3,612 research outputs found
Knowledge-Based Nonuniform Crossover
We present a new knowledge-based non-uniform crossover (KNUX) operator for genetic algorithms (GA\u27s) that generalizes uniform crossover. We extend this to Dynamic KNUX (DKNUX), which constantly updates the knowledge extracted so far from the environment\u27s feedback on previously generated chromosomes. KNUX can improve on good solutions previously obtained by using other algorithms. The modifications made by KNUX are orthogonal to other changes in parameters of GA\u27s, and can be pursued together with any other proposed improvements. Whereas most genetic search methods focus on improving the move-selection procedures, after having chosen a fixed move-generation mechanism, KNUX and DKNUX make the move-generation process itself time-dependent. The same parents may give rise to different offspring at different moments in the evolutionary process, based on the past experience of the species. Simulation results show orders of magnitude improvement of KNUX over two-point and uniform crossover, on three NP optimization problems: graph partitioning, soft-decision decoding of linear block codes, and the traveling salesperson problem. KNUX has been applied to variants of the graph partitioning problem that cannot be solved easily using non-GA approaches, and to improve quality of solutions obtained using non-GA methods. DKNUX opens up the field of applying GA\u27s to Incremental Optimization problems, characterized by a slow change in problem structure with time. DKNUX also achieves some of the goals of diploid representations with adaptive dominance, with smaller computational requirements
Dimensional crossover of the fundamental-measure functional for parallel hard cubes
We present a regularization of the recently proposed fundamental-measure
functional for a mixture of parallel hard cubes. The regularized functional is
shown to have right dimensional crossovers to any smaller dimension, thus
allowing to use it to study highly inhomogeneous phases (such as the solid
phase). Furthermore, it is shown how the functional of the slightly more
general model of parallel hard parallelepipeds can be obtained using the
zero-dimensional functional as a generating functional. The multicomponent
version of the latter system is also given, and it is suggested how to
reformulate it as a restricted-orientation model for liquid crystals. Finally,
the method is further extended to build a functional for a mixture of parallel
hard cylinders.Comment: 4 pages, no figures, uses revtex style files and multicol.sty, for a
PostScript version see http://dulcinea.uc3m.es/users/cuesta/cross.p
Initial static susceptibilities of nonuniform and random Ising chains
Within the conventional framework of standard linear response theory we have
derived exact results for the initial static susceptibilities of nonuniform
spin-1/2 Ising chains. The results obtained permit one to study regularly
alternating-bond and random-bond Ising chains. The influence of several types
of nonuniformity and disorder on the temperature dependence of the initial
longitudinal and transverse static susceptibilities is discussed.Comment: LaTeX, 7 figure
Nematic crossover in BaFeAs under uniaxial stress
Raman scattering can detect spontaneous point-group symmetry breaking without
resorting to single-domain samples. Here we use this technique to study
, the parent compound of the "122" Fe-based
superconductors. We show that an applied compression along the Fe-Fe direction,
which is commonly used to produce untwinned orthorhombic samples, changes the
structural phase transition at temperature into a crossover
that spans a considerable temperature range above . Even in
crystals that are not subject to any applied force, a distribution of
substantial residual stress remains, which may explain phenomena that are
seemingly indicative of symmetry breaking above . Our results
are consistent with an onset of spontaneous nematicity only below
.Comment: 4 pages, 4 figure
Mesoscopic Noise Theory: Microscopics, or Phenomenology?
We argue, physically and formally, that existing diffusive models of noise
yield inaccurate microscopic descriptions of nonequilibrium current
fluctuations. The theoretical shortfall becomes pronounced in quantum-confined
metallic systems, such as the two-dimensional electron gas. In such systems we
propose a simple experimental test of mesoscopic validity for diffusive
theory's central claim: the smooth crossover between Johnson-Nyquist and shot
noise.Comment: Invited paper, UPoN'99 Conference, Adelaide. 13 pp, no figs. Minor
revisions to text and reference
An investigation of messy genetic algorithms
Genetic algorithms (GAs) are search procedures based on the mechanics of natural selection and natural genetics. They combine the use of string codings or artificial chromosomes and populations with the selective and juxtapositional power of reproduction and recombination to motivate a surprisingly powerful search heuristic in many problems. Despite their empirical success, there has been a long standing objection to the use of GAs in arbitrarily difficult problems. A new approach was launched. Results to a 30-bit, order-three-deception problem were obtained using a new type of genetic algorithm called a messy genetic algorithm (mGAs). Messy genetic algorithms combine the use of variable-length strings, a two-phase selection scheme, and messy genetic operators to effect a solution to the fixed-coding problem of standard simple GAs. The results of the study of mGAs in problems with nonuniform subfunction scale and size are presented. The mGA approach is summarized, both its operation and the theory of its use. Experiments on problems of varying scale, varying building-block size, and combined varying scale and size are presented
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Genetic algorithm approach to find the best input variable partitioning
Conference PaperThis paper presents a variable partition algorithm which combines the quasi-reduced ordered multiple-terminal multiple-valued decision diagrams and genetic algorithms (GAs). The algorithm is better than the previous techniques which find a good functional decomposition by non-exhaustive search and expands the range of searching for the best decomposition providing the optimal subtable multiplicity. The possible solutions are evaluated using the gain of decomposition for a multiple-output multiple-valued logic function. The distinct feature of GA is the possible solutions being coded by real numbers. Here the simplex-based crossover is proposed to use for the recombination stage of GA. It permits to increase the GA coverag
Finite Temperature Theory of Metastable Anharmonic Potentials
The decay rate for a particle in a metastable cubic potential is investigated
in the quantum regime by the Euclidean path integral method in semiclassical
approximation. The imaginary time formalism allows one to monitor the system as
a function of temperature. The family of classical paths, saddle points for the
action, is derived in terms of Jacobian elliptic functions whose periodicity
sets the energy-temperature correspondence. The period of the classical
oscillations varies monotonically with the energy up to the sphaleron, pointing
to a smooth crossover from the quantum to the activated regime. The softening
of the quantum fluctuation spectrum is evaluated analytically by the theory of
the functional determinants and computed at low up to the crossover. In
particular, the negative eigenvalue, causing an imaginary contribution to the
partition function, is studied in detail by solving the Lam\`{e} equation which
governs the fluctuation spectrum. For a heavvy particle mass, the decay rate
shows a remarkable temperature dependence mainly ascribable to a low lying soft
mode and, approaching the crossover, it increases by a factor five over the
predictions of the zero temperature theory. Just beyond the peak value, the
classical Arrhenius behavior takes over. A similar trend is found studying the
quartic metastable potential but the lifetime of the latter is longer by a
factor ten than in a cubic potential with same parameters. Some formal
analogies with noise-induced transitions in classically activated metastable
systems are discussed.Comment: European Physical Journal B EDP Sciences, Societ`a Italiana di
Fisica, Springer-Verlag 200
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