6,032 research outputs found
Graph isomorphism completeness for trapezoid graphs
The complexity of the graph isomorphism problem for trapezoid graphs has been
open over a decade. This paper shows that the problem is GI-complete. More
precisely, we show that the graph isomorphism problem is GI-complete for
comparability graphs of partially ordered sets with interval dimension 2 and
height 3. In contrast, the problem is known to be solvable in polynomial time
for comparability graphs of partially ordered sets with interval dimension at
most 2 and height at most 2.Comment: 4 pages, 3 Postscript figure
Single-wavenumber Representation of Nonlinear Energy Spectrum in Elastic-Wave Turbulence of {F}\"oppl-von {K}\'arm\'an Equation: Energy Decomposition Analysis and Energy Budget
A single-wavenumber representation of nonlinear energy spectrum, i.e.,
stretching energy spectrum is found in elastic-wave turbulence governed by the
F\"oppl-von K\'arm\'an (FvK) equation. The representation enables energy
decomposition analysis in the wavenumber space, and analytical expressions of
detailed energy budget in the nonlinear interactions are obtained for the first
time in wave turbulence systems. We numerically solved the FvK equation and
observed the following facts. Kinetic and bending energies are comparable with
each other at large wavenumbers as the weak turbulence theory suggests. On the
other hand, the stretching energy is larger than the bending energy at small
wavenumbers, i.e., the nonlinearity is relatively strong. The strong
correlation between a mode and its companion mode is
observed at the small wavenumbers. Energy transfer shows that the energy is
input into the wave field through stretching-energy transfer at the small
wavenumbers, and dissipated through the quartic part of kinetic-energy transfer
at the large wavenumbers. A total-energy flux consistent with the energy
conservation is calculated directly by using the analytical expression of the
total-energy transfer, and the forward energy cascade is observed clearly.Comment: 11 pages, 4 figure
Weak and strong wave turbulence spectra for elastic thin plate
Variety of statistically steady energy spectra in elastic wave turbulence
have been reported in numerical simulations, experiments, and theoretical
studies. Focusing on the energy levels of the system, we have performed direct
numerical simulations according to the F\"{o}ppl--von K\'{a}rm\'{a}n equation,
and successfully reproduced the variability of the energy spectra by changing
the magnitude of external force systematically. When the total energies in wave
fields are small, the energy spectra are close to a statistically steady
solution of the kinetic equation in the weak turbulence theory. On the other
hand, in large-energy wave fields, another self-similar spectrum is found.
Coexistence of the weakly nonlinear spectrum in large wavenumbers and the
strongly nonlinear spectrum in small wavenumbers are also found in moderate
energy wave fields.Comment: 5 pages, 3 figure
Large-scale lognormality in turbulence modeled by Ornstein-Uhlenbeck process
Lognormality was found experimentally for coarse-grained squared turbulence
velocity and velocity increment when the coarsening scale is comparable to the
correlation scale of the velocity (Mouri et al. Phys. Fluids 21, 065107, 2009).
We investigate this large-scale lognormality by using a simple stochastic
process with correlation, the Ornstein-Uhlenbeck (OU) process. It is shown that
the OU process has a similar large-scale lognormality, which is studied
numerically and analytically.Comment: 7 pages, 5 figures, PRE in pres
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