We give
examples of supersymmetric parameters for which large departures from the SM
expectations can be found and emphasize that the present 90% C.L. experimental
upper bound on Br(K^+ -> pi^+ nu nu-bar) gives a non trivial constraint on the
MSSM parameter space. Unlike previous analyses, we find that chargino box
diagrams can give, already for moderately light charged sleptons, a significant
contribution. As a byproduct we find that the ranges for the angles beta and
gamma in the unitarity triangle are relaxed due to the presence of new
CP-violating phases in K^0 - K^0-bar and B^0_d - B^0_d-bar mixing to 12 degrees
<= beta <= 27 degrees and 20 degrees <= gamma <= 110 degrees.Comment: 36 pages, 27 figures, latex, uses axodraw.st
Anomalously Weak Dynamical Friction in Halos
A bar rotating in a pressure-supported halo generally loses angular momentum
and slows down due to dynamical friction. Valenzuela & Klypin report a
counter-example of a bar that rotates in a dense halo with little friction for
several Gyr, and argue that their result invalidates the claim by Debattista &
Sellwood that fast bars in real galaxies require a low halo density. We show
that it is possible for friction to cease for a while should the pattern speed
of the bar fluctuate upward. The reduced friction is due to an anomalous
gradient in the phase-space density of particles at the principal resonance
created by the earlier evolution. The result obtained by Valenzuela & Klypin is
probably an artifact of their adaptive mesh refinement method, but anyway could
not persist in a real galaxy. The conclusion by Debattista & Sellwood still
stands.Comment: To appear in "Island Universes - Structure and Evolution of Disk
Galaxies" ed. R. S. de Jong, 8 pages, 4 figures, .cls and .sty files include
Bar-Halo Friction in Galaxies II: Metastability
It is well-established that strong bars rotating in dense halos generally
slow down as they lose angular momentum to the halo through dynamical friction.
Angular momentum exchanges between the bar and halo particles take place at
resonances. While some particles gain and others lose, friction arises when
there is an excess of gainers over losers. This imbalance results from the
generally decreasing numbers of particles with increasing angular momentum, and
friction can therefore be avoided if there is no gradient in the density of
particles across the major resonances. Here we show that anomalously weak
friction can occur for this reason if the pattern speed of the bar fluctuates
upwards. After such an event, the density of resonant halo particles has a
local inflexion created by the earlier exchanges, and bar slowdown can be
delayed for a long period; we describe this as a metastable state. We show that
this behavior in purely collisionless N-body simulations is far more likely to
occur in methods with adaptive resolution. We also show that the phenomenon
could arise in nature, since bar-driven gas inflow could easily raise the bar
pattern speed enough to reach the metastable state. Finally, we demonstrate
that mild external, or internal, perturbations quickly restore the usual
frictional drag, and it is unlikely therefore that a strong bar in a galaxy
having a dense halo could rotate for a long period without friction.Comment: 13 pages, 11 figures, to appear in Ap
Adaptive weight estimator for quantum error correction
Quantum error correction of a surface code or repetition code requires the
pairwise matching of error events in a space-time graph of qubit measurements,
such that the total weight of the matching is minimized. The input weights
follow from a physical model of the error processes that affect the qubits.
This approach becomes problematic if the system has sources of error that
change over time. Here we show how the weights can be determined from the
measured data in the absence of an error model. The resulting adaptive decoder
performs well in a time-dependent environment, provided that the characteristic
time scale of the variations is greater than , with the duration of one error-correction cycle and
the typical error probability per qubit in one cycle.Comment: 5 pages, 4 figure
Cooperation and Self-Regulation in a Model of Agents Playing Different Games
A simple model for cooperation between "selfish" agents, which play an
extended version of the Prisoner's Dilemma(PD) game, in which they use
arbitrary payoffs, is presented and studied. A continuous variable,
representing the probability of cooperation, [0,1], is assigned to
each agent at time . At each time step a pair of agents, chosen at
random, interact by playing the game. The players update their using a
criteria based on the comparison of their utilities with the simplest estimate
for expected income. The agents have no memory and use strategies not based on
direct reciprocity nor 'tags'. Depending on the payoff matrix, the systems
self-organizes - after a transient - into stationary states characterized by
their average probability of cooperation and average equilibrium
per-capita-income . It turns out that the model
exhibit some results that contradict the intuition. In particular, some games
which - {\it a priory}- seems to favor defection most, may produce a relatively
high degree of cooperation. Conversely, other games, which one would bet that
lead to maximum cooperation, indeed are not the optimal for producing
cooperation.Comment: 11 pages, 3 figures, keybords: Complex adaptive systems, Agent-based
models, Social system
Dynamics of Interacting Neural Networks
The dynamics of interacting perceptrons is solved analytically. For a
directed flow of information the system runs into a state which has a higher
symmetry than the topology of the model. A symmetry breaking phase transition
is found with increasing learning rate. In addition it is shown that a system
of interacting perceptrons which is trained on the history of its minority
decisions develops a good strategy for the problem of adaptive competition
known as the Bar Problem or Minority Game.Comment: 9 pages, 3 figures; typos corrected, content reorganize
Multiobjective adaptive symbiotic organisms search for truss optimization problems
This paper presents a multiobjective adaptive symbiotic organisms search (MOASOS) and its two-archive technique for solving truss optimization problems. The SOS algorithm considers the symbiotic relationship among various species, such as mutualism, commensalism, and parasitism, to live in nature. The heuristic characteristics of the mutualism phase permits the search to jump into not visited sections (named an exploration) and allows a local search of visited sections (named an exploitation) of the search region. As search progresses, a good balance between an exploration and exploitation has a greater impact on the solutions. Thus, adaptive control is now incorporated to propose MOASOS. In addition, two-archive approach is applied in MOASOS to maintain population diversity which is a major issue in multiobjective meta-heuristics.
For the design problems, minimization of the truss� mass and maximization of nodal displacement are objectives whereas elemental stress and discrete cross-sectional areas are assumed to be behaviour and side constraints respectively. The usefulness of these methods to solve complex problems is validated by five truss problems (i.e. 10-bar truss, 25-bar truss, 60-bar truss, 72-bar truss, and 942-bar truss) with discrete design variables. The results of the proposed algorithms have demonstrated that adaptive control is able to provide a better and competitive solutions when compared against the previous studies
- …